Observations of orographically enhanced precipitation along the West Coast of the United States during the CALJET field program

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 release of potential instability 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. Correlation analyses between these time series will be presented for both blocked and unblocked flow regimes. Vertical profiles of the correlation coefficient will reveal the vertically varying impact of upslope flow 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. When available, the offshore thermodynamic stratification measured by NOAA's P-3 research aircraft will be shown, and the impact of this stratification on rain rates will be discussed.