AbstractA remote sensing approach to retrieve the degree of non-sphericity of ice hydrometeors using scanning polarimetric Ka-band radar measurements from a US Department of Energy Atmospheric Radiation Measurement (ARM) program cloud radar operated in an alternate transmission – simultaneous receiving mode is introduced. Non-sphericity is characterized by aspect ratios representing the ratios of particle minor-to-major dimensions. The approach is based on use of circular depolarization ratio (CDR) proxy which is reconstructed from differential reflectivity ZDR and copolar correlation coefficient ρhv linear polarization measurements. Essentially combining information contained in ZDR and ρhv, CDR-based retrievals of aspect ratios are fairly insensitive to hydrometeor orientation if measurements are performed at elevation angles of around 40°-50°. The suggested approach is applied to data collected using the third ARM mobile facility (AMF-3), deployed to Oliktok Point (Alaska). Aspect ratio retrievals were also performed using ZDR measurements, which are more strongly (compared to CDR) influenced by hydrometeor orientation. The results of radar-based retrievals are compared with in situ measurements from the tethered-balloon based Video Ice Particle Sampler and the ground-based Multi-Angle Snowflake Camera. The observed ice hydrometeors were predominantly irregular-shaped ice crystals and aggregates, with aspect ratios varying between approximately 0.3 and 0.8. The retrievals assume that particle bulk density influencing (besides the particle shape) observed polarimetric variables can be deduced from the estimates of particle characteristic size. Uncertainties of CDR based aspect ratio retrievals are estimated at about 0.1 - 0.15. Given these uncertainties, radar-based retrievals generally agreed with in situ measurements. Advantages of using the CDR proxy compared to linear depolarization ratio are discussed.
Journal of Atmospheric and Oceanic Technology – American Meteorological Society
Published: Sep 29, 2017
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