In this contribution, we discuss regional variations of wind ahead of severe storms. The region of interest is the Swiss midlands, a hilly area that is surrounded by several mountain chains. Measurements from operational radiosoundings, radar (VAD) and a dense ground mesonetwork are considered. Data from 1990-1997 are evaluated. Special emphasis is given on days with severe propagating thunderstorms, producing large hailswaths, severe winds, and floods.
The wind regimes above 4 km altitude are similar in western and eastern Switzerland. At lower altitudes, one finds substantial differences. In western Switzerland, the typical profile of horizontal wind ahead of severe storms shows weak wind from SW near the ground and moderate wind from SW at 3 km altitude. In eastern Switzerland, one often finds weak wind from W-NW near the ground and moderate wind from south at 3 km altitude. As a consequence, wind shear and storm-relative helicity can change from western to eastern Switzerland, thus modifying the conditions for the development, internal structure and evolution of severe storms. Severe storms in eastern Switzerland often move fast, rotate and are typically outflow-dominated. These characteristics are consistent with the observed wind profile.
The discussed regional differences in the wind field can be explained with drainage or channeling of airflow. In western Switzerland, channeling of wind in SW-NE direction between the Jura and the Alps is evident. In eastern Switzerland, two gaps between mountain chains are important: one producing drainage flow from NW near the ground, and one producing drainage flow from south at 3 km altitude. The drainage flow can extend to the lee of a gap between mountain chains. Hence, it is the combination of two drainage flows that may produce favorable conditions for severe storms in eastern Switzerland. It is suggested that the concept of drainage flow can be applied to many regions around the world: i.e. to all regions that are affected by neighboring mountain chains.