An investigation of the Ora del Garda wind in the Alps by means of Kriging of airborne and surface measurements

The present study investigates a coupled lake-valley wind, known as Ora del Garda, which typically arises in the late morning on clear-sky days along the northern shorelines of Lake Garda and then channels northward into the Sarca and Lakes valleys, until breaking out into the Adige Valley north of Trento city (southeastern Italian Alps), where it interacts with the local up-valley wind in a complex fashion. This thermally-driven circulation displays great regularity, marking the local climate with a strongly mildening influence. A series of targeted measurement flights, performed by means of an instrumented motorglider, allowed to explore the valley atmosphere thermal structure at selected vertical sections, namely over the lake’s shore, at half valley, at the end of the valley, and at the junction with the Adige Valley. Dominant vertical profiles of potential temperature were inferred from airborne data, while 3D potential temperature fields were mapped over high-resolution regular grids for each explored section, through the application of a Residual Kriging (RK) technique both to airborne and to surface data from weather stations disseminated along the valley floor. These procedures allowed to identify atmospheric boundary layer (ABL) features typical of most diurnal valley winds. In particular, rather shallow convective mixed layers, surmounted by deep stable layers, occur up-valley. On the other hand, closer to the lake the advection of colder air tends to stabilize the atmosphere throughout the whole valley ABL depth. Small-scale features of the thermally-driven wind field produced by the coupling between the lake breeze and the up-valley circulation into a unified mesoscale flow were revealed by RK-interpolated potential temperature 3D fields; the development of a lake-breeze front structure was captured in the shoreline area, while up-valley sections displayed cross-valley thermal asymmetries, mostly amenable to the different irradiation of the valley sidewalls and to inhomogeneities in the surface coverage, but also to the curvature of the valley in its final part. Lastly, RK-interpolated fields suggested the occurrence of a hydraulic jump structure at the end of the Lakes Valley, where the Ora del Garda potentially cooler air overflows from an elevated saddle down to the underlying Adige Valley floor, producing there an anomalous and gusty cross-valley flow.