The Single Air Mass Disturbance and Precipitation Characteristics at San Francisco

Abstract

Precipitation in California is generally associated with frontal systems. The author's observations suggest that although the three major frontal cyclone categories are responsible for the bulk of the annual precipitation in the San Francisco Bay Area, a fairly large proportion can be ascribed to non-frontal, single air mass disturbances related only to positive vorticity advection aloft. Daily precipitation at San Francisco International Airport for the period 1966?75 was compared with the 1200 GMT surface and 500 mb charts. Each 24 h precipitation amount was associated with either a frontal or a, non-frontal disturbance. The analysis showed that between 20% and 50% of the total precipitation at San Francisco during the 10-year period was non-frontal in nature. In addition, the temporal variability of frontal precipitation was much higher than that of non-frontal precipitation. The results suggest that the ?wetness? or ?dryness? of a given season in California is related to frontal characteristics and that non-frontal precipitation, being more reliable across time, has a damping effect on the total variability of precipitation in California.