Rotor Formation in the Inn Valley: A Modeling Study

The aim the study is to investigate rotor formation in the surroundings of the city of Innsbruck, located in the Alpine Inn Valley, Austria. Foehn, a downslope windstorm, regularly develops in Innsbruck and can lead to hazardous conditions for aircrafts approaching the local airport. This study attempts to determine under which synoptic conditions foehn can be accompanied by turbulent rotors. For this purpose, the impact of several idealized atmospheric conditions on the realistic terrain surrounding Innsbruck was investigated using numerical simulations. The Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) was used. The employed atmospheric conditions are derived from upstream radiosonde measurements of past foehn events.

The simulation results suggest, that southerly foehn in the Inn Valley can be accompanied by rotors under the condition that an inversion above the Alpine crest is present. The large-scale wind direction can influence the probability for rotor formation. No rotors form, if the upper level wind is southwesterly, since the mountain wave forcing is reduced. In contrast, southeasterly upper level wind is favorable for rotor formation, due to significant mountain wave activity. In the presence of an inversion, the strongest rotors develop in connection with trapped lee waves, developing due to strong vertical wind shear. Rotors developing in the surroundings of Innsbruck are spatially variable. The simulations show, that rotors are more likely to develop in the lee of peaks surrounding Innsbruck, where the mountain wave forcing is stronger, than over the city. High amplitude trapped lee waves leading to rotors over two kilometers in height, connected with extreme turbulence, can form close to Innsbruck airport.