Effects of cyclone track on precipitation distribution along the California Coastal Range and Sierra Nevada

Precipitation distribution in California can vary widely. The complex terrain surrounding the Central Valley plays a significant role in this distribution. We hypothesize that the rainfall patterns over the California mountain ranges are influenced by low-level flow, and are thus closely related to the track of approaching cyclones. Therefore it is important to characterize the track and flow associated with mid-latitude cyclones moving toward the California coastal range and Sierra Nevada.

In this study, we use North American Regional Reanalysis (NARR) data for the months from December to March, 1997-2005 to analyze the track of cyclones in the vicinity of California. Cyclones considered are only those which move eastward passing 135°W and produce precipitation in the mountains of California. Three track types within the domain are classified: I) cyclones moving from southwest to northeast, II) cyclones moving from west to east, III) cyclones moving from northwest to southeast. Precipitation distributions associated with each track are analyzed. Initial analysis of the precipitation distributions indicate that the heaviest precipitation associated with track I occurs in the mountains surrounding northern California, including the northern portions of the Sierra Nevada range. Track II type of precipitation is marked by a similar distribution, but with heavy precipitation also along the western slopes of the coastal range. Track III type of precipitation features less precipitation, and the precipitation is focused in along the mountains in southern California. The complex terrain in California modifies the impinging flow associated with each track type, which in turn effects the distribution of precipitation. Therefore the nature and extent of this modification is investigated. The synoptic conditions for each track type and how the conditions contribute to the precipitation distributions are also discussed.