This thesis describes spectrally-resolved radiance measurements, recorded using
the Imperial College-built Tropospheric Airborne Fourier Transform Spectrometer (TAFTS), to validate the current parameterisation of the far-infrared foreign-
broadened continuum of atmospheric water vapour in line-by-line radiative transfer
models. The current parameterisation is not based on measurement through the
centre of the far-IR spectral region, but is constructed by interpolation from measurements either side of the band. Accurate knowledge of this parameterisation is
essential as the far-IR contribution to the planetary outgoing longwave radiation
(OLR) is significant.
The research takes the form of a radiative closure study, comparing the radiance
measurements with simulated radiances modelled using the measured atmospheric
state (temperature, humidity) profile as input. The profile information was recorded,
alongside the radiance measurements, aboard the UK Met Office / FAAM (Facility
for Airborne Atmospheric Measurements) BAe-146 research aircraft, during the first
of two field campaigns as part of the CAVIAR (Continuum Absorption in the Visible
and Infrared and its Atmospheric Relevance) project.
This thesis provides an overview of the relevant atmospheric physics and Fourier
Transform Spectroscopy, including a description of TAFTS; the construction of the
atmospheric profiles from the various in-situ, radiosonde and model datasets; the
calibration of the radiances measured by TAFTS and derivation of the foreign-
broadened water vapour continuum between 114−540 cm−1 within the framework of
the line-by-line radiative transfer model (LBLRTM v12.0). The resulting continuum
is compared to other relevant recent studies. The measurements from this study
suggest an increase in continuum strength up to 11% at 238.6 cm−1, decreasing
steadily to a reduction of 55% at 114 cm−1 and 54% at 513 cm−1. However, the
uncertainties in these measurements are considerable, and might equally suggest an
increase in continuum strength.