Fragmentation of stratospheric intrusions

Evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels. These features are related to, and evolve from, the classical V‐shaped intrusions of stratospheric air down to tropospheric elevations. It is shown that intrusions can develop into elongated (∼2000–3000 km) and slender (∼200 km) streamers, and that thereafter such a streamer can roll up to form a train of stalactite‐shaped vortex subentities with an accompanying substantial thinning of the intervening filament. In addition there are indications that the vortices themselves can develop a spirallike interior structure of interleaved stratospheric and tropospheric air. These inferences are based upon two independent but complementary sources: analysis of the potential vorticity distribution on tropopause transcending isentropic surfaces derived from the analysis fields of the European Centre for Medium‐Range Weather Forecasts either directly, or indirectly using a contour advection technique; and imagery from the water vapor channel of the European Space Agency Meteosat 4 satellite. Streamers were observed to occur with a frequency of approximately one per week over central and southern Europe during the winter of 1991–1992. The fragmentation is linked to the instability or self‐development of a filament of enhanced potential vorticity and it can modify or instigate surface weather systems. Moreover, by inducing a substantial and rapid enlargement of the intrusion's surface area it greatly enhances the potential for local irreversible mixing of stratospheric and tropospheric air.