Radar and Microphysical Properties in the “Dendritic Growth” Layer of Winter Storms: Findings from the IMPACTS Field Campaign

Radar scans of winter storms frequently show enhancements in equivalent radar reflectivity factor (Z e ) with in the -18°C to -12°C cloud layer, often referred to as the “Dendritic Growth Layer” (DGL). However, the microphysical structures responsible for these radar signatures remain poorly understood due to limited in-cloud in situ validation. This study leverages coordinated airborne radar and in situ observations collected during the NASA Investigation of Microphysics and Precipitation in Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. We analyzed 581 vertical profiles of Ku-band Z e gradient (dZ Ku /dz), each averaged over 10 km (∼1 minute) segments and grouped into five clusters using a k-means clustering algorithm. Two clusters exhibited local maxima in the magnitude of dZ Ku /dz ≥ 10 dBZ e km −1 and corresponding increases in Ku-Ka dual-frequency ratio (DFR) ≥ 1.5 dB with in the DGL. Coincident in situ observations from one of these clusters revealed larger particle sizes and the presence of aggregates across the DGL. However, habit analyses from three independent imaging probes showed that pristine dendrites were rare across all clusters, comprising only a small fraction of observed particles. Instead, complex polycrystals, particularly side planes and polycrystalline plates, dominated the habit population in the DGL, with large aggregates prevalent only in the cluster exhibiting enhanced radar signatures. Thermodynamic observations showed that ice supersaturation largely remained below the criterion for dendritic growth. We hypothesize that the aggregation of predominately non-dendritic polycrystals produced the observed radar enhancements in the DGL, with only a minor contribution from dendrites.