LOW-FROUDE-NUMBER STABLE FLOWS PAST MOUNTAINS

A new approximate analysis is presented for stably stratified flows at low Froude number F past mountains of height H. In the ''top'' layer where the streamlines pass above the surface of the mountain, there is a perturbation flow. This approximately matches the lower flow in the ''middle'' 'horizontal' layer [M] in which the streamlines pass round the mountain in nearly horizontal planes, as in Drazin's (DRAZIN P. G ., On the steady flow of a fluid of variable density past an obstacle, Tellus, 13 (1961) 239-251) model. The pressure associated with the di verging streamlines on the lee side of the summit layer flow drives th e separated flow in the horizontal layer (which is not included in Dra zin's model). This explains the vortical wake flow in experiments and in the ''inviscid'' computations of Smolarkiewicz and Rotunno (SMOLARK IEWICZ P. K. and ROTUNNO R., Low Froude number flow past three-dimensi onal obstacles. Part I: Baroclinically generated lee vortices, J. Atmo s. Sci., 46 (1989) 1154-1164). A method for estimating the height H-T approximate to FH of the cut-off mountain is derived, as a function of upstream shear, mountain shape and other parameters. Recent laborator y experiments have confirmed how the curvature of the oncoming shear f low profile (-partial derivative U-2/partial derivative z(2)) can prod uce a significant reduction in the cut-off height H-T and in the dista nce downstream of the crest where the lee flow separates. This effect may reduce the wave drag of groups of mountains of similar height. The extension of the analysis to the movement of weak fronts past mountai ns is briefly described.