Hydraulic Interpretation of the Footprints of Sierra Nevada Windstorms Tracked with an Automobile Measurement System

Abstract

This article reports results from the Sierra Rotors Project, which took place in the central part of Owens Valley, California, east of the Sierra Nevada in March and April 2004. The aim of the study is to describe the footprints of cross-mountain and downslope airflow by mobile surface measurements and radiosoundings. An instrumented car measured wind, temperature, pressure, and humidity. Four case studies cover the spectrum of forcings behind the foehn-like downslope windstorms. Hydraulic theory as a conceptual model was used to explain the data from the car in combination with radiosoundings. All four cases had a colder air mass on the upstream side, thus creating a hydrostatic pressure forcing. With weak flow parallel to the sierra, no downslope windstorm developed and a valley-slope circulation was documented, which for the first time related continuous pressure measurements to the thermal wind system. A second case with a stronger wind component perpendicular to the sierra caused the flow to plunge to the Owens Valley floor. Signatures indicating supercritical regions with accelerated flow reverting to a subcritical state in a hydraulic jump were found. In the third case, the flow separated from the lee slope and subsequently reattached. In the last case, a downslope windstorm developed ahead of a cold front. The downslope windstorm and cold front coexisted in the valley for several hours, with the latter being confined to its eastern side and the storm riding up over it.