Valley-wind characteristics in the Alpine Rhine Valley: Measurements with a wind-temperature profiler in comparison with numerical simulations

Abstract

This study investigates the valley-wind system of the Alpine Rhine Valley on the basis of measurements with a wind-temperature radar combined with high-resolution numerical simulations. It is found that the valley-wind circulation follows a very regular pattern on synoptically undisturbed days. In particular, the times at which the wind direction switches from up- to downvalley and vice versa and the wind speeds in the lower part of the valley-wind layer exhibit only a minor day-to-day variability. The model results indicate that in the exit region of the Rhine Valley, the nocturnal downvalley flow is much stronger than the daytime upvalley flow. Towards the interior of the valley, this asymmetry diminishes. These differences are related to variations of the valley width and to the impact of the tributaries. Along-valley variations of the near-surface wind speed are found to be associated with variations of the nocturnal surface temperatures. Significant near-surface winds reduce the nocturnal cooling due to increased vertical mixing.