Polarimetric Radar Based Retrievals of Ice Hydrometeor Shapes and Intercomparisons between Retrievals and in Situ Results

A remote sensing approach to retrieve the degree of non-sphericity of atmospheric ice hydrometeors, which is characterized by the particle aspect ratio, using polarimetric Ka-band radar measurements from a scanning US Department of Energy Atmospheric Radiation Measurement (ARM) program cloud radar (SACR) is introduced. This approach is based on use of circular depolarization ratio (CDR) values which are reconstructed from fully polarimetric SACR measurements. The main advantage of CDR for retrieving particle shapes lies in its weak sensitivity to particle orientations. The suggested approach is applied to a case study conducted at the ARM’s Oliktok Point (Alaska) mobile facility. The results of the radar based retrievals are compared to in situ measurements of particle shapes from the balloon based Video Ice Particle Sampler (VIPS) and the ground based Multi-Angle Snowflake Camera (MASC). The observed ice hydrometeors were predominantly irregular-shaped ice crystals and aggregates, with aspect ratios varying approximately between 0.3 and 0.8. The radar retrievals assume that particle bulk density influencing (besides the particle shape) observed polarimetric variables can be deduced from the estimates of particle characteristic size. With reasonable uncertainties in particle size estimates, the errors of aspect ratio retrievals from radar measurements, were expected to be about 0.15. Given the retrieval errors there was a general agreement between the radar-based estimates of aspect ratios and those observed in situ.