Thermal circulation in South Bavaria: climatology and synoptic aspects

The stronger diurnal temperature variation of air within Alpine valleys compared to the adjacent plain causes a thermal wind system, which in summer often propagates far north into the foreland of South Bavaria. Observations at 39 surface stations in South Bavaria between 1996-2000 are used to derive climatological surface wind fields for weather conditions with strong solar irradiance at ground level. It is shown that the thermal wind system propagates about 100 km into the Alpine foreland in 42 % of all days from April through September during which daily total global radiation exceeds 20 MJ m -2 . The thermal wind system also develops into the vertical direction starting at ground level. Between 1500-1600 CET the inflow layer increases up to 1 km above ground near the Alpine margin and up to 2 km within valleys of the Northern Alps causing a pronounced development of convective clouds. Here we propose the term Alpine Pumping for this well-defined dynamical process. During afternoon subsiding air motion suppresses convective cloud formation north of the Alpine margin. A classification by the 500 hPa geostrophic wind shows that the thermal wind system in the Alpine foreland is stronger influenced by the large-scale synoptic flow than valley wind systems within the Alps. A regional peculiarity can be observed in the Allgau under conditions with weak or southerly synoptic-scale flows. Under these conditions north-westerly slope-winds converge in the Allgau with north-easterly winds prevailing in the eastern part of the Alpine foreland. The well-known higher thunderstorm frequency of this region could be a consequence of the convergence of both flow patterns.