THE ATMOSPHERIC MOMENTUM BUDGET OVER A MAJOR MOUNTAIN-RANGE - 1ST RESULTS OF THE PYREX FIELD PROGRAM

The PYREX program is a major field study of the dynamical influence of the Pyrenees mountains (on the border between France and Spain) on th e atmospheric circulation. In October and November 1990 a large number of experimental means had been deployed in the area, including additi onal sounding systems, automated stations at high elevation sites, win d profilers, sodars, constant level balloons, and four research airpla nes. The main focus was on the quantification of the retardation of th e cross-mountain flow by the range, but a number of related mesoscale phenomena had been captured, e.g. the formation of lee waves, lee eddi es, sheltering area's, and local surface winds. The data interpretatio n was supported by an extensive effort in meso-scale numerical modelli ng, and it is argued that only numerical models, conveniently qualifie d by observations, can document the momentum budget in a consistent wa y. Improvements in the field of mountain waves and mountain roughness representation in the large-scale atmospheric models are expected from the PYREX results, as well as improvements in the performance of meso scale models for the operational forecasts. The present paper presents a broad introduction to the experiment. It reviews the experimental s et-up and the available data sets. Finally, two more specific topics a re discussed in depth, as examples of the available data and modelling strategy: the lee wave event of 15 October 1990 is presented with bot h experimental and model results. Some of the findings concerning the Tramontana wind are also discussed.