Supported by 
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