Investigating and Forecasting the Erosion of the Marine Layer during Santa Ana Wind Episodes

Santa Ana Winds (SAWs), which are common to Southern California, occur during the non-summer months and can lead to catastrophic wildland fires when the native vegetation is dry. Our study focuses on the spatial-temporal characteristics of SAWs, which can vary significantly with each event, depending on the type and amount of synoptic scale forcing that is present in the atmosphere. Specifically, we want to examine the interaction that occurs between SAWs and the marine layer, and determine the extent and duration of offshore flow along the coast. We have reviewed a number of cases, using the analysis period of the North American Mesoscale Forecast System (NAM) model at a 12 km horizontal resolution, in which we compared and contrasted the marine layer erosion process for different SAW events. Vertical cross sections over Los Angeles County (Palmdale to Santa Monica) of potential temperature were used to reveal the depth and structure of offshore flow and the presence of gravity waves, while 850 hPa temperature advection plots were utilized in detecting areas of subsidence in the vicinity of the Transverse Ranges. Our investigation shows that when the latter is present to a greater degree, SAWs can effectively erode and dislodge the marine layer in an expeditious manner. Through the course of our study, we hope to confirm and improve upon some of the pre-established meteorological forecast methods to better determine the spatial extent of future SAWs. More accurate forecasts in this regard will ultimately help fire agencies pre-position resources more strategically for the preparation of wind-driven fire activity.