Orographically trapped disturbances in the lower atmosphere: Scale analysis and simple models

Abstract

Orographically trapped disturbances (OTD) in the lower atmosphere propagate along sufficiently large mountain barriers and are trapped vertically by stable stratification and, laterally, by Coriolis effects against the mountains. Observed events have occurred along the coastal mountains of western North and South America, the Southern African coastal escarpment, the Australian Great Dividing Range, the Rockies and the Himalayas. Scale analysis indicates that, during generation, the fundamental parameters are the Rossby number and the ratio of the Rossby to the Froude number where the length scales in these numbers are the mountain half-width and height. Evaluation of these parameters indicates that only the Southern African case possibly remains in the quasi-geostrophic regime. For propagating disturbances, the observed asymmetry in the horizontal length scales implies that only the momentum balance in the across-mountain direction is likely to be geostrophic. In the along-mountain direction, the inertial, advective and frictional terms are also significant in the momentum balance.