AN EXPERIMENTAL-STUDY OF STABLY STRATIFIED FLOW IN THE LEE OF HIGH MOUNTAINS

A complex flow situation over a frozen lake during winter in the north ern part of Sweden during 1987 and 1989 has been studied. The high mou ntains on the order of 1100 m surrounding the lake dominate the flow, which can be characterized by three regimes. For very stable stratific ation (Froude number less than 1) blocking occurs on the upwind slope of the mountains, while in the lee there are calm wind conditions. Sec ond, for less stable stratification the wind blows down the slope of t he mountains at all levels. When there is an uplifted inversion with n ear neutral stratification below, a recirculation flow pattern occurs in the lower part of the boundary layer. Third, for slightly stable st ratification and with a pressure gradient oriented along the lake, a m esoscale flow pattern will create strong ''gap winds'' near the surfac e, preferably from an easterly direction. One case has near-surface wi nds of 30 m s(-1). At higher levels, a turbulent wake with very low me an wind speed will be present. The wake oscillates vertically and inte racts with the flow in the surface layer. The layer near the ground is thus characterized by alternating periods with high wind speed and lo w turbulence and with low wind speed and high turbulence.