Relationship of Synoptic Winds and Complex Terrain Flows during the MISTRAL Field Experiment

Abstract

The relationship between the surface and synoptic wind direction is examined climatologically in a complex terrain region. Surface winds were observed over a 1-yr period during the MISTRAL project in the Basel, Switzerland, area. 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 approximated by upper-level winds from routine rawinsonde observations. The relationship between this synoptic wind and the surface wind at each site was compared to a conceptual model. According to the conceptual model used, there are four mechanisms for the forcing of near-surface winds by synoptic winds. Three of the four forcing mechanisms leading to channeled flow are identified in the MISTRAL area. In this region with its complex terrain, different channeling mechanisms act at different locations as well as different mechanisms may occur at the same location. The study shows that the type of channeling depends on the topography surrounding the observation site. The combination of several channeling mechanisms with the multitude of valley orientations in a complex terrain setting explains the variety of observed flow patterns. One mechanism, the thermal forcing of valley winds, is examined in more 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 forced wind reverses?one in the morning and one in the evening. The daytime upvalley flow is, on average, stronger than the nighttime downvalley flow. In the MISTRAL region, the frequency of days with thermally driven flow does not have a significant annual cycle.