Using the PSU/NCAR MM5 model initialized with ETA-model reanalysis data, the climate of Southern California was simulated from May 1995 to present at a 6-km grid space resolution. Because of the regions' intricate coastline and various mountain ranges, Southern California’s climate patterns tend to be complex with numerous small-scale variations. We focus on how topography interacts with large-scale forcings to shape the climatological precipitation patterns in Southern California.

Scatterplots of monthly-mean precipitation vs. elevation show that high altitudes generally receive more precipitation than low altitudes. This effect is especially pronounced on the coastal side of the major mountain ranges in Southern California, prompting us to divide our domain into a coastal zone and an inland zone. For an equal increase in elevation, the coastal zone experiences a precipitation enhancement that is one-third larger than that received at inland locations. In both zones, this orographic enhancement is only evident during the wet season (October through May), but is most significant in February. The coastal zone experiences enhanced precipitation during the wet season at all elevations, including the low-elevation coastline locations. Because orographic enhancement is minimal at the coastline, the presence of a wet season at such locales indicates that large-scale forcing produces some portion of the coastal zone precipitation. In contrast, a wet season only becomes apparent with increasing elevation at inland locations, as low-elevation locations receive little or no precipitation throughout the year. This finding suggests that inland precipitation is produced almost exclusively through orographic forcing; however, the extent to which this forcing occurs is still governed by synoptic-scale activity.