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  • Terrain-Induced Rotor Experiment (T-Rex) x
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Juerg Schmidli, Gregory S. Poulos, Megan H. Daniels, and Fotini K. Chow

Abstract

The dynamics that govern the evolution of nighttime flows in a deep valley, California’s Owens Valley, are analyzed. Measurements from the Terrain-Induced Rotor Experiment (T-REX) reveal a pronounced valley-wind system with often nonclassical flow evolution. Two cases with a weak high pressure ridge over the study area but very different valley flow evolution are presented. The first event is characterized by the appearance of a layer of southerly flow after midnight local time, sandwiched between a thermally driven low-level downvalley (northerly) flow and a synoptic northwesterly flow aloft. The second event is characterized by an unusually strong and deep downvalley jet, exceeding 15 m s−1. The analysis is based on the T-REX measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction System (ARPS). Using horizontal grid spacings of 1 km and 350 m, ARPS reproduces the observed flow features for these two cases very well. It is found that the low-level along-valley forcing of the valley wind is the result of a superposition of the local thermal forcing and a midlevel (2–2.5 km MSL) along-valley pressure forcing. The analysis shows that the large difference in valley flow evolution derives primarily from differences in the midlevel pressure forcing, and that the Owens Valley is particularly susceptible to these midlevel external influences because of its specific geometry. The results demonstrate the delicate interplay of forces that can combine to determine the valley flow structure on any given night.

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Thomas Raab and Georg Mayr

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.

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