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Satellite images from yr.no valid 9,12 and 18 UTC on the 28 October.
Animation - IR |
Reading wind trace |
Reading temp/humidity trace | |
|---|---|---|---|
Animation - Reading WCB phase |
Animation - Reading SJ phase |
Animation - time of Reading max SJ gust | |
Part of Sting Jet conceptual model |
Damage Swathe conceptual model |
The 8 panels above relate to the mesoscale structure of the storm as it passed over the UK (and beyond). We refer here to the warm conveyor belt (WCB), sting jet (SJ) and cold conveyor belt (CCB).
The IR The satellite sequence shows, perhaps unsurprisingly, all the hallmarks of an extreme extra-tropical cyclone, including evidence of a sting jet SJ event. Notably, fingers of cold top cloud continue to emerge forwards from the tip of the cloud head, but evaporate as they descend (the 'smoking gun' effect). This happens first across S England, then on across the N Sea, and finally over S Denmark; thereafter there is more evidence of cold top cloud surrounding the cyclone, as the cold conveyor phase of the maturing cyclone takes over. The cyclone seems to fit the model proposed by Keith Browning in his October 1987 sting jet (QJRMS) paper very well, and also the model of related 'damage swathes' shown on the final panel. As well as the evaporating cloud fingers note the evidence on IR of sheets curved slabs of slantwise upward motion extending NW from the low centre, with dry gap(s) inbetween. A second sheet new 'slab' develops at the start of the IR sequence, and relates also to a propogating band of intense rain at the surface - note the narrow band of bright echoes in the northern portion of all the radar sequences. In the SJ conceptual model this equates to one of the dark-shaded arrows on the top panel. The lighter shaded arrows, meanwhile, denoting evaporating descending branches, correspond to the dry gaps on the radar sting jet sequence, two of which are highlighted on the 'Reading max SJ gust' panel. It is expected from modelling studies performed by Pete Clark, that these descending branches will be the source of the main SJ gusts at the surface, and so it proves for this case, at least at the Reading university site, as the traces and radar show. Detailed analysis of the storm of October 30th 2000 has shown a similar correspondance.
The maximum Reading gust for this case came in the warm sector, in WCB flow. It would appear from radar sequence that this may have been convectively enhanced. The sudden nature of the gust and the corresponding lack of increase in mean winds suggests that this was perhaps a finely balanced situation, where stability was, for the most part, just inhibiting the downward propogation of momentum needed for strong gusts.
So why was the maximum Reading gust not in the SJ phase? It is probably because the SJ was only just starting to form. Later on SJ gusts exceeded 40m/s in places, as charts below show.
Notable for this event, compared to others I have looked at, is the duration of the apparent sting jet period - almost 12 hours it seems, rivalled only by Oct '87 (also about 12 hours). Caveats are need here though, as I am basing all this largely on IR sequence interpretation - ideally we would like to see trajectory analysis in very high resolution runs to complement this.
A key point for ECMWF is whether or not the IFS can capture the strength of gusts characteristic of the sting jet phaseobserved, be it in the WCB or SJ phases (and indeed CCB, though that is not really looked at here).
The figure above shows the maximum reported wind gusts (coloured numbers) for the 28 October and the MSLP for every 6th hour (contours).
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