Showing papers by "Heini Wernli published in 1998"
TL;DR: In this paper, an analysis of four separate autumnal events of very heavy precipitation on the southern slopes of the European Alps indicate that each was accompanied by a strikingly similar anomaly at upper-tropospheric levels.
Abstract: Analyses of four separate autumnal events of very heavy precipitation on the southern slopes of the European Alps indicate that each was accompanied by a strikingly similar anomaly at upper-tropospheric levels. It took the form of a narrow (∼500km), deep (∼4km) and elongated (∼2000 km) streamer of intruded stratospheric air [sic. high potential vorticity air] extending north-south from the British Isles to the western Mediterranean. In each case the streamer translated comparatively slowly eastward, and the storm event ensued as its forward flank approached the Alpine ridge. Empirical indicators and theoretical considerations support the thesis that the streamer is an ubiquitous, dynamically significant, and distinctive precursor of storms on the Alpine southside.
227 citations
TL;DR: In this paper, the effect of barotropic shear on upper-level induced cyclogenesis was studied using idealized numerical experiments within the frameworks of semigeostrophic and primitive equation dynamics.
Abstract: Idealized numerical experiments within the frameworks of semigeostrophic and primitive equation dynamics were performed to study the effect of barotropic shear on idealized upper-level induced cyclogenesis. Localized finite-amplitude potential temperature anomalies were used as initial perturbations, and the atmosphere was considered as a dry frictionless fluid of uniform quasigeostrophic potential vorticity on an f-plane. It is demonstrated that the main features of the numerical simulations are in essence unaffected by the choice of the dynamical framework. They comprise, for instance, the development of elongated cold fronts under anticyclonically sheared conditions, a “T-bone” shaped frontal palette in the unsheared case (cf. Shapiro and Keyser), and a Bergen-type occlusion process in the simulations with cyclonic shear. This confirms the profound dynamical influence of lateral shear in the background environment upon the resulting surface cyclone and frontal structures (and the accompanying ...
47 citations