MODIFICATION OF AIR-FLOW OVER AN ESCARPMENT - RESULTS FROM THE HJARDEMAL EXPERIMENT

Mean flow, turbulence, and surface pressure measurements over an escar pment are presented. The speed-up in the mean wind field shows the kno wn dependence on stratification. Cross-sections of the standard deviat ion of horizontal and vertical wind components and of the friction vel ocity are derived from the data and compare favorably with the numeric al model of Zeman and Jensen (1987). The modification of turbulent pow er spectra at intermediate frequencies can be explained by rapid disto rtion theory. At very low frequencies, there is a quasi-stationary res ponse to the disturbance. Except for speed-up and standard deviations of the wind components, which are also shown for downslope wind, all r esults in this paper refer to upslope winds. An analysis of the vertic al momentum flux reveals that upstream of the escarpment, most of the flux is transported in sweeps of fast, sinking motion to the ground. D ownstream of the escarpment, ejections of slow, rising motion dominate the turbulent transport.