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

Abstract

Radar scans of winter storms frequently show enhancements in equivalent radar reflectivity factor (Ze) within the -18˚C to -12˚C cloud layer, often referred to as the “Dendritic Growth Layer” (DGL). However, the microphysical processes 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 over the Northeastern U.S. We analyzed 591 vertical profiles of Ku-band Ze gradient (dZku/dz), each averaged over 10 km (~1 minute) segments and grouped into five clusters using a k-means clustering algorithm. Two of the five clusters exhibited local maxima in the magnitude of dZku/dz ≥ 10 dBZe/km and corresponding increases in Ku-Ka dual-frequency ratio (DFR) ≥ 1.5 dB within the DGL. Coincident in situ observations revealed enhanced particle growth with temperature for these clusters. However, habit imagery and relative humidity measurements showed that dendrites were largely absent and that the RHw supersaturation criterion for dendritic growth was unmet across all clusters. Instead, side planes and polycrystalline plates dominated with RHw subsaturated in the DGL. These findings suggest that the growth of plate-like polycrystals, not dendrites, was primarily responsible for observed radar enhancements in these IMPACTS cases, offering new insight into the microphysical drivers of radar signatures in winter storms.