Observing and modelling the impact of arctic and tropical cirrus clouds on far-infrared radiance spectra

• Main Author: Humpage, Neil
• Other Authors: Harries, John, Natural Environment Research Council
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Imperial College London 2010
• Subjects: Arctic; Barrow; Alaska
• Online Access: http://hdl.handle.net/10044/1/5652; https://doi.org/10.25560/5652

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.