Linear Results on the Barrier Effects of Mesoscale Mountains

Abstract

We examine the factors determining whether a mountain acts as a strong barrier to an impinging flow. A primary concern is the extent to which the barrier effect is reduced when smoothed orography is used in a numerical model. These questions are addressed within the model of linear, rotating, stratified flow over topography first studied by Queney. The ground level Green's functions for this model are derived and their near- and far-field asymptotic behavior is discussed. Using the asymptotic results, the upstream deceleration is estimated as a function of Ro=U/fL and hm=Nhm/U, where U is the far upstream speed of the cross-mountain flow, f the Coriolis parameter, L the mountain width, N the Brunt-Väisälä frequency, and hm is the maximum mountain height. Effects of terrain shape are also considered. The Green's functions are evaluated numerically and used to calculate the response to a family of mountain profiles; a comparison with asymptotic results shows the latter to be very useful. It is concluded that preserving maximum terrain height of ridgelike features is superior to preserving an integrated quantity such as mountain volume. Both the Alps and the Rocky Mountains exert a pronounced barrier effect which would not generally be preserved in present numerical models using smoothed orography.