135. Phillipson, L. M.; Toumi, R. The Crossover Time as an Evaluation of Ocean Models Against Persistence GEOPHYSICAL RESEARCH LETTERS 45  Issue: 1 250-257 2018


A new ocean evaluation metric, the crossover time, is defined as the time it takes for a numerical model to equal the performance of persistence. As an example, the average crossover time calculated using the Lagrangian separation distance (the distance between simulated trajectories and observed drifters) for the global MERCATOR ocean model analysis is found to be about 6 days. Conversely, the model forecast has an average crossover time longer than 6 days, suggesting limited skill in Lagrangian predictability by the current generation of global ocean models. The crossover time of the velocity error is less than 3 days, which is similar to the average decorrelation time of the observed drifters. The crossover time is a useful measure to quantify future ocean model improvements.


134. Bruneau, Nicolas; Toumi, Ralf; Wang, Shuai Impact of wave whitecapping on land falling tropical cyclones SCIENTIFIC REPORTS   Vol: 8  Article Number: 652   2018


Predicting tropical cyclone structure and evolution remains challenging. Particularly, the surface wave interactions with the continental shelf and their impact on tropical cyclones have received very little attention. Through a series of state-of-the-art high-resolution, fully-coupled ocean-wave and atmosphere-ocean-wave experiments, we show here, for the first time, that in presence of continental shelf waves can cause substantial cooling of the sea surface. Through whitecapping there is a transfer of momentum from the surface which drives deeper vertical mixing. It is the waves and not just the wind which become the major driver of stratified coastal ocean ahead-of-cyclone cooling. In the fully-coupled atmosphere-ocean-wave model a negative feedback is found. The maximum wind speed is weaker and the damaging footprint area of hurricane-force winds is reduced by up to 50% due to the strong wave induced ocean cooling ahead. Including wave-ocean coupling is important to improve land falling tropical cyclone intensity predictions for the highly populated and vulnerable coasts.


133. Bruneau, Nicolas; Zika, Jan; Toumi, Ralf Can the Ocean's Heat Engine Control Horizontal Circulation? Insights From the Caspian Sea GEOPHYSICAL RESEARCH LETTERS 44  19   9893-9900 2017


We investigate the role of the ocean's heat engine in setting horizontal circulation using a numerical model of the Caspian Sea. The Caspian Sea can be seen as a virtual laboratorya compromise between realistic global models that are hampered by long equilibration times and idealized basin geometry models, which are not constrained by observations. We find that increases in vertical mixing drive stronger thermally direct overturning and consequent conversion of available potential to kinetic energy. Numerical solutions with water mass structures closest to observations overturn 0.02-0.04x10(6)m(3)/s (sverdrup) representing the first estimate of Caspian Sea overturning. Our results also suggest that the overturning is thermally forced increasing in intensity with increasing vertical diffusivity. Finally, stronger thermally direct overturning is associated with a stronger horizontal circulation in the Caspian Sea. This suggests that the ocean's heat engine can strongly impact broader horizontal circulations in the ocean.


132. Li, Yi; Toumi, Ralf A balanced Kalman filter ocean data assimilation system with application to the South Australian Sea OCEAN MODELLING  Vol: 116  159-172  2017


In this paper, an Ensemble Kalman Filter (EnKF) based regional ocean data assimilation system has been developed and applied to the South Australian Sea. This system consists of the data assimilation algorithm provided by the NCAR Data Assimilation Research Testbed (DART) and the Regional Ocean Modelling System (ROMS). We describe the first implementation of the physical balance operator (temperature-salinity, hydrostatic and geostrophic balance) to DART, to reduce the spurious waves which may be introduced during the data assimilation process. The effect of the balance operator is validated in both an idealised shallow water model and the ROMS model real case study. In the shallow water model, the geostrophic balance operator eliminates spurious ageostrophic waves and produces a better sea surface height (SSH) and velocity analysis and forecast. Its impact increases as the sea surface height and wind stress increase. In the real case, satellite-observed sea surface temperature (SST) and SSH are assimilated in the South Australian Sea with 50 ensembles using the Ensemble Adjustment Kalman Filter (EAKF). Assimilating SSH and SST enhances the estimation of SSH and SST in the entire domain, respectively. Assimilation with the balance operator produces a more realistic simulation of surface currents and subsurface temperature profile. The best improvement is obtained when only SSH is assimilated with the balance operator. A case study with a storm suggests that the benefit of the balance operator is of particular importance under high wind stress conditions. Implementing the balance operator could be a general benefit to ocean data assimilation systems.


131 Corsaro, C. M. and Toumi, R. A self-weakening mechanism for tropical cyclones. Q.J.R. Meteorol. Soc, 143: 2585-2599. doi:10.1002/qj.3109 2017


A mechanism leading to the self-weakening of tropical cyclones is proposed using the Weather and Research and Forecasting model. A comparison between an experiment with variable Coriolis parameter f and one on an f-plane shows that, after the initial intensification, the former is characterized by a smaller intensity. As opposed to the tropical cyclone on the f-plane, the one with variable f weakens significantly after reaching maturity. Analyses of the 3D circulation show that the main reason for the weakening is dry intrusion in the mid-upper troposphere from the west. Once the dry intrusion reaches the inner vortex, strong downdraughts reduce the high equivalent potential temperature in the boundary-layer inflow. The subsequent updraughts in the eyewall, characterized by lower equivalent potential temperature, are considerably reduced and, consequently, the secondary circulation weakens. Back-trajectories are used to determine the origin of the dry intrusion. It is found that the air parcels expelled from the storm deep convection into the outflow layer recirculate anticyclonically back into the vortex, causing a self-weakening of the tropical cyclone. A time span for the recirculation of at least 48 h allows the air parcels to sink substantially before reaching the vortex circulation. Some implications of the intrinsic nature of this process are briefly discussed.



130. Phillipson, Luke; Toumi, R Impact of data assimilation on ocean current forecasts in the Angola Basin OCEAN MODELLING Vol: 114 45-58 2017


The ocean current predictability in the data limited Angola Basin was investigated using the Regional Ocean Modelling System (ROMS) with four-dimensional variational data assimilation. Six experiments were undertaken comprising a baseline case of the assimilation of salinity/temperature profiles and satellite sea surface temperature, with the subsequent addition of altimetry, OSCAR (satellite -derived sea surface currents), drifters, altimetry and drifters combined, and OSCAR and drifters combined. The addition of drifters significantly improves Lagrangian predictability in comparison to the baseline case as well as the addition of either altimetry or OSCAR. OSCAR assimilation only improves Lagrangian predictability as much as altimetry assimilation. On average the assimilation of either altimetry or OSCAR with drifter velocities does not significantly improve Lagrangian predictability compared to the drifter assimilation alone, even degrading predictability in some cases. When the forecast current speed is large, it is more likely that the combination improves trajectory forecasts. Conversely, when the currents are weaker, it is more likely that the combination degrades the trajectory forecast.


129. Arcucci, Rossella; D'Amore, Luisa; Pistoia, Jenny; Toumi, R, Murli, A. On the variational data assimilation problem solving and sensitivity analysis J. COMP. PHYS. Vol: 335  311-326  2017

We consider the Variational Data Assimilation (VarDA) problem in an operational framework, namely, as it results when it is employed for the analysis of temperature and salinity variations of data collected in closed and semi closed seas. We present a computing approach to solve the main computational kernel at the heart of the VarDA problem, which outperforms the technique nowadays employed by the oceanographic operative software. The new approach is obtained by means of Tikhonov regularization. We provide the sensitivity analysis of this approach and we also study its performance in terms of the accuracy gain on the computed solution. We provide validations on two realistic oceanographic data sets.

128. Cahill, B., R. Toumi, G. Stenchikov, S. Osipov, H. Brindley. Evaluation of thermal and dynamic impacts of summer dust aerosols on the Red Sea, J. Geophys. Res. Oceans, 122, doi:10.1002/2016JC011911 2017.

The seasonal response of upper ocean processes in the Red Sea to summer-time dust aerosol perturbations is investigated using an uncoupled regional ocean model. We find that the upper limit response is highly sensitive to dust-induced reductions in radiative fluxes. Sea surface cooling of −1 oC and −20C is predicted in the northern and southern regions, respectively. This cooling is associated with a net radiation reduction of −40 W m−2 a nd −90 W m−2 over the northern and southern regions, respectively. Larger cooling occurs below the mixed layer at 75 m in autumn, −1.2 oC (north) and −1.9 oC (south). SSTs adjust more rapidly (ca. 30 days) than the subsurface temperatures (seasonal time scales), due to stronger stratification and increased mixed layer stability inhibiting the extent of vertical mixing. The basin average annual heat flux reverses sign and becomes positive, +4.2 W m−2 (as compared to observed estimates −17.3 W m−2) indicating a small gain of heat from the atmosphere. When we consider missing feedbacks from atmospheric processes in our uncoupled experiment, we postulate that the magnitude of cooling and the time scales for adjustment will be much less, and that the annual heat flux will not reverse sign but nevertheless be reduced as a result of dust perturbations. While our study highlights the importance of considering coupled ocean-atmosphere processes on the net surface energy flux in dust perturbation studies, the results of our uncoupled dust experiment still provide an upper limit estimate of the response of the upper ocean to dust-induced radiative forcing perturbations.

127. Nissan, H. and Toumi, R., Regional modelling of rainfall erosivity: sensitivity of soil erosion to aerosol emissions. Q.J.R. Meteorol. Soc., 143: 265-277. doi:10.1002/qj.2919 2017

A rainfall erosivity scheme is implemented into the widely used regional atmosphere-land model, WRF. Rainfall erosivity is parametrized from hourly precipitation and surface runoff in a high resolution (4 km) convection-permitting model. The scheme is used to examine the potential effects of changes in atmospheric aerosol concentrations on soil erosion in a case-study of northern India and the surrounding countries for the 2010 monsoon season, using a model which isolates the indirect effect on cloud microphysics only. This study offers a preliminary investigation into this emerging topic, but longer simulations would be needed to establish a robust signal. Summer precipitation is reduced in most areas and the monsoon circulation weakens for increases in cloud condensation nuclei concentrations. This can be attributed to localised cloud microphysical changes in the northeast of India, which induce a dynamic response opposing the monsoon circulation. The two regions of greatest decrease in erosion with increasing aerosol are in the Western Ghats and the Ganges Delta, both significant cropland areas. However, the effect is not uniform, with isolated local increases in soil erosion. These results suggest that, while efforts to reduce anthropogenic aerosol emissions may improve water availability for crops through enhanced rainfall, these benefits are likely to be tempered by an increase in soil erosion, though robust local changes were difficult to predict.

126. Trivedi, A. and Toumi, R., Mechanisms of bottom boundary fluxes in a numerical model of the Shetland shelf. Ocean Dynamics, 67(1),1-21, 2017

Across-slope bottom boundary layer (BBL) fluxes on the shelf-edge connect this region to deeper waters. Two proposed ways in which across-slope BBL fluxes can occur, in regions that have a slope current aligned to the bathymetry, are the frictional veering of bottom currents termed the 'Ekman drain' and through local wind-forced downwelling (wind-driven surface Ekman flow with an associated bottom flow). We investigate the variability, magnitude and spatial scale of BBL fluxes on the Shetland shelf, which has a prominent slope current, using a high-resolution (similar to 2 km) configuration of the MITgcm model. Fluxes are analysed in the BBL at the shelf break near the 200 m isobath and are found to have a seasonal variability with high/low volume transport in winter/summer respectively. By using a multivariate regression approach, we find that the locally wind-driven Ekman transport plays no explicit role in explaining daily bottom fluxes. We can better explain the variability of the across-slope BBL flux as a linear function of the speed and across-slope component of the interior flow, corresponding to an Ekman plus mean-flow flux. We estimate that the mean-flow is a greater contributor than the Ekman flux to the BBL flux. The spatial heterogeneity of the BBL fluxes can be attributed to the mean-flow, which has a much shorter decorrelation length compared to the Ekman flux. We conclude that both the speed and direction of the interior current determines the daily BBL flux. The wind does not explicitly contribute through local downwelling, but may influence the interior current and therefore implicitly the BBL fluxes on longer timescales.

125. Bruneau, N. and Toumi, R., A fully-coupled atmosphere-ocean-wave model of the Caspian Sea. Ocean Modelling, 107, pp.97-111, 2016.

Located in the mid-latitudes, the Caspian Sea is the largest enclosed basin in the world. A fully-coupled atmosphere-ocean-wave model of the Caspian Sea at high resolution (8 km) for a period of three years is presented. After validating each component of the modelling platform, the wave state of the Caspian Sea is studied. Results show very different wave regimes between the three different basins, a strong seasonality and an almost swell-free state. It is shown here that waves modify the horizontal eddy viscosity and vertical heat diffusion. However, due to a reasonably weak annual wave state, these effects are restricted to the upper-ocean layer (< 30 m) except during the most severe events (100 m). Three main experiments are conducted: 1) the ROMS ocean model forced by atmospheric reanalysis (CFSR), 2) ROMS coupled with the atmospheric model WRF and 3) the impact of wave-induced processes. The seasonality of the Caspian Sea is accurately captured in each experiment which highlights a rapid warming of the sea surface temperature (SST) in spring while the mixed layer depths (MLD) become very rapidly shallow (shifting from over 100 m to 15 m in two months). Contrarily, a gentle cooling of the SST accompanied with a deepening of the MLD is modelled during autumn and winter. The results also show a significant improvement of the model skill in the representation of the dynamics when ROMS is coupled to WRF. Finally, as ocean surface waves imply feedback at the interface atmosphere-ocean through the transfer of momentum, mass and heat, we investigate their potential effects on the Caspian Sea dynamics. Results are mixed and show a reasonably weak impact of wave-induced processes. While waves have a negligible effect during the winter as wave-induced mixing is confined to the MLD, the summer global SST are less accurately modelled due to the enhancement of mixing in shallow MLDs. However the SST bias, temperature at a subsurface location are improved. (C) 2016 Elsevier Ltd. All rights reserved.

124. Wang S. and Toumi R., On the relationship between hurricane cost and the integrated wind profile. Environ. Res. Lett., 11, 114005 2016.

It is challenging to identify metrics that best capture hurricane destructive potential and costs. Although it has been found that the sea surface temperature and vertical wind shear can both make considerable changes to the hurricane destructive potential metrics, it is still unknown which plays a more important role. Here we present a new method to reconstruct the historical wind structure of hurricanes that allows us, for the first time, to calculate the correlation of damage with integrated power dissipation and integrated kinetic energy of all hurricanes at landfall since 1988. We find that those metrics, which include the horizontal wind structure, rather than just maximum intensity, are much better correlated with the hurricane cost. The vertical wind shear over the main development region of hurricanes plays a more dominant role than the sea surface temperature in controlling these metrics and therefore also ultimately the cost of hurricanes.

123. Wong, Benjamin; Toumi, Ralf Effect of extreme ocean precipitation on sea surface elevation and storm surges QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Vol:142  699 2541-2550 Part: B  2016


Ocean models that neglect mass and momentum contributions from precipitation can have a systematic bias in sea surface height (SSH). Here, a new rainfall scheme is introduced into the Regional Ocean Modelling System (ROMS) to incorporate the effects of precipitation mass. When precipitation is added to the sea surface, it spreads out via surface gravity waves that increase in propagation speed with increasing water depth. Over several days, the SSH increase due to the precipitation mass added creates a geostrophic adjustment, generating anticyclonic geostrophic currents around the SSH increase. The transfer of momentum from precipitation to the sea surface, or rain stress, can also be important. In the case-study of a real tropical cyclone, Monica, passing North Australia, the effect of incorporating precipitation mass is compared with other processes affecting the storm surge: surface wind, inverse barometer effect and rain stress. The maximum SSH response is 170.6 cm for the wind effect, 61.5 cm for the inverse barometer effect, 7.5 cm for the effect of rain stress and 6.4 cm for the effect of rain mass. Each process has been shown to have different spatial influences. The effect of rain mass has a strong remote influence compared to the inverse barometer effect and the effect of rain stress. This is particularly seen in semi-enclosed bays.


122. Phibbs, Samuel; Toumi, Ralf The dependence of precipitation and its footprint on atmospheric temperature in idealized extratropical cyclones JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES Vol: 121 15 8743-8754  2016


Flood hazard is a function of the magnitude and spatial pattern of precipitation accumulation. The sensitivity of precipitation to atmospheric temperature is investigated for idealized extratropical cyclones, enabling us to examine the footprint of extreme precipitation (surface area where accumulated precipitation exceeds high thresholds) and the accumulation in different-sized catchment areas. The mean precipitation increases with temperature, with the mean increase at 5.40%/degrees C. The 99.9th percentile of accumulated precipitation increases at 12.7%/degrees C for 1h and 9.38%/degrees C for 24h, both greater than Clausius-Clapeyron scaling. The footprint of extreme precipitation grows considerably with temperature, with the relative increase generally greater for longer durations. The sensitivity of the footprint of extreme precipitation is generally super Clausius-Clapeyron. The surface area of all precipitation shrinks with increasing temperature. Greater relative changes in the number of catchment areas exceeding extreme total precipitation are found when the domain is divided into larger rather than smaller catchment areas. This indicates that fluvial flooding may increase faster than pluvial flooding from extratropical cyclones in a warming world. When the catchment areas are ranked in order of total precipitation, the 99.9th percentile is found to increase slightly above Clausius-Clapeyron expectations for all of the catchment sizes, from 9km(2) to 22,500km(2). This is surprising for larger catchment areas given the change in mean precipitation. We propose that this is due to spatially concentrated changes in extreme precipitation in the occluded front.


121. Poulain, V.; Bekki, S.; Marchand, M.; et al. Evaluation of the inter-annual variability of stratospheric chemical composition in chemistry-climate models using ground-based multi species time series J. ATMOS. SOLAR-TERR. PHYSICS  Vol: 145   61-84   2016


The variability of stratospheric chemical composition occurs on a broad spectrum of timescales, ranging from day to decades. A large part of the variability appears to be driven by external forcings such as volcanic aerosols, solar activity, halogen loading, levels of greenhouse gases (GHG), and modes of climate variability (quasi-biennial oscillation (QBO), El Nino-Southern Oscillation (ENSO)). We estimate the contributions of different external forcings to the interannual variability of stratospheric chemical composition and evaluate how well 3-D chemistry-climate models (CCMs) can reproduce the observed response-forcing relationships. We carry out multivariate regression analyses on long time series of observed and simulated time series of several traces gases in order to estimate the contributions of individual forcings and unforced variability to their internannual variability. The observations are typically decadal time series of ground-based data from the international Network for the Detection of Atmospheric Composition Change (NDACC) and the CCM simulations are taken from the CCMVaI-2 REF-B1 simulations database. The chemical species considered are column O-3, HCl, NO2, and N2O. We check the consistency between observations and model simulations in terms of the forced and internal components of the total interannual variability (externally forced variability and internal variability) and identify the driving factors in the interannual variations of stratospheric chemical composition over NDACC measurement sites. Overall, there is a reasonably good agreement between regression results from models and observations regarding the externally forced interannual variability. A much larger fraction of the observed and modelled interannual variability is explained by external forcings in the tropics than in the extratropics, notably in polar regions. CCMs are able to reproduce the amplitudes of responses in chemical composition to specific external forcings. However, CCMs tend to underestimate very substantially the internal variability and hence the total interannual variability for almost all species considered. This lack of internal variability in CCMs might partly originate from the surface forcing-of these CCMs by analysed SSTs. The results illustrate the potential of NDACC ground-based observations for evaluating CCMs.


120. Wong, Benjamin; Toumi, Ralf Model study of the asymmetry in tropical cyclone-induced positive and negative surges ATMOSPHERIC SCIENCE LETTERS  Vol: 17   Iss: 5   334-338   2016


Storm surges pose significant threats to coastal communities, yet negative surges are not as well understood as positive surges. In this study, idealized experiments of a tropical cyclone forcing a 3D ocean model are conducted to investigate the asymmetry of positive and negative surges. Negative surges are larger in magnitude and extend further across the coastline than positive surges. While positive surges are driven by wind blowing onshore, negative surges are largely dominated by alongshore winds, with horizontal divergence as the main mechanism. This asymmetry also increases with decreasing depth and increasing latitude.



119. Wang, Shuai; Toumi, Ralf; Czaja, Arnaud; et al. An analytic model of tropical cyclone wind profiles QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY  Vol: 141   Iss: 693  3018-3029   Part: B   2015


A physically based analytic model (lambda model) is presented to describe the wind profile of tropical cyclones in terms of the pressure deficit and a single shape parameter (lambda). To test the model, an idealized full-physics numerical model is employed to provide wind-profile samples and also to show the influence of environmental temperature and the properties of initial vortices on tropical cyclone size. It is found that the lambda model provides an accurate fit of the azimuthal wind profile at the top of the boundary layer. In the simulations, tropical cyclone size is sensitive to sea-surface temperature, upper tropospheric temperature and initial vortex structure. The numerical model confirms the assumed Gaussian distribution with width lambda of the moist entropy in the boundary layer. A linear relationship between model cyclone size and root lambda is found, in agreement with the lambda model. The lambda model predicts a weak relationship between tropical cyclone size and intensity, as is observed. In addition, the lambda model suggests that change in tropical cyclone size should be closely related to angular momentum transport near the boundary layer, as has been found in observations. The good agreement of the lambda model with the numerical model shows that the lambda model could be a reasonable alternative for characterizing the wind structure of tropical cyclones with only one scaling parameter.



118. Tamura-Wicks, Helen; Toumi, Ralf; Budgell, W. Paul Sensitivity of Caspian sea-ice to air temperature QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY  Vol: 141   Iss: 693  3088-3096   Part: B   2015


Caspian sea ice concentration from satellite passive microwave data and surface daily air temperatures are analysed from 1978 to 2009. Relationships between mean winter air temperatures, cumulative freezing degree days (CFDD) and the sum of daily ice area (cumulative ice area) are found. These show that mean monthly air temperature of less than 5.5-9.5 degrees C, and a minimum CFDD of 3.6 +/- 11.2 degrees C, are required for ice formation in the northern Caspian. Examination of climate projections from multi-model ensembles of monthly mean air temperatures suggest that the northern Caspian may be largely ice-free by 2100 for the highest emission scenario. An ocean-ice-atmosphere model of the Caspian shows weak sensitivities of the minimum CFDD to varied sea ice albedo and ice compressive strength. Sea level decline is found to reduce the minimum CFDD as well as promote the formation of higher concentration or closed ice.



117. Broadbridge, Maria B.; Toumi, Ralf The deep circulation of the Faroe Shetland Channel: Opposing flows and topographic eddies JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  Vol: 120   Iss: 9 5983 5996    2015


New insights into the deep circulation of the Faroe-Shetland channel are gained from a high-resolution regional ocean model. The simulation shows a more complex structure of the deep flow field than previously thought: a flow reversal of the deep and intermediate waters to the northeast on the Faroese flank of the channel and persistent topographic eddies that force the deep currents into a gyre-like structure. This flow reversal opposes the previously accepted understanding of a purely southwestward deep flow but is in agreement with velocity measurements. The southwestward transport of the overflow waters is found to be facilitated almost exclusively by a strong and narrow current on the Shetland side of the channel. Float release experiments show that up to 38% of the overflow water takes longer than a purely southwestward flow regime suggests and up to 13% takes twice as long. From the release of tracers, a substantial amount of lateral mixing is evident within the channel, predominantly facilitated by the topographic eddies.


116. Bossay, S. ; Bekki, S.; Marchand, M.; Poulain, V.;Toumi, R. Sensitivity of tropical stratospheric ozone to rotational UV variations estimated from UARS and Aura MLS observations during the declining phases of solar cycles 22 and 23 JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS  Vol: 130  96-111 2015.


The correlation between tropical stratospheric ozone and UV radiation on solar rotational time scales is investigated using daily satellite ozone observations and reconstructed solar spectra. We consider two 3-year periods falling within the descending phases of two 11-year solar cycles 22 (1991-1994) and 23 (2004-2007). The UV rotational cycle is highly irregular and even disappears for half a year during cycle 23. For the 1991-1994 period, ozone and 205 nm UV flux are found to be correlated between about 10 and 1 hPa with a maximum of 0.29 at similar to 5 hPa; ozone sensitivity (percentage change in ozone for 1% change in UV) peaks at similar to 0.4. Correlation during cycle 23 is weaker with a peak ozone sensitivity of 0.2. The correlation is found to vary widely, not only with altitude, but also from one year to the next with a rotational signal in ozone appearing almost intermittent. Unexpectedly, the correlation is not found to bear any relation with the solar rotational forcing. For instance, solar rotational fluctuations are by far the strongest during 1991-1992 whereas the correlation peaks at the end of 1993, a rotationally quiescent period. When calculated over sliding intervals of 1-year, the sensitivity is found to vary very strongly within both 3-year periods; it is almost negligible over the entire vertical profile during some 1-year intervals or reaches close to 1 around 2-5 mb for other intervals. Other sources of variability, presumably of dynamical origin, operate on the rotational spectral range and determine to a large extent the estimated solar rotational signal. Even considering 3 years of observations (corresponding to about 40 solar cycles), the extraction of the rotational solar signal does not appear to be robust during declining phases of 11-year solar cycles. As observational studies cover at best three 11-year solar cycles, it must be challenging to produce a reliable estimation of the 11-year solar cycle signal in stratospheric ozone, especially in the presence of decadal climate variability.



115. Newinger, C.; Toumi, R ; Potential impact of the colored Amazon and Orinoco plume on tropical cyclone intensity. JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS Vol: 120   Is: 2   1296-1317 2015


The Amazon and the Orinoco river plumes modulate ocean stratification and color in the tropical North Atlantic. This changes air-sea interactions and may thus be important for tropical cyclones (TCs). Using a regional ocean model, we try to separate the potential impact of river freshwater and light absorption on ocean temperatures, stability, and TC intensity. While the freshwater plume stabilizes the water column, there is no significant change in sea surface temperatures. However, increased stability and temperature inversions may reduce surface cooling feedbacks. The cooling inhibition index (CI) is +2.2 (J/m(2))(1/3) larger when the river freshwater plume is present. Ocean color in the river plume on the other hand, blocks the deeper ocean from sunlight, leading to moderate surface warming (+0.1 degrees C) and substantial subsurface cooling (-0.3 degrees C 100 m mean temperature). As a consequence cold water is more readily available to passing storms and the CI decreases by -2.1 (J/m(2))(1/3). Using simple, idealized relationships between expected surface cooling and TC intensity, we find that river-induced stability enhances strong TCs by up to -5 to -12 hPa, while ocean color may reduce intensity by +8 hPa to +16 hPa. The net impact of the colored plume is negligible for weak storms and a slight intensity reduction for stronger cyclones. Within the Amazon and Orinoco plume, the river freshwater effect may thus be substantially reduced or even offset by light absorbing particles.


114. Hardwick SR, Toumi R, Pfeifer M, Turner EC, Nilus R, Ewers RM The relationship between leaf area index and microclimate in tropical forest and oil palm plantation: Forest disturbance drives changes in microclimate, Agricultural and Forest Meteorology, Vol:201, ISSN:0168-1923, 187-195 2015.


Land use change is a major threat to biodiversity. One mechanism by which land use change influences biodiversity and ecological processes is through changes in the local climate. Here, the relationships between leaf area index and five climate variables air temperature, relative humidity, vapour pressure deficit, specific humidity and soil temperature are investigated across a range of land use types in Borneo, including primary tropical forest, logged forest and oil palm plantation. Strong correlations with the leaf area index are found for the mean daily maximum air and soil temperatures, the mean daily maximum vapour pressure deficit and the mean daily minimum relative humidity. Air beneath canopies with high leaf area index is cooler and has higher relative humidity during the day. Forest microclimate is also found to be less variable for sites with higher leaf area indices. Primary forest is found to be up to 2.5 degrees C cooler than logged forest and up to 6.5 degrees C cooler than oil palm plantations. Our results indicate that leaf area index is a useful parameter for predicting the effects of vegetation upon microclimate, which could be used to make small scale climate predictions based on remotely sensed data.


113. Sparks, N.; Toumi, R Numerical Simulations of Daytime Temperature and Humidity Crossover Effects in London . BOUNDARY-LAYER METEOROLOGY Vol: 154   Issue: 1   Pages: 101-117 2015


The effect of the London urban area on vertical profiles of temperature and humidity was analyzed using a mesoscale model. It was found that the near-surface warming and drying effects usually associated with the urban heat island in London in the summer daytime are reversed at heights near the top of the boundary layer. This effect has previously been observed for nighttime temperatures above cities and termed a 'crossover'. The mechanism proposed here to explain this new phenomenon, the daytime crossover, is similar to the previously suggested cause of the nighttime effect, that is, increased entrainment of warm dry air into the top of a cooler, more humid, boundary layer. The median summer daytime temperature crossover was found to be 1.1 K. The cooling was shown to be of a similar magnitude to the warming near the surface and extends up to 100 km downwind with a maximum magnitude at about 1500 UTC in summer. The moistening occurred over a similar spatial scale and peak values were typically two times greater than the near-surface drying effect.


112. Farley Nicholls, J., Toumi, R. and Stenchikov, Effects of unsteady mountain-gap winds on eddies in the Red Sea. Atmosph. Sci. Lett.. doi: 10.1002/asl2.554, 2014


Atmosphere and ocean simulations capture the Tokar mountain-gap wind jet and associated eddy dipole in the Red Sea. Results suggest that, once generated by a wind event, the anticyclonic eddy is further intensified through the summer by subsequent events. An asymmetry in magnitude of the two eddies is observed with the anticyclonic eddy attaining greater magnitude. A series of simulations with idealized wind forcing show that this asymmetry is not present under steady wind forcing. This effect depends on the intermittency of the wind forcing and the presence of a background northerly wind field.


111. Phibbs, Samuel; Toumi, Ralf Modeled dependence of wind and waves on ocean temperature in tropical cyclones GEOPHYSICAL RESEARCH LETTERS  Volume: 41   Issue: 20   Pages: 7383-7390   2014


A coupled ocean-atmosphere-wave model is used to investigate the sensitivity of surface wind speed and significant wave height to ocean temperature for idealized tropical cyclones (TCs). More intense and larger TCs, with higher waves, form when ocean temperature is increased. The maximum significant wave height increases more than the maximum wind speed for TCs up to hurricane force wind. However, above hurricane force wind the change in maximum wind speed is similar or greater than the change in maximum significant wave height. This can be explained by the wind drag coefficient decreasing as wind speed exceeds hurricane force wind, so that the growth of waves is dampened. The areal footprint of wave height grows considerably more than the maximum as ocean temperature is increased. This suggests a large increase in the surface area of damaging waves generated by TCs may be the dominant impact of a future warmer ocean.



110. Radu, Raluca; Toumi, Ralf; Phau, Jared Influence of atmospheric and sea surface temperature on the size of hurricane Catarina QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY  Volume: 140  Issue: 682   Pages: 1778-1784  Part: A 2014


The factors that influence the size of tropical cyclones (TCs) are not completely understood. High-resolution numerical simulations of hurricane Catarina in the South Atlantic indicate that the TC size increases proportionally to the surface latent heat flux, when atmospheric and sea surface temperature (SST) are increased. The TC size is defined as the area enclosed by three wind thresholds: gale-force winds, damaging-force winds and hurricane-force winds. The enlargement increases sharply with the wind threshold. Depending on the wind threshold, the area can increase by more than an order of magnitude when air and sea temperature are both increased by 2 degrees C. There is a clear linear relationship between the size increase and the latent heat flux during the TC lifetime. The size effect is driven initially by the enhanced air-sea moisture contrast and the surface winds become predominant only in the later stages. Temperature changes may therefore have a profound impact on cyclone size.


109. Nicholls, James Farley; Toumi, Ralf On the lake effects of the Caspian Sea QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Vol: 140  Iss: 681   Pages: 1399-1408   Part: B   2014


The impact of the Caspian Sea on precipitation and circulation is investigated. The control regional climate model simulation accurately represents the spatial and temporal pattern of observed precipitation and the upper atmospheric jet. From a simulation without the Caspian, it can be deduced that it causes a year-round increase in evaporation and increases in precipitation in autumn and winter with little change in spring and summer. Over half of the over-lake precipitation is attributable to the lake effect. These findings are confirmed through coupled simulation with a three-dimensional lake model. The Caspian Sea's effect on surface air temperatures (an increase in winter and decrease in summer) causes an increase in cyclonicity in winter and anticyclonicity in spring and summer. These surface air temperature effects also penetrate higher up in the atmosphere, causing a perturbation of up to 25m in the geopotential height at 200 hPa, leading to an increase in the maximum speed of the subtropical jet over western Asia during summer of 6.5%. Anaccurate description of the Caspian Sea in general circulation models is thus important for both local and regional climates.


108. White, R. H.; Toumi, R. River flow and ocean temperatures: The Congo River JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS Volume: 119   Issue: 4   Pages: 2501-2517  2014


Using a regional ocean model, the impacts of the Congo River on ocean temperatures, mixed layer depths (MLDs), and barrier layers are studied. A sensitivity simulation with no Congo flow shows that the river shallows the mixed layer by up to 7 m and creates barrier layers up to 6 m thick and temperature inversions of up to 0.5 K. Contrary to previous modeling studies, but in agreement with observational analysis, simulation results show no substantial SST signal from the Congo River in the eastern Atlantic. A cooling signal, <0.25 K, is simulated in localized regions. A heat budget analysis shows that in these regions the cooling effect of reducing shortwave absorption within the mixed layer, by the shallowing of this layer, exceeds the warming impact of reducing vertical mixing. This is shown to be partly a consequence of the shallow mixed layer (10-15 m) in this region. The direct impact of the temperature of the Congo River input is confined to the region within 1 degrees of the river mouth.


107. Nissan, Hannah; Toumi, Ralf On the impact of aerosols on soil erosion GEOPHYSICAL RESEARCH LETTERS  Volume: 40   Issue: 22   Pages: 5994-5998   2013


Soil erosion is a serious threat to agricultural productivity and the sustainable provision of food to a growing world population. No connection has hitherto been established between aerosols and rainfall-induced soil erosion on the ground. Here we use a cloud resolving model to simulate the effect of aerosols on rainfall erosivity (an indicator of the erosive potential of rain). Increased atmospheric aerosol concentrations tend to suppress precipitation in warm rain clouds, while in cold or mixed-phase systems, invigoration of surface rainfall can occur. We show that in both these cases, the response of erosivity to an increase in aerosol is in the same direction as, but amplified beyond the change in total rainfall. We also show that aerosols can impact erosivity through changes in raindrop size. Our results suggest that anthropogenic aerosol emissions affect erosivity and thus may have important consequences for agricultural productivity.


106. White, R. H.; Toumi, R. The limitations of bias correcting regional climate model inputs GEOPHYSICAL RESEARCH LETTERS  Volume: 40   Issue: 12   Pages: 2907-2912  2013.


We investigate the theoretical limitations of applying statistical bias corrections to general circulation model data prior to downscaling with a regional climate model (RCM). Two methods from recent literature are compared: linear (LC) and quantile-quantile (QQ) corrections. An LC method is limited to correcting biases only in the distribution mean but retains all first-order spatial and intervariable dependencies. Conversely, the QQ method corrects any bias in the distribution function but cannot retain intervariable dependencies and adds variability to smooth spatial fields. An RCM's boundary relaxation scheme dampens the effect of imbalanced variables but may not remove the spurious variability from the QQ method. Case study results of precipitation over a South African river catchment show that both correction methods improve RCM-simulated monthly climatology, but the QQ correction creates a 20% bias in interannual monthly variability. We recommend the LC method as an effective bias correction for RCM inputs.


105. Nissan, H.; Toumi, R. Dynamic simulation of rainfall kinetic energy flux in a cloud resolving model GEOPHYSICAL RESEARCH LETTERS  Volume: 40   Issue: 12   Pages: 3331-3336 2013.


We present the first simulation of rainfall kinetic energy flux in a cloud resolving model. This demonstrates the potential for conducting erosion prediction studies using a regional climate model. Soil erosion is already a major global problem, and there is growing pressure on our land to deliver food and stability. Rainfall kinetic energy flux is an important variable in erosion prediction but is generally parameterized from intensity due to measurement difficulties. We show that a cloud resolving model can be used to dynamically simulate the kinetic energy of rain from basic physics, using four microphysics schemes. Results are within the range of observations and also capture the observed variability in kinetic energy for a given intensity. Large drops are shown to contribute disproportionately to total kinetic energy compared with their number, suggesting that several existing relations between terminal velocity and size of raindrops are poorly suited for kinetic energy modeling.




104. Denamiel, C., Budgell, W.P., Toumi, R. The Congo River plume: Impact of the forcing on the far-field and near-field dynamics JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS  Vol: 118   Issue: 2    964-989   DOI: 10.1002/jgrc.20062   2013


The first numerical simulations of the Congo River plume dynamics are presented in this study. The different forcing mechanisms responsible for the seasonal variations of the plume extend are separately analyzed and the Regional Ocean Modeling System (ROMS) is employed to carry out both a process orientated study-with simple baseline simulations and a sensitivity study-with realistic 1 year runs setup in 2005. The baseline model is forced only by the river flow, in the presence of realistic bathymetry. Tides, wind stress, surface heat flux, and ocean boundary conditions are the forcing added to the realistic model. The typical seasonal orientation of the Congo River plume is found to be northward during most of year except for the February-March (FM) season when the plume has a large westward extension (about 800 km) and its area nearly doubles. The northward extension of the plume is explained by a buoyancy-driven upstream coastal flow-due to the unique geomorphology of the Congo River estuary-and the combined influences of the ambient ocean currents and the wind. During the FM season, the surface ocean circulation is driving both (1) the westward extension of the plume and (2) the southward transport of the Nyanga River fresh waters which feed the Congo River plume. In the near-field region of the plume, the presence of the deep Congo canyon has two main effects: (1) its depth increases the intrusion of sea water into the river mouth and (2) its orientation initiates the formation of the upstream flow.



103. Singleton, A. and Toumi, R., Super Clausius-Clapeyron scaling of rainfall in a model squall line. QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY  Vol: 139   Iss: 671   Special Issue: SI   Pages: 334-339   DOI: 10.1002/qj.1919   Part: B   2013


The sensitivity of squall rainfall to changes in atmospheric temperature is investigated. For instantaneous rainrates and accumulations up to one hour, extreme rainfall scales with Clausius-Clapeyron (CC) for temperatures below 24 degrees C and at up to twice CC above 24 degrees C. For longer accumulation periods and higher temperatures the scaling breaks down due to increased propagation of the squall line. For all periods, the storm average rainfall is found to scale at approximately 1.5 times CC over the entire range of temperatures. These results have implications for design parameters for infrastructure that is vulnerable to flooding and for climate change projections



102. Nicholls, J. Farley; Toumi, R.; Budgell, W. P. Inertial currents in the Caspian Sea

GEOPHYSICAL RESEARCH LETTERS Vol: 39 L18603  DOI: 10.1029/2012GL052989 2012


We present the first simulation of near-inertial oscillations in the Caspian Sea. Model amplitude is in good agreement with observations. Annual mean near-inertial oscillations are found to be up to 14 cm/s with a seasonal maximum in the summer approximately twice as great as in the winter. The energy increases away from the coast at a rate of up to 0.8 cm(2) s(-2) km(-1); the strongest relationship is with distance from the 50 m depth contour. The peak amplitude also occurs later further from the coastline with a delay of the order of 1 day per 100 km distance. These features are consistent with propagating baroclinic and barotropic waves.


101. Benn, D. I.; Bolch, T.; Hands, K.; et al.Response of debris-covered glaciers in the Mount Everest region to recent warming, and implications for outburst flood hazards EARTH-SCIENCE REVIEWS  Vol: 114  Iss: 1-2  156-174   DOI: 10.1016/j.earscirev.2012.03.008   2012


In areas of high relief, many glaciers have extensive covers of supraglacial debris in their ablation zones, which alters both rates and spatial patterns of melting, with important consequences for glacier response to climate change. Wastage of debris-covered glaciers can be associated with the formation of large moraine-dammed lakes, posing risk of glacier lake outburst floods (GLOFs). In this paper, we use observations of glaciers in the Mount Everest region to present an integrated view of debris-covered glacier response to climate change, which helps provide a long-term perspective on evolving GLOF risks.

In recent decades, debris-covered glaciers in the Everest region have been losing mass at a mean rate of similar to 0.32 m yr(-1), although in most cases there has been little or no change in terminus position. Mass loss occurs by 4 main processes: (1) melting of clean ice close to glacier ELAs; (2) melting beneath surface debris; (3) melting of ice cliffs and calving around the margins of supraglacial ponds; and (4) calving into deep proglacial lakes. Modelling of processes (1) and (2) shows that Everest-region glaciers typically have an inverted ablation gradient in their lower reaches, due to the effects of a down-glacier increase in debris thickness. Mass loss is therefore focused in the mid parts of glacier ablation zones, causing localised surface lowering and a reduction in downglacier surface gradient, which in turn reduce driving stress and glacier velocity, so the lower ablation zones of many glaciers are now stagnant. Model results also indicate that increased summer temperatures have raised the altitude of the rain-snow transition during the summer monsoon period, reducing snow accumulation and ice flux to lower elevations.

As downwasting proceeds, formerly efficient supraglacial and englacial drainage networks are broken up, and supraglacial lakes form in hollows on the glacier surface. Ablation rates around supraglacial lakes are typically one or two orders of magnitude greater than sub-debris melt rates, so extensive lake formation accelerates overall rates of ice loss. Most supraglacial lakes are 'perched' above hydrological base level, and are susceptible to drainage if they become connected to the englacial drainage system. Speleological surveys of conduits show that large englacial voids can be created by drainage of warm lake waters along pre-existing weaknesses in the ice. Roof collapses can open these voids up to the surface, and commonly provide the nuclei of new lakes. Thus, by influencing both lake drainage and formation, englacial conduits exert a strong control on surface ablation rates.

An important threshold is crossed when downwasting glacier surfaces intersect the hydrological base level of the glacier. Base-level lakes formed behind intact moraine dams can grow monotonically, and in some cases can pose serious GLOF hazards. Glacier termini can evolve in different ways in response to the same climatic forcing, so that potentially hazardous lakes will form in some situations but not others. Additionally, the probability of a flood is not simply a function of lake volume, but depends on the geometry and structure of the dam, and possible trigger mechanisms such as ice- or rockfalls into the lake. Satellite-based measurements of glacier surface gradient and ice velocities allow probable future locations of base-level lakes to be identified. A base-level lake has begun to grow rapidly on Ngozumpa Glacier west of Mount Everest, and could attain a volume of similar to 10(8) m(3) within the next 2 or 3 decades. Unless mitigation efforts are undertaken, this lake could pose considerable GUN hazard potential.


100. Giannakopoulou, E.-M. and Toumi, R. , Impacts of the Nile Delta land-use on the local climate. Atmosph. Sci. Lett., 13: 208-215. doi: 10.1002/asl.381 2012


This is a numerical study of the effects of the Nile Delta (ND) man-made greening on local climate. We compared a control simulation, which employs the present-day ND vegetation, with a desertification experiment. It was found that the low surface albedo of the agricultural ND increases net radiation, which in turn raises potential evapotranspiration (PET). This suggests that agricultural use increases the water demand by enhancing PET. Non-local effects are also observed on a frontal system over the eastern Mediterranean Sea that shifted farther away from the coast. This shift is attributed to a stronger land breeze in the present-day land-cover.


99. White, R.; Toumi, R. A tightly bound soil-water scheme within an atmosphere-land-surface model. Journal of Hydrology Volume: 452/453   Pages: 51-63   DOI: 10.1016/j.jhydrol.2012.05.028  2012



The concept of tightly bound water, in which a reservoir of soil water is bound tightly within small soil pores but is still available for evapotranspiration, is parameterised for the first time within the land surface scheme of a fully-coupled regional-scale atmosphere-land surface model. The Weather Research and Forecasting (WRF) regional climate model and the NOAH land surface scheme are selected and a case study is performed on the Olifants River Basin in the Limpopo region of South Africa. Accurate knowledge of water availability in this water-stressed region is of great importance for adaptation and future water policy. Results of a simulation forced by ERA40 re-analysis show that the standard land surface scheme is unable to reproduce the observed runoff despite rainfall and atmospheric conditions similar to observed. This version of the model over-estimates mean annual runoff by 120%. The tightly bound water scheme shows a significant improvement, reducing the bias to 22%. The inclusion of the tightly bound water scheme has little effect on the basin average annual rainfall despite increasing annual evapotranspiration. The tightly bound water physics dampens the response of runoff to precipitation and provides additional de-coupling between precipitation and runoff, increasing the variability in this relationship. Simulations with the WRF model forced with both 1980s and 2040s CCSM3 data show that the tightly bound water scheme significantly reduces runoff in different climates and projects a greater relative future decrease in runoff, from 4% to 10% for the same precipitation decrease of 2.5%. The scheme also affects the projected changes in spatially averaged 100-year return precipitation and runoff with significance at the 0.9 confidence level.



98. Sondergaard Roar R.; Makris Theodoros; Lianos Panagiotis; et al. The use of polyurethane as encapsulating method for polymer solar cells-An inter laboratory study on outdoor stability in 8 countries SOLAR ENERGY MATERIALS AND SOLAR CELLS  Vol: 99   Special Issue: SI   Pages: 292 300   DOI:10.1016/j.solmat.2011.12.013    2012


 A new encapsulation method for organic solar cells has been tested on flexible solar modules and cells embedded in polyurethane, sandwiched between a tempered glass plate and a polycarbonate plate. Panels, each containing 10 organic solar modules/cells, were fabricated and installed for outdoor exposure in eight different countries for 41/2 months. In order to minimize potential deviations in procedures and equipment, one person was responsible for the fabrication, installation and initial and final IV-measurements of the panels using the same equipment for all measurements and calibrations. The encapsulated modules/cells showed significantly reduced degradation compared with previous studies, with final average efficiencies around 40% of the original after 41/2 months outdoor exposure. Photodegradation was furthermore found not to be the primary source of degradation.



97. Smedley Andrew R. D.; Rimmer John S.; Moore David; Toumi, R. Total ozone and surface UV trends in the United Kingdom: 1979-2008 INTERNATIONAL JOURNAL OF CLIMATOLOGY  Volume: 32  Issue: 3   Pages: 338-346   DOI: 10.1002/joc.2275 2012 

In this study we present the UK's total ozone and UV data records, their measurement and preliminary processing, and discuss the observed trends. Notably, we combine the records from the Camborne and Reading sites to obtain a single ozone data series for southern Englanda total duration of 30 years.




96. Spurny Pavel; Bland Philip A.; Shrbeny Lukas; et al. The Bunburra Rockhole meteorite fall in SW Australia: fireball trajectory, luminosity, dynamics, orbit, and impact position from photographic and photoelectric records  METEORITICS & PLANETARY SCIENCE  Volume: 47   Issue:2   Pages: 163-185   DOI: 10.1111/j.1945-5100.2011.01321.x  2012 


We report an analysis of the first instrumentally observed meteorite fall in Australia, which was recorded photographically and photoelectrically by two eastern stations of the Desert Fireball Network (DFN) on July 20, 2007. The meteoroid with an initial mass of 22 kg entered the atmosphere with a low speed of 13.36 km s(-1) and began a luminous trajectory at an altitude of 62.83 km. In maximum, it reached -9.6 absolute magnitude and terminated after a 5.7 s and 64.7 km long flight at an altitude of 29.59 km with a speed of 5.8 km s(-1). The angle of the atmospheric trajectory to the Earths surface was 30.9 degrees. The first organized search took place in October 2008 and the first meteorite (150 g) was found 97 m southward from the predicted central line at the end of the first day of searching (October 3, 2008). The second stone (174 g) was recovered 39 m northward from the central line, both exactly in the predicted mass limits. During the second expedition in February 2009, a third fragment of 14.9 g was found again very close (100 m) from the predicted position. Total recovered mass is 339 g. The meteorite was designated Bunburra Rockhole (BR) after a nearby landscape structure. This first DFN sample is an igneous achondrite. Initial petrography indicated that BR was a brecciated eucrite but detailed analyses proved that BR is not a typical eucrite, but an anomalous basaltic meteorite (Bland etal. 2009). BR was delivered from an unusual, Aten type orbit (a < 1 AU) where virtually the entire orbit was contained within Earths orbit. BR is the first achondrite fall with a known orbit and it is one of the most precise orbits ever calculated for a meteorite dropping fireball.



95. Ganshin A.; Oda T.; Saito M.; et al. A global coupled Eulerian-Lagrangian model and 1 x 1 km CO2 surface flux dataset for high-resolution atmospheric CO2 transport simulations  GEOSCIENTIFIC MODEL DEVELOPMENT  Volume: 5  Issue: 1   Pages: 231-243   DOI: 10.5194/gmd-5-231-2012   2012 


We designed a method to simulate atmospheric CO2 concentrations at several continuous observation sites around the globe using surface fluxes at a very high spatial resolution. The simulations presented in this study were performed using the Global Eulerian-Lagrangian Coupled Atmospheric model (GELCA), comprising a Lagrangian particle dispersion model coupled to a global atmospheric tracer transport model with prescribed global surface CO2 flux maps at a 1x1 km resolution. The surface fluxes used in the simulations were prepared by assembling the individual components of terrestrial, oceanic and fossil fuel CO2 fluxes. This experimental setup (i. e. a transport model running at a medium resolution, coupled to a high-resolution Lagrangian particle dispersion model together with global surface fluxes at a very high resolution), which was designed to represent high-frequency variations in atmospheric CO2 concentration, has not been reported at a global scale previously. Two sensitivity experiments were performed: (a) using the global transport model without coupling to the Lagrangian dispersion model, and (b) using the coupled model with a reduced resolution of surface fluxes, in order to evaluate the performance of Eulerian-Lagrangian coupling and the role of high-resolution fluxes in simulating high-frequency variations in atmospheric CO2 concentrations. A correlation analysis between observed and simulated atmospheric CO2 concentrations at selected locations revealed that the inclusion of both Eulerian-Lagrangian coupling and highresolution fluxes improves the high-frequency simulations of the model. The results highlight the potential of a coupled Eulerian-Lagrangian model in simulating high-frequency atmospheric CO2 concentrations at many locations worldwide. The model performs well in representing observations of atmospheric CO2 concentrations at high spatial and temporal resolutions, especially for coastal sites and sites located close to sources of large anthropogenic emissions. While this study focused on simulations of CO2 concentrations, the model could be used for other atmospheric compounds with known estimated emissions.


94. Giannakopoulou E. M.; Toumi R. The Persian Gulf summertime low-level jet over sloping terrain QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Volume: 138 Issue: 662 Pages: 145-157   Part: A 2012 


There are few observational and numerical studies of the summertime low-level jet (LLJ) over the Persian Gulf, known as the shamal. In this study, the Weather Research and Forecasting (WRF) model accurately simulates the LLJ's vertical structure, nocturnal features and strong diurnal variation of the wind. The case-study period is divided into a period during which the shamal is spatially extended (strong winds over the Gulf and Iraq) and a period during which it is less spatially extended (strong flow only over the Gulf). Orography, mountain slope and land/sea breeze are found to be key factors for the shamal diurnal variation of wind speed and wind direction; the Blackadar mechanism appears to be secondary to the heating in forcing the shamal. The Zagros Mountains not only channel the northwesterly winds but also provide a barrier for the easterly monsoon airflow, which maximizes the wind speed. Only during the less extensive period does the Zagros Mountains slope significantly affect the shamal wind intensity and direction. Both shallow and steep mountain slope sensitivity studies feature a jet-like flow over the Persian Gulf. The steep slopes cause larger wind speeds; however, the shallow slopes reveal a stronger diurnally varying wind direction due to larger heating and cooling of the sloping terrain. It is also shown that, during the less extensive period, the land breeze and the lower friction over the sea increase the intensity of the nocturnal jet over the Gulf.



93. Ryder C. L.; Toumi R An urban solar flux island: Measurements from London. ATMOSPHERIC ENVIRONMENT  Volume: 45   Issue: 20   Pages: 3414-3423   DOI: 10.1016/j.atmosenv.2011.03.045  2011


Solar irradiance measurements from a new high density urban network in London are presented. Annual averages demonstrate that central London receives 30 +/- 10 W m(-2) less solar irradiance than outer London at midday, equivalent to 9 +/- 3% less than the London average. Particulate matter and AERONET measurements combined with radiative transfer modelling suggest that the direct aerosol radiative effect could explain 33-40% of the inner London deficit and a further 27-50% could be explained by increased cloud optical depth due to the aerosol indirect effect. These results have implications for solar power generation and urban energy balance models. A new technique using 'Langley flux gradients' to infer aerosol column concentrations over clear periods of 3 h has been developed and applied to three case studies. Comparisons with particulate matter measurements across London have been performed and demonstrate that the solar irradiance measurement network is able to detect aerosol distribution across London and transport of a pollution plume out of London.


92. Sparks, N; Toumi, R, Remote sampling of a CO2 point source in an urban setting ATMOSPHERIC ENVIRONMENT   Volume: 44   Issue: 39   Pages: 5287-5294  2010 


High frequency CO2 and wind speed measurements were used to examine the urban baseline eddy covariance CO2 flux and analyse the CO2 rich plume from a local power station. A reliable relationship between high frequency CO2 maxima and the rate of CO2 emission at the power station was established. This relationship was shown to be highly dependant on wind speed. The ensemble mean plume was found to be Gaussian in horizontal profile with a width dependant on wind speed. The relationship between peak CO2 mixing ratio and averaging time was shown to be a simple power law with a time exponent of approximately 0.5. The large, short pulses in CO2 mixing ratio in the power plant plume were found to have an approximately Lorentzian shape. These pulses generated negative vertical eddy flux measurements so data from the plume sector were necessarily excluded from the flux baseline results. The plume-excluded flux had a similar magnitude and variability to those reported in other urban CO2 flux studies despite this site not being ideal due to the proximity of roughness elements to the measurement point. 




91. Haigh JD, Winning AR, Toumi R, et al. An influence of solar spectral variations on radiative forcing of climate NATURE   Volume: 467   Issue: 7316   Pages: 696-699    2010


The thermal structure and composition of the atmosphere is determined fundamentally by the incoming solar irradiance. Radiation at ultraviolet wavelengths dissociates atmospheric molecules, initiating chains of chemical reactions-specifically those producing stratospheric ozone-and providing the major source of heating for the middle atmosphere, while radiation at visible and near-infrared wavelengths mainly reaches and warms the lower atmosphere and the Earth's surface(1). Thus the spectral composition of solar radiation is crucial in determining atmospheric structure, as well as surface temperature, and it follows that the response of the atmosphere to variations in solar irradiance depends on the spectrum(2). Daily measurements of the solar spectrum between 0.2 mu m and 2.4 mu m, made by the Spectral Irradiance Monitor (SIM) instrument on the Solar Radiation and Climate Experiment (SORCE) satellite(3) since April 2004, have revealed(4) that over this declining phase of the solar cycle there was a four to six times larger decline in ultraviolet than would have been predicted on the basis of our previous understanding. This reduction was partially compensated in the total solar output by an increase in radiation at visible wavelengths. Here we show that these spectral changes appear to have led to a significant decline from 2004 to 2007 in stratospheric ozone below an altitude of 45 km, with an increase above this altitude. Our results, simulated with a radiative-photochemical model, are consistent with contemporaneous measurements of ozone from the Aura-MLS satellite, although the short time period makes precise attribution to solar effects difficult. We also show, using the SIM data, that solar radiative forcing of surface climate is out of phase with solar activity. Currently there is insufficient observational evidence to validate the spectral variations observed by SIM, or to fully characterize other solar cycles, but our findings raise the possibility that the effects of solar variability on temperature throughout the atmosphere may be contrary to current expectations.



90. Anderson BT, Reifen C, Toumi R Consistency in Global Climate Change Model Predictions of Regional Precipitation Trends
 Volume: 13 Article Number: 9   Published: 2009


Projections of human-induced climate change impacts arising from the emission of atmospheric chemical constituents such as carbon dioxide typically utilize multiple integrations (or ensembles) of numerous numerical climate change models to arrive at multimodel ensembles from which mean and median values and probabilities can be inferred about the response of various components of the observed climate system. Some responses are considered reliable in as much as the simulated responses show consistency within ensembles and across models. Other responses-particularly at regional levels and for certain parameters such as precipitation-show little intermodel consistency even in the sign of the projected climate changes. The authors' results show that in these regions the consistency in the sign of projected precipitation variations is greater for intramodel runs (e. g., runs from the same model) than intermodel runs (e. g., runs from different models), indicating that knowledge of the internal "dynamics" of the climate system can provide additional skill in making projections of climate change. Given the consistency provided by the governing dynamics of the model, the authors test whether persistence from an individual model trajectory serves as a good predictor for its own behavior by the end of the twenty-first century. Results indicate that, in certain regions where intermodel consistency is low, the short-term trends of individual model trajectories do provide additional skill in making projections of long-term climate change. The climate forcing for which this forecast skill becomes relatively large (e. g., correct in 75% of the individual model runs) is equivalent to the anthropogenic climate forcing imposed over the past century, suggesting that observed changes in precipitation in these regions can provide guidance about the direction of future precipitation changes over the course of the next century


89. Pejovic T, Noland RB, Williams V, et al. A tentative analysis of the impacts of an airport closure
JOURNAL OF AIR TRANSPORT MANAGEMENT   Volume: 15   Issue: 5   Pages: 241-248   Published: SEP 2009


This paper identifies the vulnerabilities of operations at Heathrow airport to a short airport closure. The system disruptions assessed are delays, flight rerouting or diversions to alternate airports, and flight cancellations. In addition, the impact on fuel consumption, and hence CO2 emissions, is quantified using the Reorganized Air Traffic Control Mathematical Simulator Plus simulation model and the Advanced Emission Model tool. The results have implications for assessing how increased severe weather events associated with climate change may disrupt airport operations.



88. Anderson BT, Reifen C, Toumi R , Identification of Nonlinear Behavior in Transient Climate Change Projections of Soil Moisture over the United States
EARTH INTERACTIONS   Volume: 13 Article Number: 1   Published: 2009


While most projections of climate change and its regional impacts focus on overall changes in the state of the climate system, useful information can also be found in the evolution of the climate system from one state to another. Here the authors introduce one method for identifying regions in which significant and systematic long-term nonlinear evolutions may be present, even given quasi-linear anthropogenic forcing. Using climate change projections taken from simulations of NCAR's Community Climate System Model, version 3 (CCSM3), the authors then employ the technique to isolate systematic nonlinear behavior of soil moisture variations over the United States. While the projections presented here only represent the results from one model system, it is argued that such nonlinear behavior is an important characteristic of future climate change that should be considered when discussing both short-term and long-term impacts of anthropogenic climate forcing


87. Reifen C, Toumi R Climate projections: Past performance no guarantee of future skill? GEOPHYSICAL RESEARCH LETTERS   Volume: 36 Article Number: L13704   Published: JUL 7 2009


The principle of selecting climate models based on their agreement with observations has been tested for surface temperature using 17 of the IPCC AR4 models. Those models simulating global mean, Siberian and European 20th Century surface temperature with a lower error than the total ensemble for one period on average do not do so for a subsequent period. Error in the ensemble mean decreases systematically with ensemble size, N, and for a random selection as approximately 1/N-alpha, where a lies between 0.6 and 1. This is larger than the exponent of a random sample (alpha = 0.5) and appears to be an indicator of systematic bias in the model simulations. There is no evidence that any subset of models delivers significant improvement in prediction accuracy compared to the total ensemble.


86. Rigby M, Toumi R, Fisher R, et al., First continuous measurements of CO2 mixing ratio in central London using a compact diffusion probe ATMOSPHERIC ENVIRONMENT   Volume: 42   Issue: 39   Pages: 8943-8953   Published: DEC 2008


We present one year of data from the first continuous measurements of carbon dioxide mixing ratio in central London. Measurements were made at an 87 m tower site using the new Vaisala CARBOCAP GMP343 instrument, which was found to provide a compact and inexpensive method for mixing ratio monitoring, in an environment where conventional CO2 sensors could not be accommodated. Measurements were compared with a monitoring site outside London, showing that the city's CO2 "dome" was an order of magnitude smaller than measured at lower levels in other cities. During the night time in the summer, the mixing ratio in central London was found to be significantly lower than at the rural site. This was thought to be explained by the proximity of biogenic sources to the rural sensor, differences between urban and rural mixing heights and/or the interception of a vertical mixing ratio gradient at two different measurement heights. Estimation of the CO2 loading of the air entering the city therefore proved problematic during the summer. As a result, we propose that monitoring of CO2 emission rate using this type of measurement may only be possible in the winter when the influence of these factors is minimal.


85. Pejovic T, Noland RB, Williams V, Toumi R., Estimates of UK CO2 emissions from aviation using air traffic data CLIMATIC CHANGE   Volume: 88   Issue: 3-4   Pages: 367-384    JUN 2008


The allocation of CO2 emissions to specific sources is a major policy issue for international aviation, especially for determining allocations for emissions trading schemes. This paper addresses the problem by recommending a possible methodology to allocate emissions to specific sources using detailed air traffic data. The basis for the calculations is an air traffic sample for one full-day of traffic from the UK. In order to analyse aircraft fuel burn use and hence CO2 emissions, the Reorganized Air Traffic Control Mathematical Simulator (RAMS Plus) and the Advanced Emission Model (AEM III) are used. The results from these detailed simulations are compared with two of the most widely-used aviation CO2 emission estimates to have been made for the UK: the SERAS study and NETCEN estimate. Their estimates for the year 2000 are 26.1 and 31.4 Mt, respectively. In addition, the most recent NETCEN estimate for the year 2003 is 34.1 Mt of CO2. Our estimate of total aviation CO2 emissions, using detailed simulations and real air traffic data, is 34.7 Mt for the year 2004. In addition, emission estimates are compared with two global aviation emission inventories: AERO2K and SAGE. Contributions of the highest-emitting flights and aircraft types are identified. International departures dominate; 6% of flights account for 50% of total emissions. The largest aircraft emit the most per flight-km, although not per passenger-km. Different methodologies and their implications are also discussed.


84. Rigby M, Toumi R London air pollution climatology: Indirect evidence for urban boundary layer height and wind speed enhancement ATMOSPHERIC ENVIRONMENT   Volume: 42   Issue: 20   Pages: 4932-4947   JUN 2008


Wind speed and boundary layer height are key variables for studies of air pollution climatology and are routinely calculated by general circulation models in reanalysis and climate prediction projects. However, these models cannot yet resolve urban areas and the associated meteorological phenomena that are important for urban pollutant dispersion. Two simple air pollution models are examined; the box model and a Gaussian dispersion model. We find that, using meteorology from European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis and operational analysis, the models most accurately reproduced the London climatology for daily mean concentrations of NO,, CO and PM10 from 1998 to 2005 if the boundary layer height was increased between 50 and 150m and high wind speeds were reduced. These corrections are consistent with the effects of the roughness and thermal properties of an urban area, which are not explicitly treated in the analyses. Perhaps more surprisingly, we find the best modelled pollution climatology by also specifying a minimum wind speed of at least 1 m s(-1). The minimum wind speed parameterises enhanced pollution ventilation under light wind conditions and could be consistent with an important role for the urban heat island circulation in reducing the severity of pollution events in London. These modifications are simple and computationally efficient, allowing offline comparison of pollution climates over long periods of time and potentially on a global scale.


83. Smith S, Toumi R Direct observation of cloud forcing by ground-based thermal imaging GEOPHYSICAL RESEARCH LETTERS   Volume: 35   Issue: 7 Article Number: L07814    APR 12 2008


Instantaneous surface Cloud Radiative Forcing (CRF) in the 7.5-13 mu m region is observed for the first time, using a thermal infrared camera. The sampling of clear sky and cloudy radiances from images of broken cloud fields allows cloud cover, CRF and effective cloud emission to be directly calculated, all within a consistent field of view. Analysis of 1300 images taken over more than two months in Central England shows that surface CRF is a nonlinear function of cloud cover, with daytime forcings larger and less linear than those at night. This nonlinearity is caused both by the increase in cloud optical thickness and the more frequent occurrence of low altitude (warm) cloud as the cloud cover increases. Even for nearly complete cloud cover, effective cloud emission remains significantly less than that of widely assumed homogeneous, optically thick cloud. Possible clear sky sampling errors associated with traditional methods of measuring CRF are also investigated.


82. Smith S, Toumi R Measuring cloud cover and brightness temperature with a ground-based thermal infrared camera JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY   Volume: 47   Issue: 2   Pages: 683-693    FEB 2008Thermal infrared cameras can be used to monitor clouds and the sky at high spatial and temporal resolutions. In particular, this study shows that, without the need for any external data, cloud cover can be retrieved both day and night over a field of view extending to zenith angles of similar to 80 degrees. Zenith clear sky temperatures are estimated for cloud cover up to 80%. During periods of 50% cloud cover or more the cloud-base brightness temperatures (CBBTs) can be calculated to an accuracy of +/-1 K. These calculations are made possible by using a new parameterization for the variation of sky brightness temperature with zenith angle. Both clear and cloudy conditions are found to follow this simple empirical equation more closely than the widely used parameterization of Unsworth and Monteith. A simple, angle-dependent threshold system based on cloud transmittance can then be used to retrieve cloud cover, and clear sky temperature and CBBT are calculated using the two parameters resulting from the fitting process.


81. Williams V, Noland RB, Majumdar A, Toumi R, Ochieng W, Molloy J Reducing environmental impacts of aviation with innovative air traffic management technologies AERONAUTICAL JOURNAL   Volume: 111   Issue: 1125   Pages: 741-749   NOV 2007


Cominercially-driven air traffic management (ATM) innovations typically aim to increase air space capacity and/or reduce delays. Here, their potential application for environmental mitigation is discussed. Both carbon dioxide (CO2) and non-CO2 climate impacts are considered, as are noise and air quality issues. We outline the technological, scientific and political barriers to an integrated approach to applying ATM technologies to environmental mitigation. These issues highlight the need to improve comparison and prioritisation of the emissions and effects of aviation.


80. Patmore N, Toumi R An entropy-based measure of mixing at the tropopause QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 132 (619): 1949-1967 Part B JUL 2006


Qualitative analysis of tracer correlations is now a well established technique for investigating mixing in the tropopause transition layer. Generally, these studies rely on in situ data retrieved over short distances and time-scales. Using satellite data from HALOE, we introduce a simple quantitative analysis technique, based on entropy, to provide a climatology of mixing, from H2O:O-3 tracer correlations. Results compare well with previous studies using Lagrangian-based estimates of effective diffusivity and equivalent lengths and also climatologies of tropopause folding. Further analysis techniques reveal characteristics of the transition layer, including its depth and position, leading to the distinction of three primary forms of mixing. Within our analysis domain, we estimate that half of total entropy can be attributed to subtropical 'Rossby-driven' tropopause folding events. The majority of the remaining entropy is associated with tropical convective mixing and a third form, possibly linked to shear-induced mixing at subtropical jet streaks.


79. Rigby M, Timmis R, Toumi R Similarities of boundary layer ventilation and particulate matter roses ATMOSPHERIC ENVIRONMENT 40 (27): 5112-5124 SEP 2006

Pollution wind sector or rose analyses show that under South-Easterly winds, many areas of the UK experience an increase in mean airborne particulate matter concentration of up to 30% over the average for all directions. This is often attributed solely to long-range transport of pollutants from continental Europe. Here, we present a rose analysis that suggests an additional influence of boundary layer ventilation. The directional increase in pollutant concentration is found to coincide with a 45-55% reduction in ventilation, obtained from ECMWF reanalysis. The reduced ventilation will increase the concentration of locally emitted particulate matter. This effect is explained by low average South-Easterly wind speeds, and advection of high-level warm air from continental Europe, which stabilises the boundary layer, and reduces the boundary layer height. A similar dependence of ventilation on wind direction was found across most of the world, so that a similar effect on pollution roses may be present at most locations. The high number of particulate matter exceedance days for Easterly flow was not found to be associated with a high frequency of extremely low ventilation conditions, suggesting that ventilation alone cannot explain these events.

78. Kennett EJ, Toumi R Temperature dependence of atmospheric moisture lifetime GEOPHYSICAL RESEARCH LETTERS 32 (19): Art. No. L19806 OCT 8 2005


In this study, the temperature dependence of atmospheric moisture lifetime is investigated using the European Centre for Medium- Range Weather Forecasts 40- year Reanalysis ( ERA40) dataset. ERA40 shows that atmospheric moisture lifetimes can vary by greater than a factor of 4 within the column. Globally, the shortest lifetimes occur at temperatures of about -40 degrees C, with mean lifetimes of just over a day at this level. Lifetimes derived independently from aircraft microphysical measurements in cirrus clouds confirm the rapid loss of moisture at about -40 degrees C. This can be understood to result from changes in the rate of nucleation and size distribution of ice particles with decreasing temperature. A similar temperature dependence is observed in water vapor scale height, with relative humidity increasing with decreasing temperature below about -40 degrees C. The variability of atmospheric moisture lifetime with temperature is found to be a fundamental microphysical control on the distribution of water vapor.


77. Wilson PS, Toumi R A fundamental probability distribution for heavy rainfall GEOPHYS RES LETT 32 (14): art. no. L14812 JUL 22 2005


There is currently no physical understanding of the statistics of heavy precipitation. These statistics are, however, central to diagnosing climate change and making weather risk assessments. This work derives a fundamental rainfall distribution. Interpretation of the water balance equation gives a simple expression for precipitation as the product of mass flux or advected mass, specific humidity and precipitation efficiency. Statistical theory predicts that the tail of the distribution of the product of these three random variables will have a stretched exponential form with a shape parameter of 2/3. This is verified for a global daily precipitation data set. The stretched exponential tail explains the apparent 'heavy' tailed behaviour of precipitation under standard assumptions used in extreme value theory. The novel implications for climate change are that the stretched exponential shape is unlikely to change, although the scale may, and precipitation efficiency is important in understanding future changes in heavy precipitation.



76. Kennett EJ, Toumi R Himalayan rainfall and vorticity generation within the Indian summer monsoon GEOPHYS RES LETT 32 (4): art. no. L04802 FEB 17 2005


The Himalayas are an integral part of the Indian monsoon dynamics. In this paper we examine the European Centre for Medium-Range Weather Forecasts 40-year Reanalysis (ERA40) data set in order to quantify the importance of Himalayan rainfall in terms of vorticity generation. We find that Himalayan rainfall is a powerful source of vorticity due to the multiplicative effect of steep vertical gradients in latent heating combined with a large Coriolis parameter. ERA40 shows a maximum in column-integrated and near-surface vorticity generation in the monsoon region over the southern slopes of the Himalayas. We estimate that the total generation of vorticity over the Himalayan region as a whole is at least half that over the Bay of Bengal. Thus Himalayan rainfall is likely to play a central role in amplifying the circulation of the monsoon.

75. McCarthy, MP, Toumi, R Observed Interannual Variability of Tropical Troposphere Relative Humidity Journal of Climate 2004 17: 3181-3191

Relative humidity fields from the High-Resolution Infrared Radiation Sounder (HIRS) flown on NOAA series satellites since 1979 have been used to study the seasonal aspects of the interannual variability of relative humidity in the tropical troposphere. The El Nino-Southern Oscillation (ENSO) is the only statistically identifiable physical mechanism of such variability. Boreal winter (December-February) relative humidity variations during an ENSO event follow patterns of anomalous convection and large-scale upper-level circulation. During El Nino (La Nina) regions of large negative (positive) relative humidity anomalies exist at subtropical latitudes over the Pacific Ocean. These are not always balanced by increases (decreases) in humidity near the equator. NCEP- NCAR reanalysis temperatures are used to separate observed changes in relative humidity into contributions from tropospheric temperature versus the contribution from changes in water vapor content. The authors find that at subtropical latitudes variations in temperature contribute between 50% and 70% of the observed change in relative humidity. It is also shown that large relative humidity anomalies exist over the equatorial Indian, Atlantic, and far east Pacific Oceans during the summer season (June-August) following an ENSO vent. Ocean- atmosphere dynamics coupled with the seasonal cycle of relative humidity explain the existence of the long-lasting effects of ENSO in the atmosphere. The authors argue that observed linear trends in regional and tropical mean relative humidity are unlikely to be due solely to ENSO or a simple intensification of the hydrological cycle.

74. Syroka J, Toumi R On the withdrawal of the Indian summer monsoon Q J ROY METEOR SOC 130 (598): 989-1008 Part A APR 2004

The withdrawal of the Indian summer monsoon (ISM) is defined in terms of 850 mb daily winds. The withdrawal of the ISM is found to be more variable than the onset. Most of the interannual variability of the total seasonal rainfall is dominated by the variability of the retreat phase. The withdrawal of the ISM follows a period of enhanced convective activity over the Indian subcontinent and is associated with a dry phase of the intraseasonal oscillation. The intraseasonal break dynamics have relatively more hemispheric symmetry than during the main monsoon season. The monthly persistence of interannual anomalies. the correlations with El Nino and remote upper-level temperature signals of a Rossby type are all Stronger during the withdrawal than in the summer months. These observations are found to be consistent with a mobile latent heat source migrating, towards the west Pacific during the monsoon withdrawal.

73. Toumi R, Qie XS Seasonal variation of lightning on the Tibetan plateau: A spring anomaly? GEOPHYS RES LETT 31 (4): art. no. L04115 FEB 27 2004

Satellite observations of lightning over the Tibetan Plateau show a maximum flash density in July. Thermodynamic parameters and rainfall obtained from meteorological reanalysis are broadly consistent with the observed seasonal cycle. However, there is more lightning in Spring than one may expect from a simple relationship with rainfall, temperature or cloud buoyancy. The surface total heat flux best accounts for the seasonal variation of lightning including the Spring lightning. The cloud buoyancy and rainfall show a better seasonal relationship when they are multiplied by the ratio of the sensible to latent heat flux (the Bowen ratio). This suggests an important role, at least in the Tibetan Plateau, for the sensible heat flux in modifying the efficiency of generating lightning from cloud buoyancy.

72. Kennett EJ, Toumi R, Asymmetry and non-linearity in upper tropospheric humidity variability, Geophys. Res. Lett., Vol. 30, No. 22, 2161,10.1029/2003GL018238 25 November 2003

Changes in upper tropospheric humidity (UTH) are important in understanding the climatic response to increased anthropogenic forcing. This paper explores the asymmetry and non-linearity in the response of subtropical UTH to tropical intraseasonal forcing. During northern winter, subtropical UTH anomalies, varying on intraseasonal timescales, propagate eastwards from northern Africa to the central Pacific. There is a discontinuity in the propagation of moist anomalies over the Tibetan Plateau, whilst dry anomalies show reduced propagation in the lee. This is consistent with the expected evolution of cyclones and anticyclones incident on high topography. Correlations of five-day mean UTH show no significant linear correlation between moist anomalies over Indonesia and subtropical anomalies to the north. This appears to be explained by the limitation of dry anomalies in the region of low ambient vorticity south of the East Asian Jet Stream.

71. Qie X Toumi R, Yuan T, Lightning activities on the Tibetan Plateau as observed by the lightning imaging sensor J. Geophys. Res., Vol. 108, No. D17, 4551 10.1029/2002JD003304 11 September 2003.

Lightning flash activities on the Tibetan Plateau are investigated using observations from the lightning imaging sensor. About 95% of the flashes are found to occur during May to September with a single peak in the summer from June to August. There is substantial lightning activity in May on the plateau, especially on the southern and eastern plateau. The diurnal variation of the lightning activity shows a prominent peak from 1500 to 1700 LT for most of the plateau with earlier activity on the eastern and southern plateau and a delay on the western, northern, and central plateau. Few lightning flashes are observed between 0000 and 1000 LT. The highest flash density is found in the grassland central plateau with a value of 4.5 flashes km(-2) yr(-1), while the minimum is found in the semiarid western plateau with a value of 1.5 flashes km(-2)yr(-1). The optical radiance of flashes fits a lognormal distribution. More energetic flashes are found on the mountainous eastern and northern plateau, and weaker flashes are found on the wet southern and semiarid western plateau. A nonlinear relationship between lightning activity and monthly averaged convective available potential energy (CAPE) is found. The flash number per CAPE on the Tibetan Plateau is much larger than it is for other regions with prominent lightning activity (but low altitude).

70. Williams V, Noland RB, Toumi R, Air transport cruise altitude restrictions to minimize contrail formation, CLIMATE POLICY 3, 3, 207-219, 2003.

While quantification of the effects of NOx and water vapor is still at an early stage there is evidence that contrail formation could make a significant contribution to global warming. This paper builds on previous research that analyzed a policy of restricting air transport cruise altitudes to eliminate contrail formation. Our previous work [Transport. Res. D 7(6) (2002) 451], examined altitude restrictions in European airspace and concluded that this could be a beneficial policy for reducing climate change impacts from aviation. Since most of the flights in European airspace are short-haul flights, this paper evaluates the trade-offs between altitude restrictions, fuel burn and journey times for longer haul flights of up to 6000 nm. Our focus is on the North Atlantic and US airspace and we examine potential contrail fraction to determine optimal cruise altitudes for reducing contrail formation. Changes in fuel burn and travel times associated with flight levels of 18,000 and 31,000 ft for different aircraft types are analyzed. We find that, in most cases, CO2 emission increases would be unlikely to entirely counteract the benefit of possible reductions in contrail formation. For some aircraft types, the percentage increase in emitted CO2 was found to be strongly dependent on journey length. In general, journey times appear not to be a major issue except for some aircraft types. Our results suggest that reducing aircraft cruise altitudes could be a beneficial policy for mitigating climate change impacts from the aviation sector. This is clearly dependent on aircraft type and the distances traveled, but more importantly on ambient atmospheric conditions which can vary significantly between regions and due to daily variation. This suggests that real time flight planning to minimize contrail formation should be investigated as a possible climate mitigation policy.

69. Wilson PS, Tomsett AC, Toumi R Long-memory analysis of time series with missing values PHYS REV E 68 (1): art. no. 017103 Part 2 JUL 2003

The estimation of long memory is often restricted by missing data. We examine the effects on the estimation of long memory of three simple gap-filling techniques: interpolation, random, and mean filling. Numerical simulations show that the gap- filling techniques introduce significant deviations from the expected scaling behavior for both persistent and antipersistent time series. For persistent time series the interpolation method provides a reliable estimation of long memory for scales longer than the largest likely gap.

68. Hadley A, Toumi R Assessing changes to the probability distribution of sulphur dioxide in the UK using a lognormal model ATMOS ENVIRON 37 (11): 1461-1474 APR 2003

This study investigates whether there has been any change in the concentration probability distribution of sulphur dioxide at 10 United Kingdom monitoring sites over time periods of up to 40 yr. It is a large and comprehensive study in terms of geographical and temporal extent. The lognormal probability plot, its associated correlation coefficient and a test for significance are used to fit and assess how well a 2-parameter lognormal distribution describes the data. Daily data are analysed at all sites on an annual basis and further seasonal and meteorological investigations are undertaken at two of the sites. The lognormal is found to be a good and robust fit for a variety of conditions. The level of lognormality has stayed relatively constant over a large range of ambient mean sulphur dioxide levels. To explain the lognormal distribution and specific departures, a physically based stochastic model is proposed which not only develops the idea of successive random dilutions (J. Air Waste Manage. Assoc. 40 (1990) 1378) but also includes the processes of deposition, chemical reaction and emission

67. Qie XS, Toumi R, Zhou YJ Lightning activity on the central Tibetan Plateau and its response to convective available potential energy CHINESE SCI BULL 48 (3): 296-299 FEB 2003

Lightning flash activities on the central Tibetan Plateau have been studied by using the satellite-based Lightning Imaging Sensor (LIS) database from January 1998 to July 2002. The lightning activity shows a clear diurnal variation on the central Plateau. The peak lightning activity appears at about 17 : 00 which is 3 h earlier than that in Jingzhou, Hubei in the same latitude belt nearby, indicating that the lightning activity is a sensitive indicator of solar heating on the Plateau. The lightning discharge is weaker on the Plateau than Jingzhou, Hubei and other low-altitude continental regions because of the lower convective available potential energy (CAPE) on the Plateau. The CAPE on the Plateau is 12 times lower than that in Jingzhou, Hubei, and 20 times lower than that in the sea-level region, such as Guangzhou and Florida. However, the sensitivity of lightning activity to CAPE changes on the Plateau is up to 30 times more sensitive than other prominent low-altitude regions.

65. Williams V, Noland R.B. and Toumi, R., Reducing the climate change impacts of aviation by restricting cruise altitudes, Transportation Research D, 7 (6), pp.451-464, 2002.

Two of the ways in which air travel affects climate are the emission of carbon dioxide and the creation of high-altitude contrails. One possible impact reduction strategy is to significantly reduce the formation of contrails. This could be achieved by limiting the cruise altitude of aircraft. If implemented, this could severely constrain air space capacity,especially in parts of Europe. In addition, carbon emissions would likely be higher due to less efficient aircraft operate at lower cruise altitudes. This paper describes an analysis of these trade-offs using an air space simulation model as applied to European airspace. The model simulates the flight paths and altitudes of each aircraft and is here used to calculate emissions of carbon dioxide and changes in the journey time. For a 1-day Western European traffic sample, calculations suggest annual mean CO2 emissions would increase by only 4% if cruise altitudes were restricted to prevent contrail formation. The change in journey time depended on aircraft type and route, but average changes were less than 1 minute. Our analysis demonstrates that altitude restrictions on commercial aircraft could be an effective means of reducing climate change impacts, though it will be necessary to mitigate the increased controller workload conflicts that this will generate.

64. Hadley A, Toumi R Non-linearities between atmospheric sulphur and sulphur emissions WATER AIR SOIL POLL 140 (1-4): 279-295 OCT 2002

Downing et al. (1995) and others have reported evidence for a non-linearity between atmospheric sulphur and sulphur emissions in the U.K. Also, there are reports of the effects of the non-linearity being concentrated in regions close to sources. Here we re-analyse the same data in sulphur dioxide versus emission space. Analysis has been carried out at three sites at varying distances from the main U.K. source regions. Annual, seasonal and meteorological categorisations were made to try and clarify observation of any non-linearity. Over long timescales linearity is not excluded. Inter-annual variability is not correlated with emissions. The relation between sulphur dioxide and emissions was found to be least linear for westerly days at a site in southern Scotland. Overall, the extent of non-linearity is not observed to increase with proximity to sources but the relation is least linear for summer days at sites closer to the main source areas.

63. Syroka J, Toumi R Recent lengthening of the south Asian summer monsoon season GEOPHYSICAL RESEARCH LETTERS 29 (10): art. no. 1458 MAY 15 2002

An increase in October precipitation since 1979 over the south Asian region has been observed. This increase is found to be associated with enhanced cross-equatorial flow and strengthened cyclonic circulation over the Indian subcontinent synonymous with a northwest shift in the tropical convergence zone. We show that October lower tropospheric flow is becoming more like September circulation, indicating an increasing delay in the retreat phase of the monsoon system. This extension of the south Asian summer monsoon may be linked to enhanced land-sea contrast.

62. Hadley A, Toumi R A simple model which predicts some non-linear features between atmospheric sulphur and sulphur emissions ENVIRON POLLUT 119 (3): 365-374 2002

This paper develops a simple model and suggests a plausible chemico-physical mechanism for a non-linear response between atmospheric sulphur and sulphur emissions. It contains simplified representations of transport, deposition and conversion processes and uses a proxy in-cloud oxidant-limited reaction along a pathway connecting an emission source with a receptor site. Individual pathway responses to emissions show linear behaviour above a threshold. However, by averaging the values of SO2 at the receptor site from different pathways a continuous non-linear relationship is obtained. As emissions reduce, distant emission sources become less significant contributors of sulphur dioxide at a receptor site but their emissions are still counted in an emission inventory, leading to an apparent non-linearity. Sulphate is always round to contribute a signal to the receptor site total. This model goes someway to explaining a proposed 'crossover' between observed proportions of wet and dry deposited sulphur in the UK as emissions have been reduced.

61. Syroka J, Toumi R, Scaling of Central England Temperature Flutuations ? , Atmospheric Science Letters (2), No 1/2/3/4/, 143-154 2001 (doi: 10.1006/asle.2002.0047)

Central England temperature fluctuations are found to be monoscaling with long-range dependence. Monoscaling can be explained in terms of the dominance of Gaussian temperature advection. Simulations of the UK Meteorological Office Hadley Centre general circulation model do not capture many of these features.

60. Toumi R , Syroka J, Barnes C, ,Lewis P, Robust non-Gaussian statistics and long-range correlation of total ozone, (doi:10.1006/asle.2001.0042)

Three long-term total ozone time series at Camborne, Lerwick and Arosa are examined for their statistical properties. Non-Gaussian behaviour is seen for all locations. There arelarge interannual fluctuations in the higher moments of the probability distribution. However, only the mean for all stations and summer standard deviation at Lerwick show significant trends. This suggests that there has been no long-term change in the stratospheric circulation, but there are decadal variations. The time series can be also characterised as scale invariant with a Hurst exponent of about 0.8 for all three sites. The Arosa time series was found to be weakly intermittent, in agreement with the non-Gaussian characteristics of the data set

59. Tomsett AC, Toumi R Annual persistence in observed and modelled UK precipitation GEOPHYS RES LETT 28 (20): 3891-3894 OCT 15 2001

This paper examines the correlation structure of inter-decadal precipitation variability across the UK by using a scaling analysis technique. The level of persistence displayed by the observations between different types of precipitation is remarkably constant. Comparison with synthetic long-memory time series suggests that the behaviour of the observed precipitation is consistent with a scaling process. The Hadley Centre HadCM2 model captures some of the observed behaviour, but underestimates the degree of persistence in all precipitation intensities. The scaling analysis method is a useful diagnostic to test for climate model variability.

58. Windsor HL, Toumi R Scaling and persistence of UK pollution ATMOS ENVIRON 35 (27): 4545-4556 SEP 2001

Three methods are used to examine the statistical characteristics of UK pollution time series: Sigma-T (ST), Hurst rescaled range and kurtosis. Hourly observations of ozone, PMIO and PM2.5 are examined. We find evidence of high persistence or long-term memory of pollutant fluctuations up to 400 days. The ST method was found to be more sensitive than the Hurst analysis. Scaling behaviour suggesting the lack of characteristic time scales was found. Kurtosis reveals the importance of intermittent pollution episodes for particulate species, but near Gaussian behaviour for ozone.

57. Syroka J, Toumi R Scaling and persistence in observed and modelled surface temperature GEOPHYS RES LETT 28 (17): 3255-3258 SEP 1 2001

We introduce a simple and effective method to test for temperature persistence on a range of time scales. Daily temperatures for three regions are examined: Central England, El-Nino region and global. On time scales of less than a year, climate model simulations reproduce the observed high persistence in all regions. However, for longer time scales the climate model systematically underestimates the persistence of global mean temperatures. The anti-persistence in the El-Nino, region is well simulated by the model. Scaling behaviour is seen for Central England temperatures but is not reproduced by the model. Some of these discrepancies may be accounted for by insufficient ocean-atmosphere coupling.

56. Williams, V Toumi, R ,The correlation between tropical total ozone and outgoing long- wave radiation ,QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY

Interannual variations in tropical column ozone, as observed by the Total Ozone Mapping Spectrometer? are analysed for the period 1979-94 using linear regression to extract signals related to the solar cycle, quasi-biennial oscillation, and El Ni (n) over tildeo Southern Oscillation (ENSO). Using time series of outgoing long-wave radiation (OLR) as a proxy for EN,variations, rather than the widely used Southern Oscillation index is found to improve the fit to observed ozone. The variability in the ozone and OLR datasets is investigated using empirical orthogonal function analysis. The most strongly correlated OLR and ozone modes exhibit ENSO-like variability.

55. Gorbunov, B Baklanov, A Kakutkina, N Windsor, HL Toumi, R ,Ice nucleation on soot particles ,JOURNAL OF AEROSOL SCIENCE

The ice-forming activity of soot particles of various sizes has been studied in a cloud chamber under temperatures ranging from -5 to -20 degreesC, It was found that the fraction of aerosol particles forming ice crystals was influenced by the temperature, the mean radius of aerosol particles and the degree of oxidising of the soot particle surface. It was suggested that oxidising affected the concentration of surface chemical groups that could form hydrogen bonds with water molecules. A decrease in the temperature and an increase in the radius of particles led to an increase in the number of ice crystals. Data obtained were parameterised and an expression was derived that enables the concentration of ice crystals to be calculated for conditions in a low cloud. Based on these experiments, this particular soot is a very potent source of ice nuclei. Data obtained were compared with Fletcher theory. It was shown that the theory contradicts experimental data and cannot be recommended for evaluation of the number of ice crystals in clouds. An application of the data obtained to aircraft condensation trail formation is discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.

54. Smith, CA Haigh, JD Toumi, R ,Radiative forcing due to trends in stratospheric water vapour ,GEOPHYSICAL RESEARCH LETTERS

Trends derived from the latest version of Halogen Occultation Experiment (HALOE) data are used in a two-dimensional atmospheric model to estimate their radiative effects over the last decade. The results show a stratospheric cooling in regions of H2O increase, of magnitude similar to that due to stratospheric ozone loss indicating a significant additional cause of observed stratospheric temperature decreases. Radiative forcings are derived and it is found that global average radiative forcing due to stratospheric water vapour changes probably lies in the range 0.12 to 0.20 Wm(-2)decade(- 1). This could have more than compensated for the negative radiative forcing due to decadal ozone loss.

53. Smith, CA Toumi, R Haigh, JD ,Seasonal trends in stratospheric water vapour ,GEOPHYSICAL RESEARCH LETTERS

Published analyses of HALOE water vapour data have shown an increase in stratospheric water vapour. Here the latest version of the data has been analysed for trends from Jan 1992 to April 1999. This shows an increase in stratospheric water vapour over this period although in the past few years the magnitude of the trend appears to have decreased. A seasonal analysis suggests that large increases in upper tropospheric water vapour in autumn are the source for the stratospheric increases. An analysis of trends in the seasonal amplitude supports this. It is suggested that an increase in the extremity of the Asian monsoon over Indonesia could be responsible.

52. Tomsett, AC Toumi, R ,Diurnal temperature range and rainfall probability over the United Kingdom ,GEOPHYSICAL RESEARCH LETTERS

Daily data from five stations in the United Kingdom covering the period 1900-1993 were used to explore the relationship between diurnal temperature range (DTR) and precipitation. A strong dependency between the probability of future rainfall and the magnitude of the previous day's DTR was found. The form of this relationship seems to be independent of location and season. DTRs greater than approximately 10 degrees C are associated with reduced probabilities of rainfall. It is argued that DTR is a good proxy for cloud base height, which, in turn, is critical in determining the probability of precipitation.

51. Geiser, JD Dylewski, SM Mueller, JA Wilson, RJ Toumi, R Houston, PL ,The vibrational distribution of O-2(X (3)Sigma(-)(g)) produced in the photodissociation of ozone between 226 and 240 and at 266 nm ,JOURNAL OF CHEMICAL PHYSICS

Resonance-enhanced multiphoton ionization coupled with time-of- flight product imaging has been used to study the O-3(X (1)A(1))+h nu --> O(2p P-3(J))+O-2(X (3)Sigma(g)(-)) product channel in the UV (ultraviolet) photodissociation of ozone at photolysis wavelengths of 226, 230, 233, 234, 240, and 266 nm. These imaging experiments, together with a measurement of the branching ratio into the different spin orbit states of the O atom, allowed the determination of the yields of the O-2 product in vibrational states greater than or equal to 26 as a function of wavelength. It was found that at 226, 230, 233, 234, and 240 nm, the yield was 11.8 +/- 1.9%, 11.5 +/- 1.2%, 8.2 +/- 2.0%, 4.7 +/- 1.8%, and 0.6 +/- 0.1%, respectively. (C) 2000 American Institute of Physics. [S0021-9606(00)01203-4].

49. Lary, DJ Shallcross, DE Toumi, R ,Carbonaceous aerosols and their potential role in atmospheric chemistry ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

This paper considers the nature of carbonaceous surfaces, the means by which they are activated, the nature of some functional groups that they support, and some reaction mechanisms that may be involved. Because of the strong affinity of carbonaceous surfaces for organic species and because of the ease with which compounds in a high oxidation state can oxidize the carbonaceous surface, it is highly likely that carbonaceous aerosols are interacting chemically with a range of organic species in ways that have, as yet, not been fully characterized but may significantly affect the oxidizing capacity of our atmosphere. If HONO is formed on the surface of carbonaceous aerosols then this could be a significant source of HOx, as HONO is readily photolyzed to give OH, and it could explain the large values of HONO often observed in the troposphere. In general, the reduction of NOy on carbonaceous aerosols is an important consideration, and it is addressed here.

48. Toumi, R Hartell, N Bignell, K ,Mountain station pressure as an indicator of climate change ,GEOPHYSICAL RESEARCH LETTERS

Surface temperature observations have been used to infer a global warming of about 0.7 degrees K over the last century. Here, we present a new way of deriving temperature trends from observations of mountain surface pressure. From the hydrostatic equation it can be shown that a warming below the mountain increases the local pressure. This signal increases with height. The derived temperature trends from a few sites are consistent with reported regional warmings and a recent acceleration. These results provide independent confirmation of regional warming of the lower troposphere on a longer time scale than is currently available from radiosondes and satellites.

47. Reeve, N Toumi, R ,Lightning activity as an indicator of climate change ,QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY

Data from the Optical Transient Detector lightning sensor are analysed to investigate the hypothesis that global lightning activity will increase should the average global temperature increase. It is shown that changes in global monthly land lightning activity are well correlated with changes in global monthly land wet-bulb temperatures. The correlation is strongest in the northern hemisphere and weak in the southern hemisphere. The conclusion is that a high land-area to sea-area ratio is necessary for a good correlation. Contrary to expectation, the tropics show no correlation. The results predict that a change in the average land wet-bulb temperature of the globe of just 1K would result in a change in lightning activity of about 40%.

46. Williams, V Toumi, R ,The radiative-dynamical effects of high cloud on global ozone distribution ,JOURNAL OF ATMOSPHERIC CHEMISTRY

Previous studies (e.g., Dessler et al., 1996; Haigh, 1984) have discussed the effect of cloud on modelled ozone distribution through changes in the radiative heating in the lower stratosphere. Here the relationship is investigated using an interactive chemical-radiative-transport 2D model. It is shown that, while similar cooling in the lower stratosphere due to high cloud is found, the effect on ozone is not as previously expected. The inclusion of high cloud is found to bring about a warming of the troposphere, resulting in a net heating in the lower stratosphere. This strengthens the circulation, leading to a decrease in total tropical ozone. Importantly, the effect of the cloud-induced temperature changes on heating rates does not combine linearly with the direct radiative effect of cloud changes. The possibility of a link between the high cloud increases and total ozone decreases observed in some regions during strong El Nino episodes is investigated. The possible impact on ozone of a global trend in high cloud cover is also discussed.

45. Kernthaler, SC Toumi, R Haigh, JD ,Some doubts concerning a link between cosmic ray fluxes and global cloudiness ,GEOPHYSICAL RESEARCH LETTERS

Svensmark and Friis-Christensen (1997, henceforth SFC) showed a strong correlation between cosmic ray flux and ISCCP total cloudiness between 1984 and 1990. They concluded that ionisation by cosmic rays, more prevalent at times of lower solar activity, might explain apparent correlations between solar activity and climate through changes in cloud radiative forcing. We have extended SFC's approach with a study of the different cloud types, restricting our analysis to the period 1985 to 1988 during which the ISCCP calibration is believed to be stable. We find no clear relationship between individual cloud types and cosmic ray flux. Inclusion of data at high latitudes decreases the amplitude of the apparent correlation although ionisation by cosmic rays is greatest at high latitudes. Thin high cloud shows an increase throughout the period such that the combined effect of the changes in cloud types suggests an almost monotonic increase in cloud radiative forcing between 1985 and 1988 which is not related to cosmic ray activity.

44. Pavelin, EG Johnson, CE Rughooputh, S Toumi, R ,Evaluation of pre-industrial surface ozone measurements made using Schonbein's method ,ATMOSPHERIC ENVIRONMENT

Ozone measurements made using Schonbein's method during the late nineteenth and early twentieth centuries have been examined and converted to modern units using a method originally developed by Linvill et al. (1980). Monthly Weather Review 108, 1880-1891 and Anfossi et al. (1991). Journal of Geophysical Research 96, 17,349-17,352. New data are presented here from sites in Europe, Asia, Africa, Australia, and South America. The values obtained lie in the range 5-15 ppb for all sites. A negative correlation between ozone and humidity is observed, which may be consistent with photochemical loss of ozone in the presence of water vapour. However, uncertainties in the humidity correction to the Schonbein reading will lead to considerable inaccuracies in the seasonal cycle established by this method. (C) 1999 Elsevier Science Ltd. All rights reserved.

43. Clegg, NA Toumi, R ,Non-sea-salt-sulphate formation in sea-salt aerosol ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

By comparing the observed non-sea-salt-sulphate:sodium (nss- SO42-:Na+) ratios with model calculations the main sources of nss-SO42- observed in sea-salt aerosol could be determined. As heterogeneous production appears to be responsible for at most half of the measured nss-SO42- [Clegg and Toumi, 1997; Chameides and Stelson, 1992] other mechanisms have been examined here. Processes such as the presence of oxidizers other than hydrogen peroxide and ozone and alternative sources of sulphur appear to make little if any difference to calculated ratios. Contamination by "pure" sulphate particles is deemed unlikely, as is a sufficient increase in the alkalinity of the sea-salt particles. However, observed ratios can be achieved in the presence of even a small amount of cloud processing. While observed ratios may possibly be due to experimental uncertainties the sulphate production produced by cloud processing is the most likely explanation; Predictions also show that smaller sea-salt particles will have much greater nss-SO42-:Na+ ratios than larger sea-salt particles. The presence of large quantities of submicron sea-salt particles which could also act as cloud condensation nuclei would make sea salt the largest sink for sulphur dioxide in the marine boundary layer.

42. Gorbunov, B Hamilton, R Clegg, N Toumi, R ,Water nucleation on aerosol particles containing both organic and soluble inorganic substances ,ATMOSPHERIC RESEARCH

A theory of the formation of water droplets on aerosol particles consisting of soluble inorganic salts and organic (soluble, insoluble and surface active) substances has been developed. The case of partially wettable spherical insoluble core is considered. The theory links the interfacial free energies of the surfaces, the size of the insoluble core, and the chemical characteristics of the soluble substances with the ability of the aerosols to form water droplets. The theory is compared with Kohler theory and major differences in the equilibrium pressure of water vapour and nucleation rate were found. The influence of various types of organic material on CCN activation is considered. It is shown that the presence of both soluble and insoluble organic substances in CCN can affect their ability to form cloud droplets. Surface active substances are found to be the most important components of organic matter that are able to considerably affect CCN activation. A CCN containing ammonium sulphate, sodium chloride or another salt and surface active agent is activated under lower supersaturation than CCN containing only salts. The presence of surface agents in CCN could lead to greater concentration of cloud droplets. Thus, varying the concentration of surface active substances in atmospheric aerosols may alter the number concentration of cloud droplets and the radiative properties of clouds. (C) 1998 Elsevier Science B.V. All rights reserved.

41. Evans, SJ Toumi, R Harries, JE Chipperfield, MP Russell, JM ,Trends in stratospheric humidity and the sensitivity of ozone to these trends ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

Measurements of stratospheric water vapor and methane from the Halogen Occultation Experiment (HALOE) mounted on the Upper Atmosphere Research Satellite (UARS) are used to investigate changes in stratospheric water vapor over the period 1992-1996 inclusive. An increase in water vapor mixing ratio is found at levels between 30 km and 65 km across the globe which fit, to first order, a linear trend varying with altitude from 40 parts per billion by volume per year (ppbv yr(-1)) to a maximum of 90 ppbv yr(-1) at 45 km. These trends appear to be greater than that expected due to the growth in tropospheric methane over the past several decades, and possible mechanisms accounting for this are discussed. The trend of the combined budget of 2 x CH4 + H2O is approximately constant with altitude with a global mean value of 61 +/- 4 ppbv yr(-1). On the basis of these estimates, sensitivity studies have been performed using a two- dimensional (2-D) radiative-chemical-dynamical model to assess the impact on concentrations of stratospheric ozone of this degree of change in stratospheric water vapor over timescales consistent with doubling CO2 scenarios. We find that the impact of increased stratospheric water vapor is to enhance the ozone increase in the midstratosphere by similar to 1 - 2% compared to the response due to a doubling of CO2 itself of similar to 5 - 10%. In the upper stratosphere the destruction of ozone is enhanced and the changeover from production to loss is lowered to similar to 50 km (from similar to 70 km). A chemical mechanism for these processes involving enhanced OH and NO2 is identified.

39. Clegg, NA, Toumi, R ,Sensitivity of sulphur dioxide oxidation in sea salt to nitric acid and ammonia gas phase concentrations ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

The role of sea-salt aerosol in the sulphur dioxide budget is investigated using a steady state box model. The model simulates, for certain species, the condensation onto, and chemical reactions within, deliquescent sea-salt particles, The chemical reactions are controlled by the pH of the solution, and results show that the presence of gas-phase nitric acid, HNO3, and ammonia, NH3, can have a significant effect on calculated concentrations of non-sea-salt (nss) sulphate, [nss- SO42-], formed within the solution. For example, in the case of high [HNO3(g)] (100 pptv) the calculated [nss-SO42-] can be decreased by similar to 40% after 2 days (with a 75% decrease at HNO3 concentrations of 1 ppbv). When NH3 is added, a decrease in calculated [nss-SO42-] of 20% is observed for 500 pptv NH3, 50 pptv HNO3. Thus nitric acid is more effective than ammonia. In conditions appropriate for the marine boundary layer (50 pptv of SO2, and HNO3, and 200 pptv of NH3), nitric acid is still the governing factor, and the [nss-SO42-], after 2, days, is decreased by approximately 25% compared to the concentration formed when conditions show no HNO, or NH3. Results also show that comparisons with nss-SO42- observations are nearly 1 order of magnitude lower, with observations giving a [nss-SO42-]:Na+ ratio of 0.01-0.04 compared to predicted values of 0.005 or lower. Other mechanisms of nss-sulphate production may have to be used to explain all the nss-sulphate found id sea-salt aerosol. Although in cases where [SO2(g)] is 10-100 pptv the decrease in [nss-SO42-] is 15-30%, at values of [SO2(g)] > 100 pptv the effects of both nitric acid and ammonia on calculated [nss-SO42-] are small. In a future scenario of increasing NOx(HNO3) emissions and decreasing SO2 emissions the role of HNO3 on the global sulphur budget could be very significant.

38. Lary, DJ Toumi, R ,Halogen-catalyzed methane oxidation ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

This paper highlights the importance of halogen-catalyzed methane oxidation in the upper troposphere and lower stratosphere. The calculated rate of methane oxidation is increased by at least 20% in the upper troposphere when halogen catalysis is included. In the lower stratosphere, approximately 25% of methane oxidation can be initiated by chlorine; the precise fraction is very temperature dependent. Including halogen-catalyzed methane oxidation increases the HOx and ClOx concentrations and decreases the NOx concentration. The calculated enhancement in the HOx concentration due to halogen- catalyzed methane oxidation is around 10-15% in the lower stratosphere; and around 20% in the upper troposphere. The decrease in the NOx concentration is around 10% in the upper troposphere. The enhancement in the ClOx concentration is around 7-10% in the lower stratosphere. The increase in the calculated HOx and ClOx concentrations and the decrease in the NOx concentration lead to a enhancement in the calculated O-3 loss. The additional O-3 loss calculated is most significant in the upper troposphere where over a 7-day simulation it was of the order of 0.1-1% for midlatitudes at equinox. As the atmospheric loading of chlorine drops the gross odd-oxygen production by NO + HO2 will increase, so there will be an accelerated ozone recovery. On a per molecule basis, bromine- catalyzed methane oxidation is approximately 2 orders of magnitude faster than chlorine catalyzed methane oxidation. In the upper troposphere bromine-catalyzed methane oxidation destroys ozone at a rate which is approximately one: third of that at which nitrogen-catalyzed methane oxidation is producing ozone. Therefore, with the increasing atmospheric bromine loading, bromine-catalyzed methane oxidation is set to become more important. It would be valuable to have kinetic studies of the reaction BrO with CH3O2 so that the role of bromine- catalyzed methane oxidation can be quantified more precisely.

37. Sinha, A Toumi, R ,Tropospheric ozone, lightning, and climate change ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

Tropospheric O-3 is an important greenhouse gas. Lightning is a major source of NOx, and thus of tropospheric O-3. It has recently been suggested that due to an apparent strong correlation between lightning strike rates and surface temperatures, tropospheric O-3 may significantly increase if the climate warms, resulting in a substantial positive climate forcing. This paper attempts to quantify the extent of this forcing and the associated positive climate feedback. Simulations incorporating a tropospheric O-3-surface temperature parameterization are performed of the last glacial maximum and of a climate in which greenhouse gas concentrations have been doubled. The O-3 parameterization is based on results from a two-dimensional chemical model. The simulations are obtained using a one-dimensional radiative-convective model, in which CO2, CH4, tropospheric O-3, and in the case of the paleoclimate simulation the surface albedo, are varied both independently and in combination. In the paleoclimate case, the tropospheric O-3 feedback has roughly two thirds of the effect on surface temperatures as the change in tropospheric O-3 due to industrialization alone. During climate warming, the effect on surface temperatures is about 60% of that due to a doubling of CH4. The results indicate that a surface temperature- lightning-O-3 feed-back, currently absent in general circulation models, could significantly affect anthropogenic climate change. Improved modeling and observations are required to confirm this.

36. Lary, DJ Lee, AM Toumi, R Newchurch, MJ Pirre, M Renard, JB ,Carbon aerosols and atmospheric photochemistry ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

Carbon aerosols are produced by all combustion processes. This paper investigates some possible effects of heterogeneous reduction of atmospheric constituents on carbon aerosols. Reduction of HNO3, NO2, and O-3 on carbon aerosols may be an important effect of increased air traffic that has not been considered to date. It is shown that if HNO3, NO2 and O-3 are heterogeneously reduced on atmospheric amorphous carbon aerosols, then a significant, lower stratospheric ozone loss mechanism could exist. This ozone loss mechanism is almost independent of temperature and does not require the presence of sunlight. The mechanism can operate at all latitudes where amorphous carbon aerosols are present. The relative importance of the mechanism increases with nightlength. The reduction of HNO3 on carbon aerosols could also be a significant renoxification process wherever carbon aerosols are present. Owing to the very different soot levels in the two hemispheres, this implies that there should be a hemispheric assymetry in the role of these mechanisms. The renoxification leads to simulated tropospheric HNO3/NOx ratios that are close to those Observed. In contrast ta the stratospheric response, the tropospheric production of NOx due to the reduction of HNO3 would lead to tropospheric ozone production.

35. Zhong, WY Toumi, R Haigh, JD ,Climate forcing by stratospheric ozone depletion calculated from observed temperature trends ,GEOPHYSICAL RESEARCH LETTERS

The radiative forcing of the surface-troposphere system caused by stratospheric ozone depletion in the 1980s is calculated using observed values of change in ozone, from TOMS data, and temperature, from MSU data. The seasonal variation of the ozone and temperature trends produces strong seasonal and latitudinal variations in radiative forcing. The balance between peak positive solar forcing and maximum negative longwave forcing shifts the peak negative forcing at the South Pole from November to October. The time difference between peak ozone loss and maximum temperature decrease results in net positive values in Southern Hemisphere mid-latitudes in August and September. Positive values also occur at low latitudes in both hemispheres. The globally and annually averaged net radiative forcing is about -0.025W m(-2) between 1979 and 1990, much less than that previously reported using fixed dynamical heating model temperature changes, although inclusion of the ozone 14 mu m band would somewhat narrow this gap. Ozone radiative forcing is very sensitive to the vertical profile of temperature change.

33. Bekki, S Pyle, JA Zhong, W Toumi, R Haigh, JD Pyle, DM ,The role of microphysical and chemical processes in prolonging the climate forcing of the Toba eruption ,GEOPHYSICAL RESEARCH LETTERS

The mega-eruption of Toba, Sumatra, occurred around 73 Ka ago, during the onset of a glaciation of the Late Quaternary. This coincidence combined with the unprecedented amount of sulphur released by this volcano has led to the hypothesis that Toba sulphate aerosols caused a transient surface cooling which may have contributed to a shift of the climate system. Because of tile self limiting effect of gravitational sedimentation, the climatic impact of extremely large sulphur injections into the stratosphere are thought to be rather limited. Here we present model calculations combining microphysical and chemical feedbacks which show that the eruption could instead have led to the formation of a long-lasting volcanic aerosol layer. Although the concentrations of radiatively active species such as O-3 or SO2 could also have been considerably perturbed, the resulting forcings should have only slightly moderated the aerosol cooling effect during the first few years following tile eruption. According to our results, extremely high stratospheric sulphur loading could lead to a more prolonged effect on the climate than previously assumed.

32. Zhong, WY Haigh, JD Toumi, R Bekki, S ,Infrared heating rates in the stratosphere due to volcanic sulphur dioxide ,QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY

A narrow-band radiative-transfer model is used to assess potential infrared cooling rates due to SO2 injected into the stratosphere by volcanic eruptions. In addition to SO2 profiles representing the eruptions of El Chichon and Pinatubo, two hypothetical SO2 profiles following a massive volcanic eruption are used. The net radiative impact of the SO2 depends on the vertical distribution as well as the altitude and magnitude of its injection. For broader distributions the magnitudes of the infrared heating rates are comparable with those due to heating caused by the absorption of solar ultraviolet by SO2 tending to enhance the hearing of the lower stratosphere and reduce it in the upper stratosphere. These results are explained in terms of the relative strengths of the SO2 bands and of those of H2O and O-3 overlapping in the same spectral regions.

31. Houston, PL Suits, AG Toumi, R ,Isotopic enrichment of heavy ozone in the stratosphere ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

A mass-independent, 3% enrichment of heavy ozone isotopes in the stratosphere can be assigned to a mechanism in which (1) short-wavelength photodissociation of ozone produces O-2(v greater than or equal to 26) + O, (2) the O-2)(v greater than or equal to 26) reacts with ground state O-2 to produce O-3 + O, and (3) each O atom recombines with O-2 to form O-3. The overall reaction scheme, O-3 + hv + 3O(2) --> 3O(3), produces more ozone than it consumes. The dissociation channel which begins this scheme is more probable for heavy ozone than for O- 48(3), and one of the oxygen atoms in the original O-3 is incorporated into a new O-3. Thus this new ozone source tends to ''distill'' heavy oxygen atoms into the O-3 pool while depleting them from the O-2 pool. The amount of enrichment produced by this mechanism, while significant, is still too small to account for the large observed stratospheric enrichment of heavy ozone.

30. Toumi, R Haigh, JD Law, KS ,A tropospheric ozone-lightning climate feedback ,GEOPHYSICAL RESEARCH LETTERS

Tropospheric ozone is an important greenhouse gas. In the upper troposphere one of the major sources of ozone are the nitrogen oxides produced by lightning [IPCC, 1995]. Recently it has been shown that the number of lightning flashes may be very sensitive to changes in the surface temperature [Williams, 1992]. Here we use a global two-dimensional atmospheric model and find that for a warmer surface and constant lightning ozone decreases globally except for in the polluted Northern Hemisphere. However, a warming with increased lightning can more than offset this decrease. We find that for a 20% increase of lightning the global mean radiative forcing by tropospheric ozone is about +0.1 Wm(-2). There is therefore a possibility of a positive climate feedback mechanism. The implied sensitivity of ozone to lightning and temperature suggests a major uncertainty in quantifying anthropogenic perturbations of upper tropospheric ozone.


In the past measurements of OClO have been used to determine the degree of chlorine activation in the Arctic and Antarctic lower stratospheres. In this study we have carried out box model calculations to investigate the behaviour of OClO concentrations at 50hPa for cold denoxified conditions, typical of the polar winter. The dependence of the OClO abundance on ClOx (ClO+2Cl(2)O(2)) and BrOx (BrO+BrCl), as well as the sensitivity to nitrogen species, latitude and temperature is investigated. The net production of OClO from the reaction ClO+BrO --> OClO+Br, the major source of OClO, is found to be first order in BrO for BrOx values from 0 to 22 pptv, whereas it becomes pseudo zero order in ClO for ClOx greater than similar to 400 pptv. This behaviour is due to the rate at which BrOx is tied up as BrCl at sunset. The insensitivity of the OClO concentration to high levels of CIO makes OClO a poor indicator for distinguishing between medium and high levels of ClO. OClO is only a qualitatively good indicator of chlorine activation but is a good quantitative indicator of BrO.

28. Sinha, A Toumi, R ,A comparison of climate forcings due to chlorofluorocarbons and carbon monoxide ,GEOPHYSICAL RESEARCH LETTERS

The direct radiative forcing of climate by carbon monoxide (CO) is generally considered to be negligible. However, a recent study of localised clear-sky surface irradiances asserts that the forcing by CO may be comparable to that by CFC-11. Nevertheless no detailed comparison of CO and CFC climate forcings has yet been made. Thus the present study estimates the radiative impact of the increases in CO, CFC-11 and CFC-12 that have occurred since industrialisation. A radiative transfer model is used to reproduce the results of the earlier study. Clouds are then added, and the stratosphere-adjusted forcing at the tropopause (the ''climate forcing'') calculated. Global-mean anthropogenic climate forcing by CO is determined to be 32% of that by CFC-11, 12% of that by CFC-12, and 9% of that by the CFCs combined. Even if the contemporary global-mean CO concentration is increased by a factor of three, the climate forcing by CO is still only 75% of that due to CFC-11. Regarding instantaneous clear-sky forcings, further analysis shows that surface measurements can give a misleading impression of effects at the tropopause. While the indirect effects of CO on climate change are not yet properly quantified, the direct radiative effects appear, as previously thought, to be of only minor significance.

27. Lary, DJ Chipperfield, MP Toumi, R Lenton, T ,Heterogeneous atmospheric bromine chemistry ,JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

This paper considers the effect of heterogeneous bromine reactions on stratospheric photochemistry. We have considered reactions on both sulfate aerosols and on polar stratospheric clouds (PSCs), It is shown that the hydrolysis of BrONO2 on sulfate aerosols enhances the HOBr concentration, which in turn enhances the OH and HO2 concentrations, thereby reducing the HCl lifetime and concentration. The hydrolysis of BrONO2 leads to a nighttime production of HOBr, making HOBr a major nighttime bromine reservoir. The photolysis of HOBr gives a rapid increase in the OH and HO2 concentration at dawn, as was recently observed by Salawitch et al. [1994]. The increase in the OH and HO2 concentration, and the decrease in the HCl concentration, leads to additional ozone depletion at all latitudes and for all season. At temperatures below 210 K the bulk phase reaction of HOBr with HCl in sulfate aerosols becomes important. The most important heterogeneous bromine reactions on polar stratospheric clouds are the mixed halogen reactions of HCl with HOBr and BrONO2 and of HBr with HOCl and ClONO2.


We call attention to the likely importance of the potential reaction OH + ClO --> HCl + O-2. It may only be a minor channel for the reaction of OH with ClO, which is often ignored in models, but if it occurs it considerably increases the rate of recovery of HCl after an air parcel has encountered a polar stratospheric cloud (PSC). The net effect of this reaction on the ozone concentration depends on the relative HCl concentration and whether the air parcel is in a PSC. When an air parcel is in a PSC and the HCl concentration is less than the sum of the HOCl and ClONO2 concentrations, heterogeneous ClOx production is rate limited by the production of HCl. Under these conditions the reaction allows HCl to be reprocessed more rapidly by the heterogeneous reactions of HCl with HOCl and ClONO2. This allows high ClOx concentration to be maintained for longer, and at a slightly higher level, than would otherwise be possible which in turn leads to more ozone depletion. When there are PSCs but HCl is in excess, or outside of the PSC regions (i.e. during the recovery phase), the reaction will always reduce the ClO/HCl ratio and hence slightly reduce the ozone loss.


The absolute rate constant for the reaction O(P-3) + HOBr has been measured between T=233K and 423K using the discharge-flow kinetic technique coupled to mass spectrometric detection. The value of the rate coefficient at room temperature is (2.5+/- 0.6) x 10(-11) cm3 molecule-1 s-1 and the derived Arrhenius expression is (1.4+/-0.5) x 10(-10) exp[(-430 +/- 260)/T] cm3 molecule-1 s-1. From these rate data the atmospheric lifetime of HOBr with respect to reaction with O(P-3) is about 0.6h at z = 25 km which is comparable to the photolysis lifetime based on recent measurements of the UV cross section for HOBr. Implications for HOBr loss in the stratosphere have been tested using a 1D photochemical box model. With the inclusion of the rate parameters and products for the O + HOBr reaction, calculated concentration profiles of BrO increase by up to 33% around z = 35 km. This result indicates that the inclusion of the O + HOBr reaction in global atmospheric chemistry models may have an an impact on bromine partitioning in the middle atmosphere.


CHEMICAL depletion of ozone in the lower stratosphere decreases the direct radiative forcing from greenhouse gases in the atmosphere(1). Here we show that ozone depletion may also exert an indirect effect on radiative forcing via its effect on the oxidation state of the atmosphere. Hydroxyl (OH) radicals in the troposphere are produced by photodissociation of tropospheric ozone in the presence of water vapour, and this process is enhanced if the absorption of ultraviolet radiation by the overlying stratospheric ozone column decreases. As OH oxidizes SO2 to sulphuric acid, which then forms cloud condensation nuclei(2), variations in tropospheric OH concentration can influence cloud albedo. We use a global two- dimensional model forced by observed changes in stratospheric ozone to calculate the consequent changes in production of sulphuric acid over the past decade, and thus to estimate the effect on cloud albedo. We find that this indirect effect of ozone depletion may decrease radiative forcing (via increased cloud reflectivity) by at least as much as the direct effect.


Highly vibrationally excited O-2(X(3) Sigma(g)(-), v greater than or equal to 26) has been observed from the photodissociation of ozone (O-3), and the quantum yield for this reaction has been determined for excitation at 226 nanometers. This observation may help to address the ''ozone deficit'' problem, or why the previously predicted stratospheric O-3 concentration is less than that observed. Recent kinetic studies have suggested that O-2(X(3) Sigma(g)(- ), v greater than or equal to 26) can react rapidly with O-2 to form O-3 + O and have led to speculation that, if produced in the photodissociation of O-3, this species might be involved in resolving the discrepancy. The sequence O-3 + h nu --> O-2(X(3) Sigma(g)(-), v greater than or equal to 26) + O; O-2(X(3) Sigma(g)(-), v greater than or equal to 26) + O-2 --> O-3 + O (where h nu is a photon) would be an autocatalytic mechanism for production of odd oxygen. A two-dimensional atmospheric model has been used to evaluate the importance of this new mechanism. The new mechanism can completely account for the tropical O-3 deficit at an altitude of 43 kilometers, but it does not completely account for the deficit at higher altitudes. The mechanism also provides for isotopic fractionation and may contribute to an explanation for the anomalously high concentration of heavy O-3 in the stratosphere.


Calculations using a photochemical box model from mid-January to early March 1992 show the return from perturbed levels of ClO(x) to near background levels, and the associated rise of ClONO2. The calculated values of ClONO2 in an in-vortex, background aerosol scenario are in good agreement with those observed by the balloon borne MIPAS-B limb sounder. Implications for ozone loss are discussed.


The atmospheric importance of the reaction of ClO with NO3 has been tested in a one-dimensional photochemical model. This reaction has two product channels which are of atmospheric importance. The channel which forms Cl and NO2 can destroy ozone in a catalytic cycle. This cycle constitutes the main ozone loss mechanism during the night near 40 km. The other channel forms OClO and is a significant source of OClO in the upper stratosphere during the night. In the polar lower stratosphere this reaction does not significantly enhance OClO.


We have used a two dimensional radiative-chemical-transport model of the stratosphere to investigate the sensitivity of trace gas distributions to absorption of oxygen in the wavelength region 175-210 nm. Two different formulations for the Herzberg continuum absorption cross sections are used. The calculated transmission of ultra-violet light in the stratosphere is lower and higher than observed, depending on the choice of absorption cross section. For the higher transmission O3, ClO, and HO2 are found to be significantly increased in the lower stratosphere. Calculated O3 in the upper stratosphere, chlorofluorocarbons, N2O and odd-nitrogen are lower. The photolysis of oxygen is considerably faster at high latitudes implying that the photochemical recovery of depleted polar ozone is faster than currently assumed.


Dimethylsulphide (DMS) is the major biogenic sulphur source gas. It is known to be oxidised by OH and NO3 to ultimately yield sulphate particles which are important in acid deposition and are also involved in the atmospheric radiative balance. In the laboratory BrO has also been shown to oxidise DMS. Here I present atmospheric model calculations based on inorganic Bromine measurements and find that oxidation by BrO is a sink for DMS.


Recent analysis of ozone trends in the upper stratosphere suggest that models overestimate the decrease of ozone in the upper stratosphere over the last decade (WMO, 1992). We have included the reaction OH + ClO -> HCl + O2 in a two-dimensional radiative-chemical-transport model and find that this reaction reduces the ClO/HCI ratio to agree better with observations. The calculated trend in the upper stratosphere is lower and agrees well with some observations. The effect of this reaction on the lower stratosphere is small. The reaction OH + ClO -> Cl + HO2) is the main sink for OH in regions of elevated ClO and low ozone such as during the Antarctic Spring.


SPRINGTIME ozone depletion over Antarctica is thought1,2 to be due to catalytic cycles involving chlorine monoxide, which is formed as a result of reactions on the surface of polar stratospheric clouds (PSCs). When the PSCs evaporate, ClO in the polar air can react with NO2 to form the reservoir species ClONO2. High concentrations of ClONO2 can also be found at lower latitudes because of direct transport of polar air or mixing of ClO and NO2 at the edges of the polar vortex. ClONO2 can take part in an ozone-depleting catalytic cycle18, but the significance of this cycle has not been clear. Here we present model simulations of ozone concentrations from March to May both within the Arctic vortex and at a mid-latitude Northern Hemisphere site. We find increasing ozone loss from March to May. The ClONO2 cycle seems to be responsible for a significant proportion of the simulated ozone loss. An important aspect of this cycle is that it is not as limited as the other chlorine cycles to the timing and location of PSCs; it may therefore play an important role in ozone depletion at warm middle latitudes.


RECENT observations suggest that the eruption of Mount Pinatubo in June 1991 has had a considerable effect on ozone concentrations in the tropical stratosphere (refs 1, 2, and J. W. Waters, personal communication). Although stratospheric ozone losses following volcanic eruptions are generally attributed to the presence of sulphate aerosol3-7, we present model calculations which demonstrate that gas-phase sulphur chemistry may have played a part in the tropical ozone perturbations that followed the Pinatubo eruption. We find that in the first month or so after the eruption, the large amount of SO2 injected into the tropical atmosphere catalyses mid- stratospheric ozone production. On the other hand, the SO2 cloud absorbs solar radiation, thereby reducing the rate of O2 photolysis (and hence of ozone production) below it. These two effects cancel each other out at an altitude of about 25 kilometres. After one or two months, most of the SO2 has been oxidized to sulphate; the efficiency of these two mechanisms then becomes negligible (although ozone remains perturbed in the lower stratosphere because of its long photochemical lifetime in this region). The model features show good agreement with initial ozone measurements following the eruption, including both the mid-altitude switch from ozone loss to ozone gain1, and the increase and subsequent decrease in the total ozone column2,7.


HO(x) (OH+HO2) and NO(x) (NO+NO2) radicals play a key role in controlling global ozone. However, several atmospheric measurements of these species are larger than current model calculations suggest. The oxidation of H-2 and N2O by the second electronically excited state of oxygen, O2(b1SIGMA(g)+), is theoretically possible and proposed here as an atmospheric source of these radicals. If only 1% of the quenching of O2(b1SIGMA(g)+) by H-2 and N2O results in the proposed oxidation products (OH and NO+NO2), then model calculations predict a significant production of these radicals in exactly those regions of the atmosphere were current models seem to fail.


The heterogeneous removal of N2O5 by sulphuric acid aerosols as been invoked to explain the decline of mid-latitude ozone in the last decade. We have used a photochemical model to study measurements of odd-nitrogen made by Spacelab 3. The gas-phase photochemical model overestimates the amount of N2O5 present. The loss of N2O5 by aerosols does reduce N2O5, but is likely to be slower than assumed in WMO (1992). The sunset measurements at 25.5 km cannot be explained by heterogeneous loss of N2O5 and is more likely to be due to a faster photolysis than assumed. New absorption cross-sections of HNO3 reduce the photolysis of HNO3 so that the model with gas-phase chemistry only gives better agreement at 19 km, than a model including heterogeneous chemistry.


A critical testing of atmospheric chemical theories requires the simultaneous measurement of many species and often a measurement of a photolysis coefficient. Following on from the work of HARRIES [(1982), Stratospheric composition measurements as tests of photochemical theory. J. atmos. terr. Phys. 44, 591] the required accuracy for these simultaneous measurements has been calculated using a photochemical diurnal model. The steady-state expressions for NO2 and HNO3 are investigated throughout the stratosphere. Calculations show that, if all observable parameters are measured to + 10%. a lest to +/- 20% can be made. Further work has shown the inadequacy of the HNO3 and HCl steady-state assumptions at certain altitudes and solar zenith angles. By calculating a rate ratio, defined as the rate of production divided by the rate of loss of a species, steady- state assumptions can be tested. An evaluation of chemical life-time, which is normally considered, does not allow a quantification of departures from steady-state. We show that the rate ratio is a useful parameter to do this. Using this approach we have verified the HCl steady-state assumption near 40 km at the time and location of the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. Based on this verification a CIO mixing ratio of 0.5 ppbv was derived using ATMOS measurements. This derived value agrees well with other observations.


The possibility of significant autocatalytic ozone production in the middle atmosphere has been demonstrated by Toumi et al. A detailed framework including five processes previously not considered is presented here. When these processes are included, particularly wavelength-dependent vibrational distributions of O2, Calculated ozone enhancements are less than 10% in the upper stratosphere and lower mesosphere. The vibrational distributions and the rate of vibrational quenching are identified as key issues which need to be addressed by experiments.


Recent measurements by Webster et al. [1990] have confirmed quantitatively the chemistry controlling the night-time decay of NO2. Simple equations describing the night-time behaviour of NO2 and N2O5 are presented here. With measurements of the night-time ozone and NO2 concentrations, these equations can be used to predict the amount of N2O5 produced at any time during the night. In this way the N2O5 night-time emission measurement of Roscoe [1982], Kunde et al. [1988] and sunrise measurements of the ATMOS experiment [Rinsland et al., 1989] are all used to test theory. The measurements are found to be both self consistent and confirm our understanding of night- time NO2 conversion to N2O5. The variation of N2O5 by a factor of two between measurements is found to be consistent with theory.


ONE of the main problems in atmospheric chemistry in recent years has been the discrepancy between theoretical model calculations of ozone concentrations and observations in the upper stratosphere and mesosphere 1. It has been suggested 2 that the photolysis of vibrationally excited oxygen could provide an extra source of ozone and new laboratory data now allows us to test this hypothesis in two ways. First, by including the proposed mechanism in a numerical model of the atmosphere, we find that the production of odd oxygen is enhanced and that the calculated ozone concentrations are significantly increased so that they agree well with observations. Second, we present a comparison between satellite observations of the daytime enhancement in the 6.9- mu-m emission from water vapour and theoretical calculations, which yields indirect evidence for the presence of vibrationally excited oxygen in the upper stratosphere and mesosphere; a significant fraction of which appears to be due to relaxation of very high vibrational levels. Both these tests demonstrate that the proposed mechanism may indeed account for most of the discrepancy between model results and observations.