Multiscale WRF Modeling of Meso- to Micro-Scale Flows During Sundowner Events

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.