Science with the Tropospheric Airborne Fourier Transform Spectrometer
TAFTS, developed and deployed by Imperial College, is a novel and unique far infra-red (FIR) spectrometer, the only high resolution FIR spectrometer capable of airborne measurements of up- and down-welling spectral radiances.
Research Aims:TAFTS was developed to improve understanding of the influence of the far-IR spectral region on climate. It is able to derive the spectrally-resolved net flux at different altitudes in Earth's atmosphere and obtain the far-IR radiative heating rates , a parameter of great importance to understanding of the energy balance of the Earth's climate. TAFTS far-IR measurements also vitally aid understanding of clear-sky and cloud-radiation interaction processes, and our research has focussed on this in recent years.
Earth's radiative balance and the importance of the Far Infrared:
The Earth is maintained in energy equilibrium or steady state by the balance of the Earth's thermal blackbody radiation that is emitted in the infrared to space with the incoming solar energy.
The structure of the infrared spectrum emitted,in addition to the underlying thermal emission Planck curve, is dominated by the major absorption bands and distribution of the main greenhouse gases (eg CO2, water etc), and leads to observed spectral bands originating from different altitudes in the atmosphere and/or surface. The mid infrared (3-15 μm) component of the Earth's atmospheric radiation to space is well understood and studied. However the far infrared contribution (wavelengths longer than 15μm) is relatively under- observed and much remains to be understood. This is shocking because the far-IR accounts for up to 40% of the total infrared emission.
The Earth seen from space could credibly be called a "far infrared planet". The effective radiative temperature of the Earth is about 258 K, meaning that the peak of its black body Planck function at this temperature is around 20μm (500 cm-1). If one considers a tropical surface at 300 K, the radiation peaks at about 17μm (588 cm-1), and for a typical tropopause (boundary between the troposphere and stratosphere) at 200 K the emission peak is below 25μm (400 cm-1).
The major far-IR greenhouse gas is water vapour, with its strong rotation band from 100 μm (100 cm-1) to 16 μm (600cm-1), peaking in absorbance about 40μm (250 cm-1).
The logarithmic drop in concentration of water vapour with altitude in the atmosphere combined with the varied strength of the water vapour rotation band at different wavelengths controls the cooling effect of this water vapour band at different altitudes in the atmosphere.
Atmospheric Cooling rate diagrams, like the one above, show that the cooling to space from the troposphere from the water vapour bands in the far IR is highly important in understanding the Earth's radiation balance.
The problem is that the global distribution of mid- to upper tropospheric water vapour and the water vapour spectral properties in the far infrared are poorly known, there are large uncertainties over the precise values. It is vitally important to make accurate measurements of the emission to space from levels within the troposphere together with in-situ measurements of the water vapour distribution at different altitudes to measure the cooling to space. The novel TAFTS instrument is designed for these measurements.
Our Research Highlights include:
The first comparison of tropospheric in-situ far-IR measurements with models from EAQUATE data (European AQUA Thermodynamic Experiment):
TAFTS made in-situ clear sky measurements flying onboard FAAM (UK Met Office/NERC Facility for Airborne Atmospheric Measurements). The TAFTS EAQUATE clear sky study [Cox et al 2007] represents the first ever comparison of high resolution in-situ measured Far IR radiances against equivalent model simulations utilising ancillary atmospheric sampling of the campaign. The variability of the atmospheric water vapour profile below the aircraft was shown to be clearly seen in the TAFTS FIR and model radiances.
The first in-situ measurement of the water vapour continuum across the entire IR, and validation of continuum models. Part of the CAVIAR(Continuum Absorption at Visible and Infrared wavelengths and its Atmospheric Relevance):
TAFTS measured clear sky far IR radiances in campaigns onboard FAAM over the UK and Swiss Alps, part of the CAVIAR project. The analysis led to the first validation of the water vapour foreign broadened continuum parameterization (WVFBCP), used internationally in radiative transfer and atmospheric modelling codes, across the entire IR [Green et al 2012].
The highest resolution ground based measurements of Far IR downwelling radiances in the arctic. Part of the RHUBC (Radiative Heating in Under-explored Bands Campaign):
TAFTS made the highest resolution FIR ground based measurements of downwelling radiances to date. Independent validation of the water vapour foreign broadened continuum models using this arctic dataset is underway.
The first in-situ high resolution measurements of the far IR radiative effects of cirrus.
TAFTS participated in WINTEX (Winter Experiment) and CAESAR (Cirrus and Anvils: European Satellite and Airborne Radiation), resulting in the first ever comparison between simulated and measured coincident high resolution FIR and mid infrared (MIR) spectra measured in the presence of cirrus [Cox et al 2010]. We are building on these results in our new CIRCCREXcampaign, to understand the role of cirrus and its affect on the Earth Radiation Budget (ERB).
Web Page updated 21st October 2013 by J C Pickering