The Atmospheric Momentum Budget over a major mountain range : first results of the PYREX field program

International audienceThe PYREX program is a major field study of the dynamical influence of the Pyrénées mountains (on the border between France and Spain) on the atmospheric circulation. In October and November 1990 a large number of experimental means had been deployed in the area, including additional sounding systems, automated stations at high elevation sites, wind profilers, sodars, constant level balloons, and four research airplanes. The main focus was on the quantification of the retardation of the cross-mountain flow by the range, but a number of related mesoscale phenomena had been captured, e.g. the formation of lee waves, lee eddies, sheltering areas, and local surface winds. The data interpretation was supported by an extensive effort in meso-scale numerical modelling, and it is argued that only numerical models, conveniently qualified by observations, can document the momentum budget in a consistent way. 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 mesoscale models for the operational forecasts. The present paper presents a broad introduction to the experiment. It reviews the experimental set-up and the available data sets. Finally, two more specific topics are discussed in depth, as examples of the available data and modelling strategy: the lee wave event of 15 October 1990 is presented with both experimental and model results. Some of the findings concerning the Tramontana wind are also discusse