The role of ice particle shape and size distributions in polarimetric radar measurands at 95 GHz

In recent years, millimeter-wave radars have been proven to be a valuable tool for deducing cirrus cloud macrophysical characteristics such as cloud base and top height as well as numbers of layers. To validate and improve cloud parametrisations in general circulation models, also microphysical information is demanded. Particularly the crystal shape is known to have a strong influence on the radiative properties. The CARL project is started as a first step towards the determination of microphysical cloud characteristics from active remote sensing. Within the frame of this project, retrieval algorithms are going to be developed to obtain quantitative microphysical cloud information from the synergy of radar and lidar and/or passive instruments. The potential of such synergy to improve cloud parameter retrieval was demonstrated e.g. by Intrieri et al. (1993). A combination of radar and lidar with passive instruments is intended for spaceborne mission planned by the international space agencies ESA, NASDA and NASA. In the present article the effect of crystal size distribution parameters as well as particle shape and orientation on radar observables at 95 GHz (3.2 mm) is assessed. Pristine ice crystal shapes are characterised by the aspect ratio a, which is the ratio of length to diameter. In the following analysis the aspect ratio is assumed to be size independent. Backscatter intensities have been computed for solid hexagonal columns and plates, hollow cylinders and two types of stellars. A randomly oriented compact column represents a first step towards considering polycrystals at mm-wavelengths, as this shape occasionally gives best agreement with measurement