Abstract : I.3
Studies of the thermal wind system and a downslope windstorm event with a weather station on wheels during the Sierra Rotors Project

Thomas Raab, Georg J. Mayr
University of Innsbruck

During the Sierra Rotors Project (SRP) surface observations of the air flow over the Sierra Nevada mountain range were conducted with an instrumented car driving across the Owens Valley and two thirds up the lee side slopes of the Sierra Nevada. Temperature, dewpoint, 3D Position, pressure and 2D wind were measured while driving. Cross valley profiles of dynamic pressure, static pressure, wind direction, virtual potential temperature and mixing ratio revealed the footprint of the overlying flow. A slanted pressure reduction method was used to calculate the horizontal pressure difference between the valley and the eastern Sierra Nevada slopes. On the synoptic scale, blocking of a colder air mass to the windward side induced a negative cross mountain pressure gradient. However, differential heating between the leeward slopes and the valley lead to the evolution of a pressure force towards the slopes, enabling thermal upslope winds under quiescent conditions, i.e. weak westerly cross barrier flow. The process of erosion of the nocturnal cold pool by solar insolation was documented by the car. Without further synoptic forcing, no downslope windstorm was observed. During IOP 14 the synoptic forcing was strong enough to enable the breakthrough of a downslope windstorm to the valley floor. Several subsequent measurement drives showed the emerging of a gap-flow at the beginning of the event and flow separation in association with a possible rotor over the upper part of the lee slopes. The flow touched down again further downstream, accelerated and formed a hydraulic jump over the valley center.