Windstorm - 201310 - St Jude / Simone / Christian

Status: Ongoing analysis Material from: Linus, Ervin, Tim H.

Discussed in the following Daily reports:

http://intra.ecmwf.int/daily/d/dreport/2013/10/23/sc/

http://intra.ecmwf.int/daily/d/dreport/2013/10/24/sc/

http://intra.ecmwf.int/daily/d/dreport/2013/10/25/sc/

http://intra.ecmwf.int/daily/d/dreport/2013/10/28/sc/

http://intra.ecmwf.int/daily/d/dreport/2013/10/29/sc/

1. Impact

On the 28 October a wind storm hit north-western Europe. In total 14 people were killed across Europe (http://www.bbc.co.uk/news/world-europe-24705734). The main affected countries where France, UK, The Netherlands, Germany, Denmark and Sweden. In Denmark the higher ever wind gust (for Denmark) was measured on Kegnäs on Als (53 m/s).

2. Description of the event

Analyses for MSLP and Eady Index (showing baroclinic zones) from 26 Oct 0 UTC to 28 Oct 12 UTC, every 12 hours. Here we see the baroclinic zone over the Atlantic and a wave in the surface pressure field moving to the east. A rapid development happened after 28 Oct 0 UTC.

Satellite images from yr.no valid 9,12 and 18 UTC on the 28 October.

The figure above shows the maximum reported wind gusts (coloured numbers) for the 28 October and the MSLP for every 6th hour (contours).

This figure shows the same as the figure above but zoomed in over Denmark.

3. Mesoscale Structure of the Storm

The 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 sequence shows, perhaps unsurprisingly, all the hallmarks of an extreme extra-tropical cyclone, including evidence of a 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 curved slabs of slantwise upward motion extending NW from the low centre, with dry gap(s) inbetween. A 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 observed, be it in the WCB or SJ phases (and indeed CCB, though that is not really looked at here).

The figures above show the maximum mean (10 minutes) wind speed (left) and wind gusts (right) for Denmark (from DMI). Here we see that the highest wind speeds occurred in the south.

The figures above shows time-series of the mean wind from Kegnaes Lighthouse in the south-west of Denmark and Drogdens Lighthouse located in the east (a little bit south of Copenhagen). The time-axis is in Central European time. For both stations we see a rapid increase in the wind speeds. The increase happened above 2 hours later in the east than the west.

For hour by hour plots of MSLP and wind gusts from SMHI surface analysis (MESAN), see  http://www.smhi.se/klimatdata/meteorologi/vind/storm-okt-2013 .

4. Predictability (including 40r1 E-suite evaluation)

4.1 HRES

The figures below shows forecasts of MSLP valid for 28 Oct 12 UTC and maximum wind gust during the 28 October. The o-suite is plotted in the left column and the e-suite in the right.

The figure above shows forecasts initialised 28 Oct 0 UTC. For southern England, the e-suite has less intense wind gusts than o-suite. For souther Scandinavia, it seems like the band of the highest wind gusts are a little bit to far north for both suites. In the morning on the 28th, SMHI (Swedish met service) issue a red warning for the Gothenburg area but later they had to move the red warning to the Malmo area.

The figure above shows forecasts initialised 27 Oct 0 UTC (+36h).

The figure above shows forecasts initialised 26 Oct 0 UTC (+60h). Here a large area west of England had very high wind gusts, what did not happened. We also see that the cyclone centre is more to the west compared to later runs. Both these results is due to a development of the cyclone too far west.

The figure above shows forecasts initialised 25 Oct 0 UTC (+84h).

4.2 ENS

The sequence of figures above shows the strike probability of cyclone with maximum wind speed (mean) over 60 kt (31 m/s) at 1 km height. The plots are valid for the 28 October (24-hours). The first forecast is from 28 Oct 0 UTC and one day is added for each plot. With longer lead time the feature is delayed (too slow) in the model, but the path of the cyclone seems very consistent up to 6 days before the landfall.

The figures above show cyclones in the ensemble (dots), where the colour indicates the strength of the maximum wind speed at 1 km height connected to the feature. All the plots are valid 28 October 12 UTC. The contour shows the MSLP for the control forecast. The forecasts are from 0UTC runs starting from 28 Oct (top-left) to 23 Oct (bottom-right). With increasing lead times, the spread of the features over the North Sea increases. We also see that the centre of gravity for the red-orange dots moves westward with increasing lead time, illustrating the too westward development in the model.

ADD EFI PLOTS

CDF for wind gusts (left) and maximum mean wind speed (right) for Reading valid on the 28 October. Different colour represents different initial times and the o-suite (38r2, solid) and e-suite (40r1, dotted). Comparing different initial times, the forecast from 26 October (green and black), were the most severe. For the forecasts from the 27 October the wind speed was less. This is partly because the development of the cyclone occurred later and therefore had not reached such a deep stage when it passed over Reading. Comparing e-suite and o-suite, we see that the winds (both mean and gusts) are less in the e-suite and o-suite.

4.3 Monthly forecasts

We cannot expect to see a strong signal for extreme cyclones in the monthly forecast. However, we can investigate whether the environment was favourable for windy conditions. The figures above show the weekly MSLP anomaly for the week starting on the 28 October. The first figure shows the forecast from the 28 October (Monday of the verifying week), followed by 24 Oct, 21 Oct, 17 Oct, 14 Oct and 10 Oct. At least for the 5 first forecasts a positive NAO signal (negative MSLP anomaly in the northern Atlantic and positive further south) is present. A positive NAO is usually leading to stronger winds over western Europe.

4.4 Comparison with other centres

CDF for 10-metre mean wind for Reading (left) and a point (55N, 9E) in western Denmark(right). The data is of the maximum for 4 time step valid 28 Oct 0, 6,12 ,18 UTC and 29 Oct 0 UTC. The different colours represents different centres in the TIGGE archive. Two different initial times are plotted, 24 Oct 00 UTC (dotted) and 26 Oct 00 UTC (solid). For Reading the maximum mean wind was 10 m/s and the point in Denmark lies within the area of 25-30 m/s. The results for ECMWF is not convincing, especially for the Danish point; the ECMWF forecast has the lowest wind speeds for both initial times. However, there could be a mixture of land points and sea point between the difference centres. The figure below shows therefore the mean wind on 850 hPa for the same point. Here, at least for the 26 October, ECMWF shows the highest wind speeds. For the 24 October it could be the case that the timing error led to that the cyclone had not reached this point within the window for the diagnostics.

5. Experience from general performance/other cases

• The sting jet of the 4 January storm over Scotland 2012. See attached poster from Tim Hewson.

6. Good and bad aspects of the forecasts for the event

• The early signal of the storm in the forecasts (from ~6 days before the event).

• The storm developed too far west in the forecasts, and therefore over-forecast the intensity for western England.
• Also for the very short forecast, the path of the strongest winds where too far north over Denmark and Sweden.
   

• The difference in wind speeds between e-suite and o-suite.

7. Additional material