Microphysical Impacts on the Relationship between Orographic Rain and Orographic Forcing in the Coastal Mountains of Northern California

Cool-season precipitation in the western United States is strongly modulated by the regions' complex terrain. Narrow corridors of concentrated horizontal water vapor transport within land-falling extratropical cyclones called atmospheric rivers impact the terrain and the associated orographic lift leads to enhanced precipitation. Considerable attention has been given to orographic precipitation enhancement for the larger inland mountain ranges of the west. Smaller terrain along the coastline can also significantly enhance precipitation. In fact, precipitation in these orographic locales unimpeded by upstream topography can lead to severe flooding that incurs millions of dollars in property damage. Previous studies have documented a noticeable dependence of coastal orographic rainfall on orographic forcing. However, this relationship is far from perfect since correlation coefficients for single- or multi-season composites are less than 0.7. Perhaps the largest potential source of uncertainty in the relationship is linked to cloud and precipitation microphysics. A given amount of orographic forcing may lead to different orographic rainfall rates depending on the dominant microphysical process in effect.

The present study addresses these uncertainties through analysis of more than 4000 h of data from profiling Doppler radars, rain gauges and a GPS receiver collected over ten cool seasons in the coastal mountains of northern California. Orographic forcing is documented by hourly upslope flow, integrated water vapor (IWV) and IWV flux derived from a wind profiler and GPS receiver located along the coast at Bodega Bay (BBY, 15 m MSL). Microphysics regime is inferred by examining data from a vertically pointing precipitation profiler in the coastal mountains at Cazadero (CZC, 478 m MSL), which allows designation of hourly periods dominated by two distinct rain types: brightband (BB) rain and nonbrightband (NBB) rain. BB rain is associated with the seeder-feeder process while NBB is associated with the warm-rain process.

The correlation coefficients for CZC rain rate versus BBY upslope flow, IWV and IWV flux are 0.61, 0.29 and 0.59, respectively, for the rain-type composite (BB+NBB rain), which are lower than those for the overall composite independent of microphysics regime (0.63, 0.41 and 0.67, respectively). These differences are attributed to the exclusion of CZC rain rates less than 1 mm h-1 in the rain-type composite. NBB rain is associated with smaller correlation coefficients (0.63, 0.24 and 0.60, respectively) compared to BB rain (0.66, 0.34 and 0.64, respectively). CZC rain rates are larger for BB rain compared to NBB rain for a given amount of orographic forcing.