Cirrus optical thickness and crystal size retrieval from ATSR-2 data using phase functions of imperfect hexagonal ice crystals

Along Track Scanning Radiometer 2 (ATSR-2) measurements made over a tropical cirrus anvil are analyzed on the basis of radiative transfer calculations for clouds consisting of imperfect hexagonal ice crystals. Reflectivity measurements made at two wavelengths (0.87 mu m, nonabsorbing; 1.6 mu m, absorbing) and two viewing directions (nadir and forward) are considered. Model calculations for a cloud consisting of single-sized imperfect hexagonal ice crystals adequately explain the gross features of the ATSR-2 reflectivity measurements. Retrieved values of optical thickness and crystal size reveal no discernible relationship between these quantities. Nadir-derived and forward-derived optical thickness and crystal size are compared for both imperfect and near-perfect hexagonal crystals. For these two crystal shapes, there appears to be a moderate trade-off in consistent retrieval of crystal size versus retrieval of optical thickness. Consistent retrieval of crystal size is found for imperfect crystals. We find an average crystal size (defined as maximum crystal dimension) of 63 +/- 4 mu m for a model cloud consisting of imperfect hexagonal columns. For imperfect hexagonal plates a somewhat larger value is retrieved: 71 +/- 3 mu m. Both retrieved sizes suggest that the cloud system consisted of relatively small ice crystals.