Large-scale-flow interactions with the Alps and their impact on the low-level temperature field in the northern foreland

Abstract

Numerical simulations are presented to investigate the impact of large-scale-flow interactions with the Alps on the temperature field in the northern Alpine foreland. The simulations use realistic topography but idealized large-scale conditions that allow for changing the wind direction without creating systematic temperature advection effects. The results show that the dependence of the surface temperatures on the wind direction differs substantially between the morning and the afternoon. In the afternoon, temperatures tend to be higher for southerly flow directions than for northerly ones, as might be expected from the fact that subsidence in the lee of the Alps causes warming while upslope flow is related to cooling. In the morning, however, the lowest temperatures are found for easterly directions while northwesterly flow shows the highest temperatures, followed by westerly flow. Part of this behaviour is related to the fact that in the northern Alpine foreland, westerly wind directions tend to be associated with higher wind speeds than easterly directions, which in turn is caused by the fact that the flow around the Alps is asymmetric. As a consequence, turbulent mixing tends to be stronger for westerly directions. For northwesterly flow, turbulent mixing is particularly strong because a barrier jet forms along the Alps. Another important contribution arises from the friction-induced wind turning in the surface layer, which causes a northerly (southerly) component for easterly (westerly) flow. Thus, the radiatively cooled near-surface air is piled up along the northern rim of the Alps for easterly flow but is advected away from the Alps for westerly flow.