Predictability and sensitivity of downslope windstorms in San Diego County

The Santa Ana winds represent a high-impact weather event owing to the intimate relationship between the extremely dry, fast winds and the wildfire threat. They are widely recognized as being a gap wind phenomenon, as the airflow is accelerated through passes and canyons. Our previous work with the Weather Research and Forecasting model has demonstrated that the infamous Santa Ana wind event of October 2007, which was responsible for starting and/or spreading more than twenty fires across Southern California, possessed many characteristics of downslope windstorms, especially in San Diego county, and that model horizontal resolution and model physics can have a material effect on the strength, location and timing of Santa Ana winds in real-data simulations.

The present work further explores the sensitivity and predictability of downslope winds in San Diego county by focusing on more recent cases that have occurred since a very high-density surface observation network was installed in the region. We are applying the state of art modeling techniques such as the Ensemble Kalman Filter to gauge the sensitivity of the event to perturbations, model physics and horizontal resolution. Idealized modeling has helped us to understand the influence of subtle changes in environmental conditions upstream of the topography on wind speed and downslope spread, which are areas of significant interest to operational forecasters, energy utilities, and public safety personnel.