S-band radar observations of orographic rain during CALJET

During the California Land Falling Jets Experiment (CALJET), scientists from the NOAA Environmental Technology Laboratory deployed a vertically pointing S-band radar at a site (510 m MSL) near the crest of the coastal mountains northwest of Cazadero, California. The extended dynamic range (~100 dB) of this radar makes it an ideal tool to study the wide range of reflectivity values associated with warm and cold rain processes that occur in mid-latitude storms. In addition, the site received nearly 100 inches of rainfall during CALJET (Dec..-Mar. 1997-1998), a significant portion of which was due to orographic enhancement. A popular theory used to explain orographic enhancement is based on the seeder-feeder mechanism. The concept involves a seeder cloud associated with synoptic-scale ascent overlying a feeder cloud produced locally by upslope flow over the terrain. Rainfall enhancement occurs when precipitation elements from the seeder cloud pass through the moisture rich feeder cloud.

In this study, profiles of vertically velocity and radar reflectivity from several storms are analyzed to describe a shallow, warm, rain process that does not fit the classic seeder-feeder theory due to the absence of the seeder cloud and ice-phase processes, yet is able to produce substantial rain rates (> 10 mm hr-1) at the surface. A surprising feature of this rain type is that it consistently occurs with lower reflectivity, by as much as 10 dB near the surface, as compared to cold rain with nearly the same rain rate. Power law Z-R relations are generated for both types of orographic rain observed during CALJET and put into context by comparing to results obtained in previous studies.