Using radar wind profilers to document orographic precipitation enhancement during the CALJET field experiment

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. The experimental objectives led to the deployment of an array of 915-MHz coastal wind profilers that measure tropospheric winds in the lower troposphere up to about 4 km, well above the highest coastal terrain. Predicated on basic theoretical concepts relating horizontal moisture convergence to orographic precipitation, and assuming two-dimensionality, our observationally based study quantitatively links rainfall rates observed by tipping-bucket rain gauges in California's coastal mountains to the upslope component of the flow measured by the profilers immediately upstream along the coast. A least-squares linear regression fit is applied to time series of upslope flow observed at each wind-profiler range gate and corresponding time series of rain rate measured in the downstream coastal mountains, thus yielding vertical profiles of correlation coefficient on a case-by-case basis and for the CALJET winter season. This quantitative analysis reveals that the correlation between upslope flow above the coast and rain rate in the coastal mountains is optimized near mountain top (~1 km MSL). Using the winter-season wind-profiler and rain-gauge data, wind-direction-dependent rain-rate regimes are ascertained, the relationship between the pre-cold-frontal low-level jet over the open ocean and orographic rainfall in the coastal mountains is quantified, and the impact of terrain characteristics (i.e., orientation, height, and steepness) on orographic precipitation enhancement is assessed. The wind profilers are ideally suited for addressing the problem of orographic precipitation enhancement because of their ability to continuously monitor the wind-flow patterns across the level of maximum correlation and above shallow, terrain-trapped (i.e., blocked) flow regimes that might be present.