A new, simple model for thermally induced airflow in an Alpine valley

A new model was developed to simulate a valley wind circulation. The model consists of a thermal and a dynamic part. In the first part the heating of air in a valley is calculated, considering the reduced air volume in the valley, compared to a plain. Starting with high resolution topography data, relative air volumes of the grid elements are calculated. These volumes are averaged on a scale which is around the valley width, to gain the along valley circulations. Smaller relative air volumes result in stronger heating or cooling of a grid element. To parametrize the slope winds during the day, superadiabatic lapse rates are compensated by adjusting the temperature gradient to adiabatic. From the temperature field in the valley, the pressure tendency is calculated hydrostatically.

In the dynamic part, the valley wind is calculated using the circulation theorem. This offers a favourable way to treat very small horizontal pressure gradients and also saves computation time. The flow in the valley is considered to be balanced with no acceleration.

The model was run with different configurations to examine the sensitivity of the wind field to modifications in the model equations. Results are presented for the temperature and pressure tendency and the derived velocity field of the Mesolcina valley, which is located in the Swiss Alps. A comparison between wind measurements from the field campaign VOTALP and model runs is shown.