An Estimate of Mountain Drag during ALPEX for Comparison with Numerical Models

Abstract

The pressure drag vector for a limited domain including the Alps is estimated for the ALPEX period of March and April 1982. All of the reported, three hourly, surface pressure data in the domain are used and maps of analyzed deviation surface pressure are obtained for the mountainous area. The very important drag values previously reported are confirmed and shown to occur when tight pressure gradients are localized above the steep orography. The peak values are found to be sensitive to the resolution of the analysis. A composite of the large drag variations occurring in relation to frontal passage and cyclogenesis is constructed. The diurnal cycle observed during periods of quiet synoptic activity is discussed. The observed drag compares favorably with the drag in the simulation of a case of cyclogenesis using a numerical model with comparable horizontal resolution.