Relationship of synoptic winds and complex terrain flows during the MISTRAL field experiment

The relationship between the surface and synoptic wind direction is examine d climatologically in a complex terrain region. Surface winds were observed over a I-yr period during the MISTRAL project in the Basel, Switzerland, a rea. The measurement sites were located in various topographical settings, namely, in broad and narrow valleys, on slopes, at hilltops, on passes, and at mountaintops. The synoptic winds above the MISTRAL area are approximate d by upper-level winds from routine rawinsonde observations. The relationsh ip between this synoptic wind and the surface wind at each site was compare d to a conceptual model. According to the conceptual model used, there are four mechanisms for the forcing of near-surface winds by synoptic winds. Th ree of the four forcing mechanisms leading to channeled how are identified in the MISTRAL area. In this region with its complex terrain, different cha nneling mechanisms act at different locations as well as different mechanis ms may occur at the same location. The study shows that the type of channel ing depends on the topography surrounding the observation site. The combina tion of several channeling mechanisms with the multitude of valley orientat ions in a complex terrain setting explains the variety of observed flow pat terns. One mechanism, the thermal forcing of valley winds, is examined in m ore detail. Two minima in the averaged diurnal cycle of the wind speed are found. Both minima occur at the time when the direction of the thermally fo rced wind reverses-one in the morning and one in the evening. The daytime u pvalley how is, on average, stronger than the nighttime downvalley Row. In the MISTRAL region, the frequency of days with thermally driven flow does n ot have a significant annual cycle.