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

Abstract

Along Track Scanning Radiometer-2 (ATSR-2) measurements made over a tropical cirrus anvil (Pacific Ocean, 14° N, 134° E; 6 September 1996) are analysed on the basis of radiative transfer calculations for clouds consisting of imperfect hexagonal ice crystals. Reflectivity measurements made at two wavelengths (0.87 µm, non-absorbing; 1.6 µm, absorbing) and two viewing directions (nadir and forward) are considered. Model calculations for a cloud consisting of single-sized imperfect hexagonal ice crystals (maximum dimension 60 µm) 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 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. Averaged over those measurements for which the optical thickness is larger than 10 we find a crystal size (defined as maximum crystal dimension) of 63 +/- 4 µm for a cloud consisting of imperfect hexagonal columns (crystal sizes for optically thin clouds are omitted because of high retrieval errors). For hexagonal plates a somewhat larger value is retrieved: 71 +/- 3 µm. Both retrieved sizes suggest that the cloud system consisted of relatively small ice crystals.