Isobaric Height Perturbations Associated with Mountain Waves Measured by Aircraft during the Terrain-Induced Rotor Experiment

It has only been in the last few years that accurate measurement of the horizontal pressure gradient has been possible over complex terrain using an airborne platform. To infer forcing mechanisms for the wind, an independent measure of the height of an isobaric surface is required. Differential GPS analyses have enabled determination of the aircraft height with sufficient accuracy to infer isobaric heights. When coupled with an accuratemeasurement of static pressure, the horizontal pressure field can be determined. To demonstrate this measurement technique, research flight legs by the University of Wyoming King Air (UWKA) conducted in support of the Terrain-Induced Rotor Experiment (T-REX) in March and April 2006 are examined. UWKA flights conducted on 14 and 25 March and 16 April 2006 encountered strong mountain waves in response to winds directed primarily normal to the Sierra Nevada ridgeline. Winds at flight level showed pronounced variation that suggested topographic influence. Themagnitude of isobaric height perturbations alongUWKA flight tracks obtained using differential GPS during case study days of 14 and 25March and 16April are shown to exceed 70 m, corresponding to horizontal pressure perturbations greater than 4 hPa. Measurements suggest that changes in wind speed are linked primarily to the perturbation height field and that the flow can be classified as Eulerian, implying that Coriolis accelerations are negligible and flows respond to the horizontal pressure gradient force.