The effects of aspect ratio and surface roughness on satellite retrievals of ice-cloud properties

This study investigates the effects of non-sphericity on satellite retrievals of ice-cloud properties including optical thickness and particle sizes. Ray-tracing technique is used to calculate single scattering phase function and single scattering albedo for both smooth and rough surfaces of hexagonal columns and plates at visible and near-infrared wavelengths. Two parameters, aspect ratio and distortion parameter, are used to simulate different, randomly oriented, ice crystal shapes and surface roughnesses in the ray-tracing process. A wide range of aspect ratio and distortion parameter is explored in the calculations. The resultant phase functions and single scattering albedos are used to compute bidirectional reflection functions by a radiative transfer model with adding-doubling technique. The results show that in a direct backscattering regima (Theta > 150 degrees), if no information of particle shape is available, the uncertainties in the retrieved optical thickness (a factor of more than ten) would make the retrieval meaningless. For images with viewing geometry outside of this region, the typical range of uncertainty of retrieved optical thickness is less than a factor of about two. Using averaged phase function will cut this uncertainty in half. That is, the uncertainty in the retrieved optical thickness is about 40%. Sensitivity tests show that aspect ratios are critical in reducing the uncertainties of the retrieved optical thickness using satellite data. The uncertainties in the retrieved ice particle sizes are also estimated in a similar way. It is found that using an averaged aspect ratio and roughness, the uncertainty of retrieved particle size is about 30% for small particles and 10% for large particles at lambda = 3.7 mu m. The corresponding uncertainties are about 70% for small particles and 35% for large particles if data at lambda = 1.6 or 2.2 mu m are used in the retrieval. However, at lambda = 1.6 and 2.2 mu m, retrieval of particle size may encounter even larger uncertainties for thin ice-clouds ( tau < 3). (C) 1999 Elsevier Science Ltd. All rights reserved.