Coriolis effects on orographic and mesoscale flows

A perturbation theory is developed for the effects of rotation on stably st ratified flow over mountains at low Froude number, F = U-0/(NH) where N is the buoyancy frequency, U-0 is the wind speed and H is the mountain height. The Rossby number, R-0 = U-0/(f D) where f is the Coriolis parameter, and D the along-wind length of the mountain, is assumed to be a large number. T he mountain width, <(<beta>)over bar>D, is assumed to be larger than D. Typ ically R-0 is found to lie in the range 3-10. The results are compared with the recent numerical simulations. It is found that as the flow impacts on the mountain, in the northern hemisphere it turns to the left (with your ba ck to the wind); also wave activity over the top of the mountain is greates t on the left side but the pressure drop is greatest on the right in the no rthern hemisphere. Over the Rossby deformation distance, L-R, of the order of HN/f, e.g. 150 km for the Pyrenees, a new wake structure develops that c an extend downwind over 1000 km (or a spin-down distance). There is a momen tum defect within the wake but the wind speed increases either side of the wake. Coriolis forces induce a deflection upwards of the isopycnals (and he nce more precipitation) on the left, and downwards on the right; this is co nsistent with some of the differences in mesoscale weather and climate phen omena that are observed on the different flanks of elongated mountains and between the different side of wide valleys, and also in the wakes downwind of mesoscale convection cells. The large perturbation pressure change predi cted by the theory is of the order of rhoU(0)(2)/F (where rho is the densit y), which is consistent with the magnitude of the terms introduced into the recent European Centre for Medium-Range Weather Forecasts orographic param etrizations, but it should be noted that these large Rossby-Froude orograph ic effects on drag and wave flux are asymmetric with respect to the mountai n's centre line. The theory shows how 'lift' forces on the mountain are cau sed, and how these are related to circulation in horizontal planes around t he mountains.