The Sierra Barrier Jet and Impacts on California's Orographic Precipitation Processes and Distribution

Precipitation estimation and forecasting in the Sierra Nevada of California are difficult because several meteorological factors impact where and how much precipitation will occur. One of the most prominent of these features is the Sierra Barrier Jet (SBJ), which can vary in intensity, depth, and horizontal extent. The SBJ generally is caused by terrain blocking and subsequent diversion of the flow in the Central Valley of California. A unique collection of wind profilers oriented both normal and parallel to the Sierra Nevada is used to examine the SBJ during several winter seasons. Vertically pointing precipitation profiling radars are also used to examine why a northern Sierra site behaves more like a coastal mountain site, in terms of the winter-season precipitation contribution from a shallow orographic rainfall process that is difficult to detect and quantify using operational scanning radars. Using a case study, the gap in coastal mountains afforded by the Carquinez Strait is shown to allow moisture-rich maritime flow in a land-falling winter storm to arrive at the northern Sierra site relatively unperturbed from any upstream terrain. In comparison, a southern Sierra site downstream from significant coastal terrain has much smaller storm and winter-season contributions from this shallow orographic rainfall process.