USING RADAR WIND PROFILERS TO DOCUMENT OROGRAPHIC PRECIPITATION ENHANCEMENT DURING THE CALJET FIELD EXPERIMENT

摘要

During a typical winter, many 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 California's coastal mountains. 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. It is commonly known that moist flow ascending a mountain barrier (i.e., upslope flow) will often enhance precipitation along that barrier, and that the magnitude of the upslope flow can impact the amount of precipitation that falls. For example, Hill et al. (1981) showed a direct relationship between the magnitude of the low-level onshore flow measured by rawinsondes 600 m above the coast of Wales and orographic rain-rate enhancement in the downstream hills. However, they used only 14 data points of wind speed versus rain rate, thus limiting the statistical significance of their findings. More recently, numerical studies along the western United States have looked into cool-season orographic precipitation on the windward and leeward slopes, but largely in the context of its sensitivity to model resolution (e.g., Colle et al. 1999). In the operational arena, a variant of the orographic precipitation model by Rhea (1978) has been coupled with operational three-dimensional models to forecast wintertime orographic precipitation in California. Though this model has proven quite beneficial, its success is often limited by inaccuracies associated with the input parameters generated by the operational models. Our study quantitatively links rainfall rates in California's coastal mountains to upslope flow measured immediately upstream along the coast during CALJET, using hourly wind-profiler and rain-gauge data from three observing couplets. Because of the availability of these wind profiles, we were able to determine the layer of upslope flow that optimally modulates mountain rainfall. A complementary study by White et al. (2001) discusses microphysical aspects of orographic precipitation observed during CALJET in an attempt to more fully understand this important process.