Numerical simulation of daytime mesoscale flow over highly complex terrain: Alps case

The numerical study of mesoscale wind field and dispersion over terrains belonging to large mountain structures (e.g. Alps) poses a number of difficulties. Narrow valleys and steep slopes can considerably determine the temperature and pressure gradients, thus affecting the mesoscale systems in a quite complex manner. The adequate description of the local wind field, incorporating the high variability of the terrain features, is very important when a numerical simulation above highly complex terrain is attempted. The mathematical models used to reproduce the mesoscale flow in such cases must possess, besides the non-hydrostatic option, the ability to resolve the ground irregularities as realistically as possible. The increase of the spatial resolution would improve the terrain representation, but might require a prohibitively large CPU time and computer memory. DELTA-ADREA-DIPCOT is a numerical prediction code system, specifically designed to perform mesoscale wind field and dispersion calculations over terrains of high complexity, using moderate spatial resolution. This is achieved by recognizing subgrid ground features, such as slope and orientation variations, through a ground surface resolution higher than the one used for the atmosphere. The code system is here applied, focusing on the mesoscale meteorological systems over the Alpine mountainous area. The simulation is based on the data obtained in the framework of the TRANSALP Experimental Campaign, on 29 September 1990. The tracer concentration distribution is also examined, mainly as a further support to the meteorological conclusions. The predictions are compared with corresponding measurements and show a favourable agreement.