Tropical Thin Cirrus in AIRS Measurements: Retrieval and Radiative Forcings

We developed an efficient thermal infrared radiative transfer model on the basis of the delta-four-stream approximation to facilitate high-spectralresolution remote sensing applications under cirrus cloudy conditions in the Atmospheric Infrared Sounder (AIRS) data. Multiple scattering contributions from cirrus cloud particles have been found to be important in thermal infrared cloudy spectra, especially when cirrus cloud optical depths are larger than 0.3. We selected a number of nighttime thin cirrus scenes over the Atmospheric Radiation Measurement program's Tropical Western Pacific sites from AIRS datasets and applied this radiative transfer model to the selected cases to determine cirrus optical depth and mean effective ice crystal size and habit factor. For retrieval, we followed an approach using look-up tables constructed from numerous in situ ice particle size and shape measurements over tropical cirrus clouds. Solar and infrared radiative forcings and heating rates produced by thin cirrus in the tropical atmosphere have been analyzed using the retrieved cirrus optical and microphysical properties along with a modified Fu and Liou broadband radiative transfer scheme. Subsequently, we carried out comparison of cirrus retrieval results and computed broadband fluxes to those determined from available groundbased cloud radar and pyrgeometer measurements to check consistency • The fast radiative transfer model for computation of thin cirrus thermal infrared spectral radiances has been improved by incorporating the deltafour-stream (D4S) approximation to account for the multiple scattering contributions of ice clouds. • Ice crystal size and habit distribution models have been constructed. Size and habit effects of ice particles on IR brightness temperature spectra, as well as cloud radiative forcing and heating rates have been presented.