A Nested Large-Eddy Simulation Study of the Ora del Garda Wind in the Alps

High-resolution numerical simulations performed with the Weather Research and Forecasting (WRF) model are analyzed to investigate the atmospheric boundary layer (ABL) structures associated with the development of a lake-breeze and valley-wind coupled system developing in the southeastern Italian Alps, the so-called “Ora del Garda” wind. Three mesoscale domains, forced by reanalysis data field, are used to drive the finest domains, in which the large-eddy technique is used, achieving a final horizontal resolution of 80 m and a vertical resolution of 50 m in the first 2000 m above ground level over three different target areas. Model results complement an existing dataset composed of a series of measurement flights and surface observations. 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. Model results display a good agreement with the experimental dataset. In particular, both the near-surface meteorological variables and the structure of the valley ABL are satisfactorily reproduced. The valley ABL is characterized by shallow or even absent mixed layers surmounted by slightly stable layers extending up to the lateral crest level, due to subsidence of potentially warmer air resulting from the downward motion occurring in the center of the valley to compensate up-slope flow on valley sidewalls. Moreover, simulations highlight peculiar phenomena associated with the Ora del Garda, such as the development of the lake breeze front at the northern shores of lake Garda and the presence of inhomogeneities in the cross-valley thermal field, caused by the propagation of the lake breeze and by the different heating between the sidewalls of the valley.