Wintertime Cold-Air Pools in the Bavarian Danube Valley Basin: Data Analysis and Idealized Numerical Simulations

This paper investigates wintertime cold-air pools in a basinlike part of the Danube Valley, located in the German state of Bavaria. Specifically, the focus is on cold-pool events restricted to the basin area, that is, not extending to the more elevated parts of the Alpine foreland. An analysis of observational data indicates that the delay of warm-air advection in the basin area relative to the Alpine foreland plays a major role in these events. However, the relationship between warming in the Alpine foreland and a temperature deficit in the northeast–southwest-oriented basin appears to depend sensitively on the ambient wind direction. A statistically significant correlation is found only for westerly and southerly wind directions but not for easterly directions. To examine the dynamical reasons for this phenomenon, idealized numerical simulations have been conducted. They are initialized with a pronounced cold pool in the basin area and examine the response of the cold pool to the dynamical forcing imposed by a geostrophically balanced large-scale wind field of various directions and strengths. Sensitivity tests consider the effects of the surrounding mountain ranges and of turbulent vertical mixing. The model results indicate that the most important dynamical processes capable of dissolving cold-air pools in a large basin are (i) ageostrophic advection of the cold air toward lower ambient pressure and (ii) downstream advection by the ambient flow. The former might also be interpreted as an adjustment of the cold air to the external pressure gradient, which can be balanced by the development of a spatial gradient in cold-pool depth. In principle, both advection processes are most effective in the along-basin direction because the advected air does not have to surmount significant topographic obstacles. However, a combination of several effects induced by the surrounding mountain ranges—for example, upstream flow deceleration and wake formation—modifies the dependence of the cold-pool persistence on the ambient wind direction. In agreement with observational data, the simulations with full topography predict a higher tendency for cold-pool persistence in the Danube basin for westerly and southerly flow than for easterly flow. Turbulent vertical mixing is found to make a significant contribution to the erosion of cold pools, but its effect is smaller than the sensitivity to the ambient wind direction.