High-resolution Spatial and Temporal Observations of Generating Cells and Velocity Waves in Colorado Snowstorms

Generating cells are small-scale (<2 km horizontal and vertical) overturning air circulations within ice-containing layers of storms which locally increase the size and mass of ice crystals. The ice crystals can continue to grow through water vapor deposition or by collecting other particles as they fall through the cloud, potentially leading to increased precipitation at the surface. Instabilities that initiate generating cells can be related to several different sources. Some examples are the motion of atmospheric waves, differences in cooling rates between the cloud interior and the cloud top surface, changes in wind velocity between adjacent layers of air, or the intrusion of low theta-e air into the cloud. Latent heat release within a generating cell is an amplifying feedback. We examine the relationship between generating cells and atmospheric waves in winter storms. We use high-resolution (beamwidth=0.3˚), rapid-update (3-5 minute) cross-sectional radar observations through four storms in Greeley, Colorado observed with the Colorado State University CHILL National Weather Radar facility (CSU-CHILL) during December 2017 to February 2018. Our target storms were cases where surface precipitation fell entirely as snow, storm evolution was relatively slow, and the storms were not associated with strong surface fronts. We also draw on data collected using similar scan strategies during previous CSU-CHILL studies of winter storms. In many cases, we observe that generating cells are observed to grow and move with the peaks of velocity waves. This association indicates waves may have important roles in initiating and maintaining generating cells. We additionally explore what types of waves are observed using reanalysis soundings from the North American Regional Reanalysis and the ECMWF ERA5 datasets. Specifically, we attempt to distinguish between gravity waves that propagate through stable layers and Kelvin-Helmholtz waves formed along shear boundaries that advect with the mean flow.