Simulating Sundowner Winds in Coastal Santa Barbara: Model Validation and Sensitivity

Abstract

This study investigates the influence of planetary boundary layer (PBL) schemes and land surface models (LSMs) on the performance of the Weather Research & Forecasting model in simulating the development of downslope windstorms in the lee of the Santa Ynez Mountains in Santa Barbara, California (known as Sundowner winds). Using surface stations, a vertical wind profiler, and a multi-physics ensemble approach, we found that most of the wind speed biases are controlled by the roughness length z 0 , and so by the choice of LSM. While timing characteristics of Sundowners are insensitive to both LSM and PBL schemes, a clear sensitivity in the horizontal extent of strong surface winds is found for both PBL parameterization and z 0 , which are related to patterns of self-induced wave-breaking near the mountaintop, and the erosion of the marine layer. These results suggest that LSMs with relatively high values of z 0 , and TKE-based or hybrid PBL schemes adequately simulate downslope windstorms in the lee of mountain ranges, specifically in areas where downslope windstorms interact with the marine boundary layer with stably stratified characteristics.

Keywords

• downslope windstorms
• model sensitivity
• PBL schemes
• roughness length
• multi-physics ensemble approach
• WRF