Microphysical insights into Ice pellet formation revealed by fully polarimetric Ka-band doppler radar

Fully polarimetric scanning and vertically pointing Doppler spectral data from the state-of-the-art Stony Brook University Ka-band Scanning Polarimetric Radar (KASPR) are analyzed for a long-duration case of ice pellets over central Long Island in New York from 12 February 2019. Throughout the period of ice pellets, a classic refreezing signature was present, consisting of a secondary enhancement of differential reflectivity Z_DR beneath the melting layer within a region of decreasing reflectivity factor at horizontal polarization Z_H and reduced copolar correlation coefficient r_hv . The KASPR radar data allow for evaluation of previously proposed hypotheses to explain the refreezing signature. It is found that, upon entering a layer of locally generated columnar ice crystals and undergoing contact nucleation, smaller raindrops prefer-entially refreeze into ice pellets prior to the complete freezing of larger drops. Refreezing particles exhibit deformations in shape during freezing, leading to reduced r_hv, reduced co-to-cross-polar correlation coefficient r_xh, and enhanced linear depolarization ratio, but these shape changes do not explain the Z_DR signature. The presence of columnar ice crystals, though apparently crucial for instigating the refreezing process, does not contribute enough backscattered power to affect the Z_DR signature, either.