Orographic precipitation enhancement in the coastal mountains of California during the CALJET field experiment

During a typical winter, numerous coastal mountain ranges of the western United States are inundated with copious amounts of orographically enhanced precipitation. The California Land-falling Jets Experiment (CALJET) was carried out from January through March of 1998, in part, to study physical processes that cause heavy orographic rains in these regions, including the seeder-feeder mechanism and the forced ascent of moist air caused by onshore or upslope flow. This presentation will focus on the latter mechanism. The experimental objectives led to the deployment of an array of coastal wind profilers that measure tropospheric winds in the lower troposphere up to about 4 km, well above the highest coastal terrain. The wind profilers are well-suited for observing the evolution of the upslope flow because of their continuous profiling capabilities, and because they can observe the onshore flow above shallow, terrain-trapped (i.e., blocked) flow regimes. To assess the physical connection between air ascending a coastal mountain barrier and the orographic enhancement of precipitation, time series of layer-mean upslope flow measured by coastal profilers will be quantitatively compared with time series of rain rates measured by rain gauges situated in the coastal mountains downstream of the profilers. Using case-study examples, correlation analyses between these time series will be presented for both blocked and unblocked flow regimes. In addition, a winter-season climatology for upslope flow versus rain rate will be presented for couplets in southern, central, and northern California. Vertical profiles of the correlation coefficient will reveal the vertically varying impact of upslope flow on orographic precipitation enhancement. This statistical approach will also show the impact of terrain orientation, height, and steepness on orographic precipitation enhancement. By modulating the temporal averaging period of the wind and precipitation measurements, we will assess the time scales that are favored during orographic precipitation events