What are the patterns of winds across Southern California during Santa Ana events and how do they affect fires? (UCLA and UCI)

The UCLA regional climate modeling group (Co-I Hall) will perform a multi-decade (1979-2009) reconstruction of climate in Southern California using the WRF model, with boundary forcing from the North American Regional Reanalysis (Mesinger et al. 2006). WRF is a community mesoscale model being developed collaboratively among several agencies (http://www.wrf-model.org). It consists of a fully compressible nonhydrostatic dynamical core and a full suite of physics and is applied routinely for real-time forecasts at NCAR and elsewhere on grid sizes down to a few kilometers. The 6-km domain will be nested within an 18-km domain covering Southern California and parts of Arizona, Nevada, and Mexico, which is nested within a 54-km domain encompassing most of the western U.S. All major mountain complexes in Southern California will be represented at this resolution. The simulation in the 6-km nest acts as a reconstruction of the local atmospheric conditions based on known large-scale atmospheric conditions. Comparison of the simulation to a network of point measurements demonstrates that the regional modeling approach can reproduce mesoscale and diurnal features of the atmospheric circulation and precipitation climatology in this region of complex topography with a high degree of fidelity (Conil and Hall 2006; Hughes et al. 2007; Hughes et al. 2009b). This regional simulation has also been used to study the mechanisms causing Santa Ana winds (Hughes and Hall, 2009c). Unlike previous studies, we 10 Fig 7. Average winds for days with Santa Ana time series > 8 m s-1. identify Santa Ana wind events by the strength of the offshore wind through the gaps in Southern California’s topography (Fig. 7). We create a Santa Ana time index that is simply the offshore wind strength at the exit of the largest gap, most notably across the Santa Monica mountains. Magnitude of the Santa Ana time index quantifies the intensity of Santa Ana events, and a threshold will be applied to identify the season and length of Santa Ana days. Both local and synoptic conditions control the formation of Santa Ana winds. The higher resolution climate model more accurately simulates small climatic variations due to the complex topography, and thus better simulate the spatial distribution of winds. We will investigate the spatial variability of winds across Southern California during Santa Ana events and how this controls the patterns of the size and occurrence of fires in the past 30 years. We will examine the interannual variations and trend in Santa Ana winds such as the seasonality and number of Santa Ana days and how these correlate temporally with or precondition the Santa Ana fires.