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. Composites of initial stages showed the surface vorticity center coupled with the poleward exit region of this upper jet and directly beneath the cyclonic-vorticity advection maximum by the 300-hPa flow relative to the translating disturbance. This net import of cyclonic vorticity acts initially to amplify the upper trough. Near the surface, the environment is characterized by locally enhanced baroclinity and heating in strong equatorward flow. Disturbance intensification is through the basic baroclinic self-development process, with strongest cyclogenesis coinciding with the passage of the jet through the equatorward semicircle. Tropospheric heating, probably related to cumulus clouds, is the dominant thermodynamic process at early stages, and there is some evidence that shallow baroclinic processes not directly related to the upper jet may result in initial disturbance formation. Diabatic heating associated with large-scale ascent contributes to development at later stages. The composite of the more vigorous cases exhibited a significantly stronger upper jet, lower static stability, and greater surface heating than weaker cases. However, development rate appears to be unrelated to upper-trough amplitude, as measured by flow curvature.