Ice/Snow Microphysics

Snow clouds, which are commonly produce a variety of ice particle types such as pristine ice crystals, aggregates, graupel, and ice pellets in the same cloudy volume, is challenging research. It is impossible to separate and quantify their relative amounts if a single radar variable such as radar reflectivity is utilized. Doppler spectra measured by profiling millimeter-wavelength radars offer the ability to identify and separate the contributions of different hydrometeor species to the radar returns and perform microphysical and dynamical retrievals. Polarimetric observables from cloud and precipitation radars offer the capability to identify ice hydrometeor species as well as their spatial distributions. These observations at different radar wavelengths utilize the frequency-dependent attenuation and backscattering properties of hydrometeors. We illustrated a new approach to qualitative and quantitative analysis of the complexity of ice and mixed-phase microphysical processes in Arctic deep precipitating systems using the combination of Ka-band zenith-pointing radar Doppler spectra and polarimetric radar variables measured by a Ka/W-band scanning radar.

Frequent occurrences of the multimodal Doppler spectra were observed in dendritic/planar ice growth layers and mixed-phase layers, indicating that slower- and faster-falling different types of ice particles coexisted in each layer. They had different contributions to the radar polarimetry: differential reflectivity (Z_DR, representing aspect ratio of ice particles) and specific differential phase (K_DP, proportional to aspect ratio of ice particles and water content). In the dendritic/planar growth layers, the slower-falling particle populations contributed to an increase of Z_DR, while an enhanced K_DP was caused by both the slower and faster-falling particle populations (A–A’ and region A in Fig. 1).  In the mixed-phase layers, particle fall speeds were higher than those in the dendritic growth layer, and the enhancements of K_DP and Z_DR were weak, suggesting that compact, high density, spherical particles produced by riming dominated and those with different sizes were mixed together (B–B’ and region B in Fig. 1).

Oue, M., P. Kollias, A. Ryzhkov, and E. P. Luke, 2018: Towards exploring the synergy between cloud radar polarimetry and Doppler spectral analysis in deep cold precipitating systems in the Arctic. Journal of Geophysical Research, 123, DOI: 10.1002/2017JD027717.

Figure 1: (Left) Example of KAZR Doppler velocity spectra showing a profile and single spectrum along A-A’ and B-B’. Color shade in the profile plot represents reflectivity. Horizontal gray dashed lines represent heights of temperature of -10°C, -15°C, and -20°C. (Right) Quasi Vertical Profiles (QVPs) of K_DP and Z_DR estimated from a Ka (black lines) and W (gray lines) SACR2 plan position indicator (PPI) scan at an elevation angle of 20°. Adapted from Oue et al. (2018)