Extended Abstract (272K)10.4
The Influence of Land-falling Low-level Jets on Rain Rate in California's Coastal Mountains during CALJET
Paul J. Neiman, NOAA/ETL, Boulder, CO; and F. M. Ralph, A. B. White, D. E. Kingsmill, and P. O. G. Persson
The California Land-falling Jets Experiment (CALJET) was carried out from November 1997 through March 1998, in part, to study physical processes that cause heavy orographic rains in the coastal mountain ranges of California. A focal point of CALJET's observing strategy was to document the low-level jet (LLJ) region ahead of land-falling cold fronts over the eastern Pacific Ocean, because it is believed that land-falling LLJs contribute to heavy orographic rainfall as they impact the windward slopes of coastal mountains. During CALJET, a prominent LLJ was observed by NOAA's P-3 research aircraft during numerous flights offshore of California. Despite the presence of well-defined LLJ characteristics over the open ocean, are LLJs readily observable at the coast or do they dissipate prior to land-fall because of terrain blocking and enhanced friction in the coastal zone? If LLJs make land-fall, do they modulate rainfall in California's coastal mountains? To answer these questions, we inspected coastal wind profiler data from Bodega Bay for evidence of land-falling LLJs, and we also examined coastal mountain rain-gauge data immediately downstream of the profiler. A total of 81 h of profiler data exhibited LLJ attributes during periods of rainfall. Characteristics derived from these LLJ cases were compared with characteristics derived from the full data set that included 468 h of data. Land-falling LLJs produced the largest enhancement of upslope flow at jet level (~1 km above sea level), despite the presence of terrain-modified flows below mountain top during some LLJ events. Based on a linear regression analysis between upslope flow measured at the coast and rain-rate observed in the coastal mountains, the steepest increase in rain-rate for a given increase in upslope flow also occurred during LLJ conditions at the altitude of the LLJ, as did the largest correlation coefficient. Therefore, the upslope-induced orographic rain-rate response associated with land-falling LLJs was maximized (2.55 mm h-1) and was statistically most robust at the altitude of those LLJs. The efficiency of the rain-rate response to upslope flow during precipitating LLJ episodes was 50 percent greater than that for all precipitating periods during CALJET.
Session 10, Orographic Precipitation V
Wednesday, 19 June 2002, 11:00 AM-1:30 PM
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