Characteristics of ice particles in an Arctic deep mixed-phase cloud observed by the X-SAPR and KAZR at Barrow

Description

Characteristics of graupel and columnar ice crystals in an Arctic deep mixed-phase cloud on 7 December 2013 were identified with observations from the NSA X-SAPR and KAZR at Barrow, in conjunction with scattering calculations. The cloud system produced generating cells and strongly sheared precipitation fall streaks. The X-SAPR observables revealed spatial sorting of polarimetric signatures: negative specific differential phase (KDP) collocated with negative differential reflectivity (ZDR) in the upper half of the fall streak, and near-zero KDP with positive ZDR at the bottom edge of the fall streak. The negative KDP and ZDR, indicating prolate particles with vertically-oriented maximum dimensions, were consistent with small, slow-falling conical graupel coexisting with low concentrations of more isometric graupel. The observed negative KDP values were best matched by scattering calculations for small, dense conical graupel with 30° to 40° cone angles. The positive KDP and ZDR and the KAZR Doppler spectra indicated that large isometric graupel coexisted with a second population of slower-falling rimed plate-like particles in the lower half of the fall streak. About a half-hour later, the KAZR recorded linear depolarization ratio (LDR) values up to -15 dB at temperatures corresponding to the columnar ice crystal growth regime. These LDR values were also consistent with scattering calculations for columnar ice crystals. Doppler velocity spectra of LDR within fall streaks distinguished populations of slower-falling particles with high LDR (> -15 dB) and faster-falling particles with much lower LDR, suggesting that columnar ice crystals with high LDR coexisted with larger isometric particles that produced low LDR while dominating the total co-polar reflectivity, thereby decreasing the total LDR. The measurements suggest that the columnar ice crystals initiated in liquid-cloud layers through secondary ice production (i.e. Hallett-Mossop mechanism).