Airborne measurements of terrain-induced pressure perturbations

Airborne measurement of the horizontal pressure field using differential GPS technology has been established during the last few years. Accurate aircraft measurement of the horizontal pressure gradient force requires an independent determination of the height of the airborne platform above some reference level. Here the authors demonstrate a differential GPS technique that uses data from a fixed reference station to refine the vertical position of the aircraft. A series of research flight legs by the University of Wyoming King Air research aircraft (UWKA) were conducted during the winter seasons of 2008 and 2009 over theMedicine Bow Mountains in southern Wyoming. Flight patterns consisted of a series of geographically fixed, parallel legs along a quasi-isobaric surface above the mountainous terrain, allowing the finescale mapping of the horizontal pressure (or geopotential height) field. The removal of the large-scale gradient and tendency isolates the terrain-induced pressure perturbation field. Results obtained using differential GPS measurements of aircraft height show that the Medicine Bow Range induces pronounced horizontal pressure perturbations, with a leeside region of low pressure downwind of the crest, in two cases: on 11 February 2008 and 20 February 2009. A wind maximum is found downwind of the elevated terrain consistent with this pressure gradient. Simulations of these two cases were performed using theWeather Research and ForecastingModel (WRF). The WRF height patterns for the time of the UWKA flight matched the general isobaric height patterns observed. Simulations and observations consistently show that the cross-mountain acceleration is stronger when the perturbation pressure gradient is larger.