Snow generating cells and fallstreaks observed during FROST

High-resolution X-band polarimetric radar observations of winter storms in northern Colorado during the Winter 2013 Front Range Orographic Storms (FROST) project reveal intriguing finescale structures within snow bands. These include ~1-km-scale generating cells observed at cloud top and attendant fall streaks produced by precipitating snow in the presence of vertical wind shear.

The generating cells have the appearance of small, upright convective turrets in Z_H observations, and are sometimes enshrouded by enhanced Z_DR indicative of pristine anisotropic crystals. Radial convergence observed in the Doppler velocities strongly suggests upward vertical motion within these generating cells. The evolution and life cycle of these generating cells are captured in rapid-scan (~70-second updates) RHI data. The fallstreaks descending from the generating cells often exhibit the microphysical evolution of the snow, including rapid anisotropic growth layers of enhanced Z_DR and K_DP as well as aggregation and riming.

The appearance of these signatures in numerous winter storms is documented, and their characteristic spatial and temporal scales are determined. A microphysical interpretation is provided. Ancillary information from vertically pointing Doppler radar and surface measurements is incorporated when available.