TRANSALP 1989 experimental campaign. Part I: Simulation of 3D flow with diagnostic wind field models

The first TRANSALP meteorological and tracer campaign was carried out in October 1989 in the Ticino river valley, southern Switzerland, aiming to study air masses and pollutant transport through the Alps. The experiment was executed in a very complex terrain area, during typical valley breeze condition, which caused a plume bifurcation along two valleys. Meteorological instrumentation included three Doppler SODARs and several ground stations measuring wind speed and direction. Two diagnostic wind field models, CONDOR and MINERVE, were applied to TRANSALP 89, to investigate their ability to reproduce the main characteristics of the advection field during the tracer dispersion, and to assess their sensitivity to the number and quality of input data. Although the two models differ in some features, especially in the numerical scheme to solve the continuity equation, they give comparable results. The models were first run with only one surface observation and one vertical profile taken near the release point as input wind data to represent the flow. Then several runs, combined with tracer dispersion simulations by 3D particle models, were executed in order to investigate the models' sensitivity with respect to the number and location of input wind data. The most significant results of the flow modelling are discussed, and the computed wind fields chosen to drive the dispersion simulation, described in the companion paper (Anfossi et al., Atmospheric Environment 32, 1157-1166), are shown.