Use of X-Band Differential Reflectivity Measurements to Study Shallow Arctic Mixed-Phase Clouds

Microphysical processes in shallow Arctic precipitation clouds are illustrated using measurements of differential reflectivity Z(DR) from the U.S. Department of Energy Atmospheric Radiation Measurement Program polarimetric X-band radar deployed in Barrow, Alaska. X-band hemispheric range height indicator scans used in conjunction with Ka-band radar and lidar measurements revealed prolonged periods dominated by vapor depositional, riming, and/or aggregation growth. In each case, ice precipitation fell through at least one liquid-cloud layer in a seeder-feeder situation before reaching the surface. A long period of sustained low radar reflectivity Z(H) (Z) and high Z(DR) (6-7.5 dB) throughout the depth of the cloud and subcloud layer, coinciding with observations of large pristine dendrites at the surface, suggests vapor depositional growth of large dendrites at low number concentrations. In contrast, Z(DR) values decreased to 2-3 dB in the mean profile when surface precipitation was dominated by aggregates or rimed dendrites. Small but consistent differences in zenith Ka-band radar Doppler velocity and lidar depolarization measurements were found between aggregation- and riming-dominated periods. The clean Arctic environment can enhance Z(DR) signals relative to more complex midlatitude cases, producing higher values.