Collaborative Research: Understanding the Influence of Turbulent Processes on the Spatiotemporal Variability of Downslope Winds in Coastal Environments

Downslope windstorms in coastal terrain have resulted in many serious wildland fires. The overall conditions leading to those windstorms are well-known, however the details about the timing and severity of the winds are difficult to forecast due to a lack of knowledge about the small-scale mechanisms that drive the most severe winds in complex terrain. This award provides funding for the analysis and numerical modeling of data from the Sundowner Wind Experiment in the coastal Santa Barbara region of California. Sundowner winds are a specific type of downslope windstorm in the Santa Barbara area and they get their name from the tendency of the winds to strengthen into the evening and overnight hours. The research team plans a multi-faceted approach to provide societal benefits from this project, including working with the local National Weather Service office and Santa Barbara County Fire Department. Students will be involved with the project and outreach will be conducted at K-12 and community colleges in California and Virginia. The overarching goal of this project is to advance the understanding of interactions between downslope windstorms and upstream/downstream atmospheric boundary layer characteristics in coastal mountains by combining observations and numerical simulations. The Sundowner Winds Experiment (SWEX) was conducted from April 1 to May 15, 2022, in the coastal topography surrounding Santa Barbara, CA. Sundowner winds are a dry and gusty downslope wind on the southern facing slope of the Santa Ynez Mountains that can be associated with critical fire weather conditions. This project will use SWEX datasets and perform numerical simulations using the Weather Research and Forecasting model (WRF-km, 1.25km grid spacing) and Large Eddy Simulations (LES, 50m grid spacing) to improve the understanding of mechanisms associated with lee slope jets and near-surface wind variability. The research team will also investigate biases in WRF-km simulations. Three main objectives are targeted: 1) Identify mechanisms responsible for the spatiotemporal variability of lee slope wind structure during Sundowners and quiescent periods using SWEX observations. 2) Examine the skill of WRF-km in representing lee slope wind structure and identify relevant processes contributing to model biases. 3) Investigate the performance of LES at 50m grid spacing in representing lee slope wind structure, and the contribution of turbulent processes to biases in simulated surface winds. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.