The 3D Structure of a Shallow Generating Cell Driven Snowstorm over the Midwest and Its Microphysical Characteristics Determined Using Particle Aspect Ratio: A Case Study from IMPACTS

Abstract Microphysical measurements within winter storms are commonly analyzed using two-dimensional radar cross sections from airborne vertically pointing radars or ground-based scanning radars. While these radars offer valuable insights, they provide limited insights into the storm’s microphysical characteristics within the context of the storm’s three-dimensional structure. To address this limitation, 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 25 February 2020 during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. The observed microphysical properties and reflectivity structures along the nadir-pointing radar cross section represent the superposition of 3D trajectories of fall streaks originating in GCs upwind of the aircraft’s flight track. GCs formed in a potentially unstable layer near cloud top based on HRRR analysis, where supercooled water formed and created a droplet-rich environment for ice crystal nucleation, growth, and fallout. 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 below cloud top, fall streaks typically contained larger ice crystals and aggregates with higher aspect ratios compared to the surrounding cloud, and increased reflectivity in nadir and plan-view scans. The southern end of the storm lacked GCs, was supercooled, and contained smaller, low-aspect-ratio ice crystals in high concentrations. Significance Statement Microphysical characteristics of winter storms are typically analyzed using ground-based plan-position indicator scans from operational radars or two-dimensional radar cross sections from research airborne vertically pointing radars or ground-based scanning radars. While informative, these approaches offer limited insight into a storm’s observed microphysical properties within the context of its three-dimensional structure. To address this limitation, this study examines the three-dimensional structure of a shallow snowstorm driven by cloud-top generating cells using airborne scanning radar data. The analysis reveals that generating cell fall streaks, not evident in the vertical two-dimensional cross section, precipitate through the cross section impacting the observed microphysical characteristics.