Numerical simulations of the Ora del Garda wind in the Alps: a comparison with surface and airborne measurements

In order to investigate the atmospheric boundary layer (ABL) structures associated with the development of a lake-breeze and valley-wind coupled system developing in the Lakes Valley (southeastern Italian Alps), the so-called “Ora del Garda” wind, a series of measurement flights were performed by means of an instrumented motorglider on four warm-season days. The flights explored specific valley sections at key locations in the study area, namely over the lake’s shore, at half valley and at the end of the valley where the breeze blows. Air pressure, temperature and relative humidity measurements were recorded. At the same time, surface observations from a number of local automated weather stations were collected. In order to get a complementary view of the phenomenon, high-resolution numerical simulations of the flights days were carried out using the model WRF. Five nested grids were used, achieving a final horizontal spacing of 0.5 km. High-resolution orography and land use datasets were adopted for the domain initialization, while NCEP reanalysis provided initial and boundary conditions for the meteorological fields. The simulations were initiated at 1800 UTC of the day preceding the flight. The Unified Noah land-surface model and the Bougeault-Lacarrere scheme for PBL physics were used. The preliminary results display a rather good agreement with the experimental dataset. In particular, the surface daily cycles of radiation, wind and air temperature are satisfactorily reproduced, despite some discrepancies in the timing of thermally-driven circulation onset and offset, probably due to initialization inaccuracies. The typical structure of the valley ABL, characterized by shallow or even absent mixed layers surmounted by slightly stable layers extending up to the lateral crest level, is also qualitatively well reproduced in the simulated fields. Moreover, the simulations confirmed characteristic local-scale features of the thermally-driven wind field that were suggested by the analysis of the airborne dataset. For example, the model shows the development of a well-defined lake breeze front in the lake’s shoreline area, as well as the formation of a hydraulic jump structure in the area north of Trento city, i.e. at the end of the Lakes Valley, where the Ora del Garda denser current flows down into the nearby Adige Valley from an elevated saddle (elevation difference: 400 m).