Classification of summer wind fields over complex terrain

Abstract

In order to identify the representative flow patterns occurring in the Fecht Valley (region of Alsace, France) in summer, an automated wind-field classification is applied to wind observations collected during a field experiment. Fourteen meteorological stations placed at various topographic sites provided the necessary data on two scales. On the mesoscale of the Fecht Valley (230 km 2 ), the groups obtained by the classification reflect the diurnal and seasonal cycles of winds. On the microscale of the Ringelbach catchment (0 36 km 2 ), no diurnal cycle is observed in the sequence of the groups. Radiation balance and thermal forcing are the main factors governing the local flows during anticyclonic periods in the Fecht Valley, whereas external factors dominate the flow in the Ringelbach catchment area. A possible explanation for the different behaviour can be found in differences in scale, topography, and exposure between the two hydrological basins. On the basis of the classifications, the principal characteristics of the wind fields are studied and matrices of transition probabilities between wind fields are estimated. For climatic research in complex terrain, with its strong channelling of wind, an automated classification is helpful in the sense that it gives a realistic overview of the representative wind flows occurring in an area, without the need of any a priori knowledge. 1997 by the Royal Meteorological Society. Int. J. Climatol. 17: 521-534, 1997.