Finite amplitude stratified air flow past isolated mountains on an f-plane

Abstract

A time-dependent non-linear model was employed to simulate the adiabatic hydrostatic flow of stably stratified air crossing an isolated orography on the f-plane. The potential vorticity of the air is specified as a positive function of its potential temperature. The numerical model integrations are performed on a limited area domain with locally-determined inflow/outflow side boundaries.

The results for nearly steady states have been obtained for a number of isolated mesoscale orographic phenomena—including flow splitting, wave amplitudes, low-level jets, strong downslope winds, and anomalously high temperatures behind the mountains. The flow pattern is considered in relation to different values for the Rossby number, Ro, and the non-dimensional mountain height, H, with the non-dimensional stability fixed. The study indicates that for a sufficiently large H and Ro, most of the low-level air is diverted around the mountain, and the downslope flow on the lee side originates from potentially warmer upwind strata above the surface. The maximum magnitude (in degrees) of this warm zone increases for constant values of H with increasing Ro, and also for constant values of Ro as H increases. The downstream extent of the zone is of the order of the Rossby radius of deformation corresponding to the height of the mountain. The location of the minimum and maximum wind speeds is most sensitive to the value of Ro, and their strength is most sensitive to H.