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

On fair-weather summer days an intense southerly lake breeze blows across the northern shorelines of Lake Garda (Italy). This wind, known as Ora del Garda, arises regularly in the late morning, and then channels northward into the adjacent Sarca Valley and Lakes Valley, coupling with the local up-valley flow. In the early afternoon, after flowing over an elevated (~400 m high) saddle, the Ora del Garda wind breaks into the Adige Valley north of Trento city; there it flows down on the valley floor, interacting with the local up-valley wind and creating a strongly turbulent flow. The characteristic diurnal cycle of surface meteorological variables determined by the lake-valley coupled circulation is rather well-known, on the basis of climatological analyses of data from surface automatic weather stations operated in the area by local institutions; on the contrary, the valley upper atmosphere structure, i.e. the structure of the atmospheric boundary-layer (ABL), associated with the Ora del Garda development has not yet been investigated. Indeed, in such a complex terrain area, the characterization of the typical structure, spatial variation and depth of the ABL, as well as a sound knowledge of local atmospheric circulation patterns, are of crucial importance for the understanding of the local climate and of air pollution transport and dispersion processes. To meet this lack of knowledge, a series of targeted measurement campaigns, including both intensive surface observations and research flights, were carried out by the Atmospheric Physics Group of the University of Trento in the study area between 1998 and 2001, providing the database for the present work. Five flights of an instrumented motorglider explored specific sections of the valley atmosphere, namely at Lake Garda’s shoreline, in the lower Sarca Valley, in the Lakes Valley, and where the Ora del Garda and the Adige Valley up-valley flow interact. Position, pressure, temperature and relative humidity were measured along spiralling trajectories performed over the above mentioned target areas. Surface observations from a number of weather stations disseminated along the valley floor provided a picture of the diurnal cycles of meteorological quantities determined at the surface by the development of the investigated wind on the flight days. The preliminary processing of the experimental dataset included the application of a suitable procedure to correct airborne temperature data for the time-delay effect induced by the slow-response behavior of the sensor, and required the determination of a proper time constant. The dominant vertical structure of the valley ABL was then deciphered on the basis of vertical “pseudo-soundings” (i.e. mean vertical profiles) of potential temperature and water vapour mixing ratio extracted from airborne data. Shallow mixed layers, surmounted by deeper stable layers, likely to be produced by local subsidence associated with up-slope flows, were detected up-valley. This characteristic pattern is indeed in good accord with ABL structures typically observed in deep Alpine valleys in connection with up-valley winds, as reported in the literature. On the other hand, closer to the lake the potential temperature profile was typically stabilized down to lower heights, due to the onshore advection of colder air from above the water surface. A residual kriging (RK) technique was adopted to map potential temperature fields over 3D high-resolution grids for each explored section of the valley atmosphere, integrating both surface and airborne observations. Exploiting a test-bed database, RK method was preliminarly tested against the interpolation methods commonly used in the literature for mapping airborne data, namely inverse distance, inverse squared distance and natural neighbor methods. The predictive performance of the different methods was assessed by means of a cross-validation procedure, and a critical comparison of the different interpolation results was carried out. Finally, RK resulted the best-performing technique for the specific application. RK-interpolated fields revealed fine-scale local features of the complex ABL thermal structures determined by the Ora del Garda in the study area valleys, revealing at the same time macroscopic features of the thermo-topographically driven wind field, mainly amenable to irregular topography and land cover heterogeneities. In particular, a non-homogeneous penetration of the lake-breeze front across the flat basin facing Lake Garda was detected in the morning, while in the afternoon the presence of a sharp discontinuity in the upper-level vertical stratification, originated by updrafts and downdrafts associated with the lake breeze circulation, was observed. Moreover, a strongly asymmetric potential temperature field, resulting from the contrast between the stable core of the Ora del Garda up-valley flow and an intense up-slope flow layer developing along a bare-rock valley sidewall, was detected in the area of Cavedine Lake in the Lakes Valley. Further up-valley, RK-interpolated fields displayed a thermal structure compatible with the occurrence of a single-cell cross-valley circulation, likely to be originated by asymmetric solar irradiation and by the local valley curvature. The valley curvature was also found to induce a preferential channeling of the up-valley flow along the northwestern sidewall at the valley end, in proximity of the elevated saddle from where the Ora del Garda overflows into the underlying Adige Valley, giving origin to an anomalous, strong katabatic wind that hinders the regular development of the local up-valley wind in the area north of Trento. Here the westerly inflow from the Lakes Valley feeds a denser wedge of potentially cooler air, which forces the local up-valley (i.e. southerly) wind to flow over it. Regridded potential temperature fields provided further insight into this flow pattern, revealing the occurrence in the area of a hydraulic jump structure, due to the blocking exerted on the flow by the eastern Adige Valley sidewall. This induced a pronounced deepening of the local mixed layer, which was likely produced by the highly-turbulent flow conditions that usually develop here following the Ora del Garda outbreak.