OROGRAPHICALLY TRAPPED DISTURBANCES IN THE LOWER ATMOSPHERE - SCALE ANALYSIS AND SIMPLE-MODELS

Orographically trapped disturbances (OTD) in the lower atmosphere prop agate along sufficiently large mountain barriers and are trapped verti cally by stable stratification and, laterally, by Coriolis effects aga inst the mountains. Observed events have occurred along the coastal mo untains of western North and South America, the Southern African coast al escarpment, the Australian Great Dividing Range, the Rockies and th e Himalayas. Scale analysis indicates that, during generation, the fun damental 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 indicat es that only the Southern African case possibly remains in the quasi-g eostrophic regime. For propagating disturbances, the observed asymmetr y in the horizontal length scales implies that only the momentum balan ce in the across-mountain direction is likely to be geostrophic. In th e along-mountain direction, the inertial, advective and frictional ter ms are also significant in the momentum balance.