Kelvin-Helmholtz waves in extratropical cyclones passing over mountain ranges

Abstract

Kelvin-Helmholtz billows with horizontal scales of 3-4km have been observed in midlatitude cyclones moving over the Italian Alps and the Oregon Cascades when the atmosphere was mostly statically stable with high amounts of shear and Ri<0.25. In one case, data from a mobile radar located within a windward facing valley documented a layer in which the shear between down-valley flow below 1.2km and strong upslope cross-barrier flow above was large. Several episodes of Kelvin-Helmholtz waves were observed within the shear layer. The occurrence of the waves appears to be related to the strength of the shear: when the shear attained large values, an episode of billows occurred, followed by a sharp decrease in the shear. The occurrence of large values of shear and Kelvin-Helmholtz billows over two different mountain ranges suggests that they may be important features occurring when extratropical cyclones with statically stable flow pass over mountain ranges.