The Role of Orography in the Dynamics of Storm Tracks

Abstract

This paper investigates the role of the Northern Hemispheric orography in the maintenance of the winter storm tracks with a dry balance three-layer hemispheric model. First, the zonal winter-mean flow in 1982?83, together with the corresponding orographically induced model planetary wave field, is shown to have comparable linear instability properties as the total observed winter mean flow. Then, the nonlinear version of this model is shown to be capable of simulating the Pacific and Atlantic storm tracks with considerable realism. The position of the Pacific storm track is slightly off and the intensity of the two model storm tracks is about 70% of the observed. While the localized baroclinic regions tend to be neutralized by the eddies in the model, they are sufficiently reestablished by the interaction between the slowly varying zonal flow and the Tibetan Plateau and the Rockies to maintain the storm tracks. Moist dynamics is therefore not believed to be essential for the existence of storm tracks. The constituent disturbances are intermittent wave packets with a well-defined baroclinic structure even at the center of the model storm tracks. They have a zonal group velocity statistically about three times faster than the zonal phase velocity. Their longitudinal length scale is significantly longer than that of the constituent disturbances in a leading unstable normal mode and their spatial structure is considerably more localized. It would therefore be oversimplifying to interpret the formation of storm tracks as directly associated with linear normal-mode instability. A complete diagnosis of the local energetics confirms the prevalence of highly localized baroclinic generation of eddy energy slightly downstream of the background jets. The redistribution of eddy energy to farther downstream, particularly by the advective effect of the background flow, is important.