Statistical Behavior of Transient Eddies near Mountains and Implications for Theories of Lee Cyclogenesis

Abstract

We examine the nature and spatial structure of meteorological high-frequency variability in two selected areas, one including the Alps and the other the Rocky Mountains. Seven years of geopotential height data, derived from ECMWF analysis set, have been filtered in order to remove periods longer than 6?7 days. Statistical analysis performed on the filtered series include one-point lag correlations and composites that reveal changes in the typical horizontal structure and path of traveling eddies as they move from the oceans to the adjacent mountainous regions. The important modifying action of the orography is shown, and lee cyclogenesis is interpreted as one aspect of such action. Cross-correlation maps between upper- and lower-tropospheric levels reveal the effect that mountains also exert on the vertical structure of baroclinic eddies. These statistical results are discussed and compared with those predicted by theories of cyclogenesis in the lee of the Alps and the Rocky Mountains, and by theories of propagation of disturbances in the vicinity of large-scale mountains. It is shown that the normal mode theory of baroclinic waves in the presence of mountains is capable of predicting most of the observed features, considering both lee cyclogenesis and eddy propagation and deformation near mountains as different aspects of the interaction of high-frequency eddies with orography.