In the present work we shall consider a valley wind system which develops in the northern part of Garda Lake (Italy) under fair weather conditions, the so called 'Ora del Garda'. The latter usually arises in the late morning as a typical lake breeze in the northern part of Garda Lake (65 m a.s.l.). Thence it channels in the Sarca Valley and in the Lakes Valley nearby, and finally, after reaching a maximum height of about 500 m a.s.l., it blows out, through an elevated saddle, in the River Adige Valley, just North of Trento (200 m a.s.l.), where it appears as a strong gusty wind.
This wind system displays very peculiar features of lake/valley breeze: the way it typically affects local microclimate and transport processes, like diffusion and dispersion of passive tracers, motivated a deeper investigations of the dynamics involved in its development.
Time series of surface data have been analyzed in order to gain an overall quantitative description of the wind: a few episodes are reported, for which special observations have been carried out in specific sites.
In particular a regular diurnal oscillation of ground pressure differences between stations, located respectively at the valley inlet and downstream, is shown. This is supposed to drive the horizontal flow, according to the well known theory that along-valley winds are the result of a greater diurnal temperature range in a vertical column within the valley than in a similar column with its base at the same elevation outside the valley (Whiteman, 1990).
This experimental picture has been compared with conceptual models of the evolution of the valley atmosphere under the action of radiative heating. To that purpose suitable maps have been obtained in order to estimate quantities related to the incoming solar radiation (such as slope exposition, day length, hourly and daily solar radiation averages) at single locations in the area.
Finally the bulk circulation characteristics are reproduced in terms of an 'hydraulic model' of the flow which develops along the valley. The model extends previous results on the diurnal evolution of convective boundary layers (Nieuwstadt and Glendening 1989, Park & Mahrt 1979, Driedonks 1982) to include the effects of a sloping valley bottom. Assuming the layer to be well mixed, suitable evolution equations are derived for the depth-averaged horizontal velocity components, potential temperature and the boundary layer depth.
References
Whiteman C.D., 1990, Observations of Thermally Developed Wind Systems in Mountainous Terrain, in "Atmospheric Processes over Complex Terrain", American Meteorological Society Monographs (W. Blumen Ed.).
Nieuwstadt, F.T.M. & Glendening, J. W., 1989, Mesoscale dynamics of the Depth of a Horizontally Non-Homogeneous, Well-Mixed Boundary Layer, Beitr. Phys. Atmosph., 62, 275-288.
Park, S. U. & Mahrt, L., 1979, Oscillating, stratified boundary layers driven by surface temperature variations, Tellus, 31, 254-268.
Driedonks, A. G. M., 1982, Models and observations of the growth of the atmospheric boundary layer , Boundary-Layer Meteorol., 23, 283-306.