Interpretation of AIRS data in thin cirrus atmospheres based on a fast radiative transfer model and ARM data

A thin cirrus cloud thermal infrared radiative transfer model has been developed to interpret AIRS spectral data in clear and cirrus cloudy conditions over the ARM TWP sites. This radiation model was constructed by combining the operational OPTRAN program, developed for the calculation of transmittances in clear atmospheres, and a thin cirrus cloud parameterization using a number of observed ice crystal size and shape distributions. Numerical simulations show that cirrus cloudy radiances in the 800-1130 cm-1 thermal IR window are sufficiently sensitive to variations in cloud optical depth and ice crystal size and habit. A number of clear and thin cirrus cloudy nighttime scenes over the ARM TWP sites of Manus and Nauru Islands and the surrounding oceans have been chosen on the basis of the method developed by Kahn et al. (2005) for this study. A χ²-minimization program was employed to infer the cloud optical depth and ice crystal size and shape from the observed AIRS spectra in the thermal IR window. The AIRS-inferred cloud parameters were compared to those determined from collocated ground-based millimeter-wave cloud radar and micropulse lidar (when available) measurements, as independent validation. It is shown that AIRS spectra in the 10 µm window contain sufficient information on cloud optical depth and ice crystal size and it appears that some aspect of ice crystal shape factor may also be inferred by selecting proper models a priori. Finally, we discuss the incorporation of AIRS spectral data for cirrus cloud parameterization improvement in climate models, employing the UCLA atmospheric GCM as a proto type.