Observations and numerical simulations of a valley-exit wind in the Alpine Bolzano basin

The characteristics of the nocturnal drainage wind flowing from the tributary Isarco Valley into the Bolzano basin, in the Italian Alps, during wintertime are investigated. Analyses are performed by combining measurements from an intensive field campaign and the output of four high-resolution numerical simulations, run with the Weather Research and Forecasting (WRF) model using different planetary boundary-layer (PBL) schemes. Two episodes are identified, based on the vertical temperature stratification in the basin and the evolution of the drainage flow at the valley exit. Numerical results show that the drainage flow behaves as a valley-exit wind, whose main structure at the exit of the valley is well captured by the model independently of the PBL scheme. However, the model struggles to correctly reproduce the temperature stratification in the basin, with better results when a PBL scheme including, among others, a prognostic equation for the temperature variance and a counter-gradient term is used. This has an impact on the simulation of the onset and duration of the valley-exit wind, which are sensitive to the temperature contrasts between the valley and the basin. Overall, the model is able to reproduce the different behavior of the drainage wind at the exit of the valley in the two case studies. It is found that the presence of a cold air pool in the basin favors an upward trajectory of the flow at the exit of the valley, resulting in unperturbed calm wind conditions in the lower levels. On the other hand, with weak temperature stratification, the drainage flow closely follows the topography, resulting in strong winds also near the surface.