Strongly nonlinear flow over and around a three-dimensional mountain as a function of the horizontal aspect ratio

The influence of the obstacle shape, expressed through the ratio of spanwis e to streamwise extension beta, on flow over and around a mesoscale mountai n is examined numerically. The initial wind U as well as the buoyancy frequ ency N are constant; the earth's rotation and surface friction are neglecte d. In these conditions the flow response depends primarily on the nondimens ional mountain height H-m = h(m)N/U (where h(m) is the maximum mountain hei ght) and the horizontal aspect ratio b. A regime diagram for the onset of w ave breaking, lee vortex formation, and windward stagnation is compiled. Wh en beta is increased, smaller H-m are required for the occurrence of all th ree features. It is demonstrated that lee vortices can form with neither wa ve breaking above the lee slope nor upstream stagnation. For beta less than or equal to 0.5 a vortex pair can appear although the isentropes above the lee slope do not overturn for any H-m. For beta> 1, on the other hand, lee vortex formation is triggered by wave breaking. On the windward side two d istinct processes can lead to a complete blocking of the flow: the piling u p of heavier air ahead of the barrier and the upstream propagation of colum nar modes, which are generated by the wave breaking process for beta> 1. "H igh-drag'' states and "downslope windstorms'' exist above the threshold of wave breaking as long as no lee vortices appear (or, at least, as long as t hey are very small). Hence, the interval of H-m where a high-drag state occ urs becomes progressively larger for larger beta. With the growth of lee vo rtices the maximum wind speed along the leeward slope is dampened. The norm alized drag drops rapidly below its linear counterpart and asymptotically a pproaches zero.