On rainfall distribution in areas of complex topography

Using a 6km resolution atmospheric simulation of Southern California from 1995 to the present, we develop an understanding of the degree to which and under which conditions orographic blocking affects the precipitation climatology. To diagnose whether blocking occurs, we categorize precipitating hours by a bulk Froude number. We find that blocked cases occur frequently in Southern California, and significantly contribute to the climatological precipitation, especially at low elevation areas. We compare the precipitation distribution for different ranges of Froude number to that predicted by a simple linear model of precipitation, and find that the bulk Froude number can be used to diagnose the extent of blocking that occurs. Our results suggest that orographic blocking contributes to a substantial percentage of the precipitation that occurs in low elevation areas adjacent to mountains. Further, in areas where blocking occurs frequently, to correctly predict the distribution of precipitation, rather than simple linear models, more complicated, possibly full dynamical, models will be necessary.