Polarimetric Observations of Snow Growth in Stratiform Clouds

While extensive research has been done on the many polarimetric signatures detected during severe convective events, there continues to be a lack of understanding of ice crystal habits, especially in stratiform clouds. Dual-polarization has provided the opportunity to observe signatures in polarimetric variables related to ice crystal habit, microphysical processes, growth mechanisms, and precipitation rate. With the completion of upgrading WSR-88D radars to dual-polarization, on an operational sense, understanding these signatures has become even more paramount. Using the McGill dual-polarization S-band radar and vertically pointing X-band radar, signatures of ice crystal growth were observed above the bright band in stratiform precipitation cases. Cases were chosen from a region 20 km to the northwest of the McGill radar site between 2011 and 2012. During this period, local maxima in Kdp and Zdr near -15°C were observed where dendritic growth is expected in stratiform clouds. Kennedy and Rutledge (2011) and Bechini et al. (2012) have also documented these patterns. Below this region, they also observed decreases in Kdp and Zdr expected to coincide with aggregation of dendrites. We observed these same signatures of Kdp and Zdr but during periods with fall speeds indicative of riming. During periods with fall speeds slower than 1.5 m/s, Kdp signatures instead remained relatively constant with height below the -15°C isotherm. Together, these signatures imply different growth mechanisms may be dominant.