Polar airstream cyclogenesis in the Australasian region: a composite study using ECMWF analyses

Abstract

Cold-air disturbances that form upstream of the main polar frontal cloud band and evolve into comma shapes are observed in almost any satellite image of the midlatitudes of the Southern Hemisphere. This study describes general features of the three-dimensional structure and dynamics of these polar airstream developments using composites from 30 typical cases from the Australasian area constructed from analyses prepared by the European Centre for Medium-Range Weather Forecasts (ECMWF). The major finding from this study is that an upstream upper-level southwesterly jet streak, present in each of the 30 cases at genesis, is necessary for development. -from Authors