Break-up of a stratospheric streamer observed by MST radar

The break-up of a stratospheric streamer over the British Isles is describe d, using satellite, Mesosphere-Stratosphere-Troposphere (MST) radar and in situ observations together with European Centre for Medium-Range Weather Fo recasts analyses. The streamer started as a thin ribbon of high potential v orticity to the west of the British Isles. As this was strained by the back ground flow it broke up into a series of mesoscale vortices. MST radar and ozonesonde observations delineated a tropopause fold to the west of the ini tial streamer, taking the form of a thin layer of stratospheric air embedde d in a deep layer of descending upper-tropospheric air. The fold appears to have been mixed out by small-scale turbulence as the streamer broke up. Th is break-up also generated strong inertia-gravity waves, which broke just a bove the tropopause introducing extensive mixing to the lowermost stratosph ere. One of the mesoscale vortices returned over the MST radar, showing a v ery indistinct tropopause typical of a synoptic-scale cut-off low. This Vor tex was also responsible for the development of a rain band on its eastern flank, which brought significant rainfall to the British Isles at a time wh en the synoptic flow pattern was strongly anticyclonic.