Cloud top phase characteristics and 3-D structure of a shallow generating cell driven snowstorm: results from IMPACTS

Microphysical measurements within winter storms are often analyzed in the context of two-dimensional radar cross sections from airborne vertically pointing radars. This approach, however, fails to examine the storm’s microphysical characteristics within the context of the storm’s three-dimensional structure. This analysis uses conically scanning X-band radar data to investigate the three-dimensional structure of a shallow generating cell (GC) driven snowstorm (<6 km deep) sampled over Central Illinois and Indiana on 5 February 2020 by the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. Fall streaks emanating from generating cells are evident in the two-dimensional nadir-point radar cross-section as regions of higher reflectivity, approximately 5-10 km wide. This study will focus on comparing the microphysical properties inside and outside of these fall streaks. GCs formed in a potentially unstable layer near the cloud top based on RAP reanalysis, where supercooled water formed and created a droplet-rich environment for ice crystal nucleation, growth, and fallout. This was determined using the Cloud Physis Lidar using a phase identification scheme developed to differentiate between cloud tops dominated by liquid or ice. Vertical motions associated with cloud top generating cells, seen in the radar radial velocity data, allowed for the formation and maintenance of SLW at cloud top, and subsequent microphysical growth. In situ microphysics measurements beneath cloud top allowed for the assessment of particle aspect ratios within and outside of GC fall streaks. When sampled 2-3 km beneath cloud top, fall streaks typically contained larger ice crystals and aggregates compared to the surrounding cloud, increasing reflectivity in plan-view scans. Larger particles within these streaks were aggregates with higher aspect ratios compared to the surrounding cloud. The southern end of the storm lacked GCs, was supercooled, and contained small, low-aspect-ratio ice crystals in high concentrations (>75 L^-1), indicative of secondary ice production. Observations showed GC fall streaks merging into a linear region of higher reflectivity within plan-view scans 2-3 km beneath cloud top, suggesting that plumes of larger ice crystals falling from GCs may merge to form mesoscale semi-linear regions of more intense snowfall, often called snowbands.