The thermally driven diurnal wind system of the Adige Valley in the Italian Alps

Abstract

The Adige Valley is one of the major corridors connecting the Po Plain with the inner Alps. A series of permanent weather stations and one wind profiler provide regular monitoring of air temperature, atmospheric pressure, global solar radiation, wind speed and direction over the 140 km valley length and in the adjacent plain. Data from these stations are analyzed for a subset of days on which weather conditions favoured full development of diurnal valley winds in the period 2012–2014. The analysis highlights typical features in the alternating patterns of diurnal up‐valley winds and nocturnal down‐valley winds. In particular, the wind intensity depends linearly on the along‐valley pressure gradient, supporting the concept of a quasi‐steady balance between the pressure gradient and surface friction. Also, in agreement with previous investigations, the amplitude of the surface pressure cycle increases in the up‐valley direction, causing the reversal of the horizontal pressure gradient twice per day. In contrast, no appreciable along‐valley variation in the diurnal temperature range is found. The analysis of surface temperature and pressure measurements suggests that the larger pressure perturbations found far into the valley are caused by the increased depth of the atmospheric layer subject to heating and cooling. Local inhomogeneities in the valley cross‐section, in particular in the vicinity of a large basin, cause temperature and pressure perturbations that are strong enough to alter the typical cycle of down‐ and up‐valley winds. Similarly, local wind convergence over the major cities during the night is explained in terms of the urban heat island effect.