The work described in this thesis concerns the effect of cirrus clouds on
far-infrared (FIR) radiance spectra. Though the importance of both FIR radiation and cirrus clouds to the Earth’s energy budget is well recognised, few
high spectral resolution measurements have been made at FIR wavelengths
to date. Observations taken during two diverse field campaigns, along with
spectra simulated using a radiative transfer model, are used here to investi-
gate the FIR signature of cirrus. The FIR observations presented are made
using the TAFTS spectrometer, which measures spectral radiances from ei-
ther an aircraft or the ground.
The deployment of TAFTS during the RHUBC campaign based in Barrow, Alaska is described. TAFTS was used to make ground-based FIR observations of the arctic atmosphere, both with and without cirrus. Comparing
these with modelled spectra, which assume a parameterised particle size distribution (PSD) when describing the cirrus microphysics, suggested that the
PSD parameterisation underestimates the fraction of ice water content contributed by small ice crystals. This conclusion is corroborated by AERI-ER
observations made simultaneously at the Barrow site during RHUBC.
TAFTS observations of convective tropical cirrus made during EMERALD-
II near Darwin, Australia are also presented here. During EMERALD-II
TAFTS was deployed on an aircraft, enabling spectral measurements of cirrus at wavenumbers between 100 and 200cm−1 to be made for the first time.
Comparisons with LBLDIS spectra calculated using PSDs measured using
cloud probes indicate that the number of small crystals measured may be
too high by a factor of three. This result is in agreement with previous
studies suggesting that small crystal populations are over-counted by in-situ
cloud probes, due to shattering of larger crystals on the probe inlets. The
results from both campaigns illustrate the sensitivity of FIR radiances to
cirrus properties, with particular emphasis on the effect of small ice crystals.