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

In winter storms, precipitation intensity and distribution is highly variable on the mesoscale. Due to the sparsity of ground-based in situ measurements, a necessary way to monitor precipitation variability in winter storms is through radar observations. In order to accurately interpret them, it is necessary to understand how microphysical processes contribute to the radar measurements in winter storms. Many studies have found enhancements in ground-based radar scans of equivalent reflectivity factor (Z_e), differential reflectivity (Z_DR), and propagational differential phase shift (K_DP) in the -20˚C to -10˚C cloud layer of winter cyclones. This region encompasses the “Dendritic Growth Zone” (DGZ) between -18˚C to -12˚C, which is known for the growth of dendritic ice crystals under supersaturated conditions. As such, studies have attributed the reflectivity intensification observed across the -20˚C to -10˚C cloud layer with the growth of dendrites despite lacking in situ observations to confirm the presence of these particle habits. Consequently, the root microphysical cause(s) of distinct radar features observed by radars in this temperature zone of winter cyclones remain poorly understood. In this presentation, we leverage ground-based radar measurements, airborne radar measurements, and in situ observations made by two aircraft, the NASA ER-2 and P-3, flown in coordination across winter storms over the Northeastern U.S. during the NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. We analyzed 546 vertical profiles of Ku-band Z_e gradient, each averaged across 10 km (~1 minute) segments sampled by radar onboard the ER-2. These profiles were grouped into five clusters based on shape similarity using a k-means clustering algorithm. The clustering results revealed distinct centroids (mean profiles) for each group, with some showing a local maximum in Z_e gradient within the DGZ temperature range, suggesting locally enhanced Z_e and thus particle growth. We focus on two clusters: one exhibiting a local Z_e gradient maximum and the other a constant Z_e gradient near zero within the DGZ. Additionally, we examine corresponding vertical profiles of ground-based S-band Z_e, Z_DR and K_DP. A statistical analysis of key microphysical variables, including liquid water content, particle number concentration, habit type, relative humidity, derived ice water content and mass-weighted mean diameter (D_m) was conducted for each cluster using in situ observations collected by cloud probes onboard the P-3. Our analysis within the DGZ temperature range indicates that the cluster with the local Z_e gradient maximum also had higher concentrations of large particles and enhanced D_m relative to the cluster with no local Z_e gradient maximum. Habit images predominantly revealed side-plane polycrystals in both clusters within the DGZ. These findings suggest that the observed local Z_e gradient maxima, alongside higher concentrations of large particles and enhanced D_m within the DGZ temperature range are mainly attributed to the growth of side-plane polycrystals rather than dendrites.