The Statistical Relationship Between Upslope Flow and Rainfall in California's Coastal Mountains: Observations during CALJET

The California Land-falling Jets Experiment (CALJET) was carried out during the winter of 1997-98, in part to study orographic rainfall in California's coastal mountains using coastal wind profilers. This observational study statistically links hourly rainfall rates observed by tipping-bucket rain gauges in California's quasi-linear coastal mountains to the hourly averaged upslope component of the flow measured by coastal wind profilers immediately upstream. Vertical profiles of the linear correlation coefficient of upslope flow versus rain rate are calculated on a case-by-case basis, for all cases containing a low-level jet (LLJ), and for the winter season of 1997-98. These correlation coefficient profiles show a direct relationship between the magnitude of the upslope flow impacting the coast and the magnitude of the rain rate in the downstream coastal mountains. The layer of upslope flow that optimally modulates orographic rainfall is at ~1 km above mean sea level, which corresponds to the mean altitude of land-falling LLJs observed by the coastal profilers. The presence of shallow, terrain-blocked flow modulates the correlation coefficient profiles below mountain top, such that the low-level flow at the coast is poorly correlated with rain rates observed in the coastal mountains. However, cases without significant blocking retain relatively large correlation coefficient values below mountain top. In all scenarios, standard surface anemometers are unable to reveal the orographic connection between upslope flow and rain rate clearly enough for forecasting purposes.