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

Along Track Scanning Radiometer 2 (ATSR-2) measurements made over a tropica l cirrus anvil are analyzed on the basis of radiative transfer calculations for clouds consisting of imperfect hexagonal ice crystals. Reflectivity me asurements made at two wavelengths (0.87 mu m, nonabsorbing; 1.6 mu m, abso rbing) and two viewing directions (nadir and forward) are considered. Model calculations for a cloud consisting of single-sized imperfect hexagonal ic e crystals adequately explain the gross features of the ATSR-2 reflectivity measurements. Retrieved values of optical thickness and crystal size revea l no discernible relationship between these quantities. Nadir-derived and f orward-derived optical thickness and crystal size are compared for both imp erfect and near-perfect hexagonal crystals. For these two crystal shapes, t here 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 (def ined as maximum crystal dimension) of 63 +/- 4 mu m for a model cloud consi sting of imperfect hexagonal columns. For imperfect hexagonal plates a some what larger value is retrieved: 71 +/- 3 mu m. Both retrieved sizes suggest that the cloud system consisted of relatively small ice crystals.