Synergy Between Cloud Radar Polarimetry and Doppler Spectra in Arctic Ice and Mixed-Phase Clouds

Arctic clouds often include mixed-phase layers, which produce a variety of ice particle types in the same cloudy volume. Clarifying how much and what types of ice particles are included in the volume is important to understand the complex ice microphysical processes over the Arctic. This study provides some important insights and methods on how to analyze mixed ice particles in Arctic clouds using the US Department of Energy Atmospheric Radiation Measurement (ARM) Ka-band zenith pointing radar (KAZR) Doppler spectra and the ARM Ka/W band scanning cloud radar (Ka/W SACR2) polarimetric quasi-vertical profiles (QVPs) from PPI measurements during ice precipitation events on April 29 and May 24, 2016.

The Doppler spectra showed frequent multimodality in a dendritic growth layer (DGL, around temperature of -15°C) and in a mixed-phase region below DGL. In DGL, gradients of the mean Doppler velocity with reflectivity dV/d(dBZ) were 0.4-1.4 cms^-1dB^-1 for fast- and slow-falling subpeaks within multimodal spectra. The small gradients indicate that even large reflectivity particles had slower fall speeds, suggesting low density ice particles. Enhanced Kdp coexisting with low Zdr in the DGL suggests that there were more than one type of ice particles composed the multimodal spectra; faster-falling, larger particles with quasi-isotropic shapes (i.e., typical for aggregates) produced low Zdr, whereas slower-falling, smaller particles with strongly anisotropic shapes produced high Zdr and low Kdp. The magnitude of dV/d(dBZ) in the mixed-phase layer has larger gradient of ~2 cms^-1dB^-1 for both slow- and fast-falling subpeaks. The QVPs showed low Zdr (<1 dB) and Kdp (near 0.0° km^-1). These measurements suggest that both fast- and slow-falling particles were compact, high density, and isotropic. These particles could be produced at an effective growth by riming in the mixed-phase layer.

To illustrate how particles contribute to the Zdr in the multimodal Doppler spectra cases, the ratio of radar reflectivities from the slow-falling peaks to from fast-falling peaks versus Zdr was examined. In the dendrite growth, Zdr tended to increase when reflectivities from slower-falling peaks approached that from faster-falling peaks, whereas in the mixed-phase layer Zdr values were lower regardless of the reflectivity ratio. The presented study introduces a new approach in analyzing ice and mixed-phase microphysics using the combination of radar Doppler spectra and polarimetric observations.