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 retrieva ls 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 hexagon al columns and plates at visible and near-infrared wavelengths. Two paramet ers, aspect ratio and distortion parameter, are used to simulate different, randomly oriented, ice crystal shapes and surface roughnesses in the ray-t racing process. A wide range of aspect ratio and distortion parameter is ex plored in the calculations. The resultant phase functions and single scatte ring albedos are used to compute bidirectional reflection functions by a ra diative transfer model with adding-doubling technique. The results show tha t in a direct backscattering regima (Theta > 150 degrees), if no informatio n of particle shape is available, the uncertainties in the retrieved optica l thickness (a factor of more than ten) would make the retrieval meaningles s. For images with viewing geometry outside of this region, the typical ran ge of uncertainty of retrieved optical thickness is less than a factor of a bout two. Using averaged phase function will cut this uncertainty in half. That is, the uncertainty in the retrieved optical thickness is about 40%. S ensitivity tests show that aspect ratios are critical in reducing the uncer tainties of the retrieved optical thickness using satellite data. The uncer tainties in the retrieved ice particle sizes are also estimated in a simila r way. It is found that using an averaged aspect ratio and roughness, the u ncertainty of retrieved particle size is about 30% for small particles and 10% for large particles at lambda = 3.7 mu m. The corresponding uncertainti es 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 uncer tainties for thin ice-clouds ( tau < 3). (C) 1999 Elsevier Science Ltd. All rights reserved.