Estimating Raindrop Size Distributions and Vertical Air Motions using S-band and KAZR Vertically Pointing Radars

Description

Vertically pointing Doppler radars measure the vertical motion of raindrops in precipitating cloud systems that pass directly overhead. The Doppler motion is the sum of two components: motion of falling raindrops and their motion due to updrafts or downdrafts of the surrounding air. It is difficult to separate these two motions using a single vertically pointing Doppler radar. But, by using two Doppler radars with different backscattering and extinction sensitivities to raindrops, it is possible to separate the two contributions in the vertical column from near the surface to just under the melting layer. This study demonstrates the retrieval method using measured reflectivity and Doppler velocity from vertically pointing 2.8 GHz (S-band) and 35-GHz (Ka-band, KAZR) radars deployed during MC3E. The four independent measurements plus Ka-band attenuation are enough pieces of information to estimate four unknowns: raindrop size distribution (DSD) (Nw, Dm, mu) and vertical air motion W. The retrieval has three key elements. First, the difference in Doppler velocity at the two frequencies is a function of raindrop size distribution (DSD) shape and is independent of vertical air motion, radar calibration and attenuation. Second, the difference in reflectivity is also a function of DSD shape plus the two-way path integrated attenuation down to the radar. And third, after determining the DSD profile, the vertical air motion is the difference between the observed and DSD estimated Doppler velocities. While the retrieval method was developed using S-band and KAZR observations, the same retrieval method can be applied to UAZR and KAZR observations. This will allow DSD and vertical air motion profiles during precipitation to be estimated using the 3+ years of UAZR and KAZR observations already collected at the SGP site.