Mesoscale simulations of thermodynamic fluxes over complex terrain

Due to photochemical processes, most of the air quality problems arise during 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 questions, knowledge about typical wind flow patterns occurring in the investigation area, leading to a classification of characteristic situations is useful. In particular, the most important of these flow fields and their generating processes can be analysed and understood by numerical simulations. Fourteen meteorological stations located at various topographic sites in the Fecht Valley (≈230 km 2 , Vosges Mountains, Alsace, France) provided the data necessary for processing an automatic wind‐field classification. This classification identifies the characteristic summer flow patterns occurring in the valley. Two broad types of wind regimes can be schematically identified: during anticyclonic periods, thermally driven wind regimes are dominant; in contrast during cyclonic periods advective wind regimes are channelled by the orography. On the basis of this classification the authors were able to choose a typical summer day with anticyclonic conditions. A 3D simulation was performed for this day with the new Meso‐Nh atmospheric 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 the model. The data collected during a measurement campaign are used to validate the simulation results. The diurnal cycle of wind fields, the thermal stratification, and the heat fluxes are compared with observations. Copyright © 2000 Royal Meteorological Society