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Abstract Two Sundowner events observed during the Sundowner Wind Experiment (SWEX) project are analyzed using a realistically forced large eddy simulation (LES) employing a multiscale Weather and Research Forecasting (WRF) model configuration with domain grid spacings ranging from 11,250 to 30 m centered over the Santa Barbara, CA region to examine their meso‐ to micro‐scale drivers. The main drivers of both events are increasing mountaintop stability and the mountain wave activity exhibiting a hydraulic jump and near‐surface critical layer. Another important finding is ascent of the downslope flows over the turbulent adiabatic layers at the coastal regions. In both events, the strong downslope flow warms and dries the air descending the southern slopes of the SYM adiabatically generating a deepening adiabatic layer that is 0.4 to as much as 1 km deep during peak Sundowner intensity over the coastal regions. This layer, exhibiting turbulence within and atop, is characterized with the strong downslope flow atop with much weaker, and at times, reversed flow beneath over the coastal regions. This flow structure, along with regions of turbulence within and atop the adiabatic layer, is indicative of a mountain lee‐wave rotor. Coastal locations in both events remain relatively unaffected. Further investigations are needed to determine whether or not this is consistent across all Sundowner events observed during the SWEX project and whether turbulence helps diffuse or accelerate the flows. 

Abstract On 7 February 2020, precipitation within the comma-head region of an extratropical cyclone was sampled remotely and in situ by two research aircraft, providing a vertical cross section of microphysical observations and fine-scale radar measurements. The sampled region was stratified vertically by distinct temperature layers and horizontally into a stratiform region on the west side, and a region of elevated convection on the east side. In the stratiform region, precipitation formed near cloud top as side-plane, polycrystalline, and platelike particles. These habits occurred through cloud depth, implying that the cloud-top region was the primary source of particles. Almost no supercooled water was present. The ice water content within the stratiform region showed an overall increase with depth between the aircraft flight levels, while the total number concentration slightly decreased, consistent with growth by vapor deposition and aggregation. In the convective region, new particle habits were observed within each temperature-defined layer along with detectable amounts of supercooled water, implying that ice particle formation occurred in several layers. Total number concentration decreased from cloud top to the −8°C level, consistent with particle aggregation. At temperatures > −8°C, ice particle concentrations in some regions increased to >100 L −1 , suggesting secondary ice production occurred at lower altitudes. WSR-88D reflectivity composites during the sampling period showed a weak, loosely organized banded feature. The band, evident on earlier flight legs, was consistent with enhanced vertical motion associated with frontogenesis, and at least partial melting of ice particles near the surface. A conceptual model of precipitation growth processes within the comma head is presented. Significance Statement Snowstorms over the northeast United States have major impacts on travel, power availability, and commerce. The processes by which snow forms in winter storms over this region are complex and their snowfall totals are hard to forecast accurately because of a poor understanding of the microphysical processes within the clouds composing the storms. This paper presents a case study from the NASA IMPACTS field campaign that involved two aircraft sampling the storm simultaneously with radars, and probes that measure the microphysical properties within the storm. The paper examines how variations in stability and frontal structure influence the microphysical evolution of ice particles as they fall from cloud top to the surface within the storm.