Investigation of the “Ora del Garda” wind in the Alps: a combined approach of surface and airborne field measurements and numerical modeling.

Results from the investigation of a coupled lake/valley local circulation, the so-called “Ora del Garda” wind in the southeastern Italian Alps, are presented. On sunny days the wind develops in the late morning as an intense southerly lake breeze along the northern shorelines of the Lake Garda (65 m MSL), then channels northward into the adjacent Sarca Valley and Valley of Lakes, and finally breaks into the adjacent Adige Valley (200 m MSL) through an elevated saddle (580 m MSL minimum height) north of the city of Trento. Here the “Ora del Garda” gets mixed with the local up-valley winds, producing a strong and gusty local flow. In order to investigate the connection between the breeze flow occurring at the lower levels and the upper thermal structure of the BL in the valley, some measurements flights have been performed in the area with an instrumented motorglider during 1998-2001 late summer months, providing the dataset for the present analysis. The flight trajectories followed spiraling paths on vertical planes oriented either in cross- or along-valley direction, allowing the exploration of some valley sections at key locations, namely over the Lake Garda shore, at half valley, and at the end of the valley. Air pressure, temperature and relative humidity measurements were recorded. Pseudo-soundings obtained from airborne data allowed to identify the dominant vertical profile of potential temperature and mixing ratio for each explored section. The comparison with routine soundings from synoptic stations at Milan, Udine and Innsbruck highlighted a pattern common to valley BLs: a shallow or even absent superficial ML, surmounted by a deep slightly stable layer, extending up to the ridge-top level (see figure 1). Furthermore, the continuous advection of colder air from Lake Garda's surface was found to further inhibit the convective growth of the ML in the adjacent area. A Residual Kriging technique was then applied to map potential temperature on 2D regular grids over selected valley sections. The mapping provided a detailed picture of the thermal structure of the valley atmosphere, such as the lake breeze flow depth (around 500 m) and the development of an Internal Boundary Layer in the shoreline area. Moreover the effect of a local discontinuity in heat and moisture fluxes (possibly associated with a small lake close to an area covered with bare rock) was outlined at half-valley cross sections, as well as the intense heating of the layer near the lateral sidewall. The flow swerving in the final stage of the wind path, where the valley bends eastward, was also revealed. To get a complementary view of the phenomenon, high resolution simulations of the flights days were carried out using the model WRF. The simulations were initiated at 1800 UTC of the day preceding the flight; five nested domains were used, with a final horizontal grid spacing of 0.5 km; the Unified Noah land-surface model and the Bougeault-Lacarrere scheme for PBL physics were used. The preliminary results presented here display good agreement with the experimental dataset, providing further insight into the investigated phenomenon, as to wind and turbulent kinetic energy fields, as well as surface heat and moisture fluxes, and the thermal structure and flow features at unexplored locations. The temporal and spatial evolution of the “Ora del Garda” complex wind field and of the connected BL structure was identified on the basis of the modeled fields, following the development of along- and across-valley structures (e.g. the propagation of the lake breeze front, the patterns of slope and up-valley circulation). Numerical results allowed for a preliminary evaluation of the contributions to the wind system and BL evolution of the different valley atmosphere heating mechanisms, that is, turbulent sensible heat fluxes from the valley floor and lateral sidewalls, latent heat fluxes, advection terms, and heat exchanges between the valley atmosphere and the Free Atmosphere. Figure 1. Comparison of observed mean vertical profiles of potential temperature to soundings and surface observations for 1 Sep 1999 flight. Thinner lines are routine soundings: solid line is 1200 UTC Milan, dashed line is 1200 UTC Udine and dotted line is 1800 UTC Milan. Bolder lines are airborne data pseudo-soundings for: Lake Garda shoreline (A), half Valley of Lakes (B/w), Valley of Lakes end (C) and Adige Valley (D). Associated scatter symbols are surface observations from the valley floor. Short horizontal lines indicate the valley floor height for the section.