Mesoscale simulations of thermodynamic fluxes over complex terrain

Due to photochemical processes, most of the air quality problems arise duri ng anticyclonic weather conditions in summer. Therefore, the problem is how thermodynamic fields and atmospheric processes (and especially wind fields ) interact with the transport of pollutants in a valley. To answer such que stions, knowledge about typical wind flow patterns occurring in the investi gation area, leading to a classification of characteristic situations is us eful. In particular, the most important of these flow fields and their gene rating processes can be analysed and understood by numerical simulations. F ourteen meteorological stations located at various topographic sites in the Fecht Valley (approximate to 230 km(2), Vosges Mountains, Alsace, France) provided the data necessary for processing an automatic wind-field classifi cation. This classification identifies the characteristic summer flow patte rns occurring in the valley. Two broad types of wind regimes can be schemat ically identified: during anticyclonic periods, thermally driven wind regim es are dominant; in contrast during cyclonic periods advective wind regimes are channelled by the orography. On the basis of this classification the a uthors were able to choose a typical summer day with anticyclonic condition s. A 3D simulation was performed for this day with the new Meso-Nh atmosphe ric mesoscale model to describe the thermodynamic fluxes and to understand the underlying physical processes. To obtain the most realistic results in this small area, high resolution surface information was introduced into th e model. The data collected during a measurement campaign are used to valid ate the simulation results. The diurnal cycle of wind fields, the thermal s tratification, and the heat fluxes are compared with observations. Copyrigh t (C) 2000 Royal Meteorological Society.