X-Band Polarimetric and Ka-Band Doppler Spectral Radar Observations of a Graupel-Producing Arctic Mixed-Phase Cloud

Abstract

Characteristics of graupel in an Arctic deep mixed-phase cloud on 7 December 2013 were identified with observations from an X-band scanning polarimetric radar and a Ka-band zenith-pointing radar in conjunction with scattering calculations. The cloud system produced generating cells and strongly sheared precipitation fall streaks. The X-band radar hemispheric RHI observables revealed spatial sorting of polarimetric signatures: decreasing (with increasing range) differential propagation phase shift φDP, negative specific differential phase KDP collocated with negative differential reflectivity ZDR in the upper half of the fall streak, and increasing or near-constant φDP 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°–40° cone angles. The positive KDP and ZDR and the Doppler spectra indicate that large isometric graupel coexisted with a second population of slower-falling rimed platelike particles in the lower half of the fall streak. Through the core of the fall streak, φDP decreased in range while ZDR was slightly positive, indicating that the prolate conical graupel dominated φDP while the isometric larger graupel dominated reflectivity (and thus ZDR). These results demonstrate the capability of polarimetric observables and Doppler spectra to distinguish different growth stages of rimed particles, allowing for the improvement of hydrometeor classification methods.

Observing ice particle growth along fall streaks in mixed-phase clouds using spectral polarimetric radar data

“…In a Doppler spectrum, a separated particle mode of larger and denser ice particles is often a clear indication of a riming process (Kollias et al, 2007;Chandrasekar et al, 2013;Oue et al, 2015a). As described in Figure 3 a) they form a separated particle mode in the Doppler spectrum (left side) that indicates larger particles fall velocities in comparison to already existing particle mode.…”

“…Ice particles grow into large, dense and almost spherical shaped particles. In terms of conical graupel the 10 resulting particles get slightly prolate (Oue et al, 2015a). Because of the sensitivity of the reflectivity to the hydrometeor…”

“…Below 4 km the sZ DR spectrogram 5 and spectra in Figure 12 e) and Figure 13 (right column) show that the larger particles could be prolate. Negative differential reflectivity signatures can be expected for conical graupel presence (Oue et al, 2015a). However, the observations of Oue et al…”

“…The closer ρ HV is to 1 the more homogeneous are the particle shapes of the measured particle population. Therefore, changes of those parameters are used to examine the microphysical evolution of the particle population tracked from cloud top to bottom (Pfitzenmaier et al, 2017;Kalesse et al, 2016;Oue et al, 2015a). The analysis of the fall streak corrected spectrograms are 5 used to investigate the changes in the spectra due to microphysical changes of the tracked ice particle distribution.…”

Observing ice particle growth along fall streaks in mixed-phase clouds using spectral polarimetric radar data

“…Ice particles grow into large, dense and almost spherical shaped particles. In terms of conical graupel the resulting particles get slightly prolate (Oue et al, 2015a). Because of the sensitivity of the reflectivity to the hydrometeor size,…”