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Break‐up of a stratospheric streamer observed by MST radar

The break‐up of a stratospheric streamer over the British Isles is described, using satellite, Mesosphere—Stratosphere—Troposphere (MST) radar and in situ observations together with European Centre for Medium‐Range Weather Forecasts analyses. The streamer started as a thin ribbon of high potential vorticity to the west of the British Isles. As this was strained by the background flow it broke up into a series of mesoscale vortices. MST radar and ozonesonde observations delineated a tropopause fold to the west of the initial streamer, taking the form of a thin layer of stratospheric air embedded 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. This break‐up also generated strong inertia‐gravity waves, which broke just above the tropopause introducing extensive mixing to the lowermost stratosphere. One of the mesoscale vortices returned over the MST radar, showing a very indistinct tropopause typical of a synoptic‐scale cut‐off low. This vortex 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 when the synoptic flow pattern was strongly anticyclonic.