Status: Ongoing analysis Finalized Material from: Linus, Ervin, Tim H., Ivan, Fernando, Mark
Discussed in the following Daily reports:
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http://intra.ecmwf.int/daily/d/dreport/2013/10/31/sc/
This case is documented in ECMWF Newletter 139 (Hewson et al): http://www.ecmwf.int/sites/default/files/Newsletter_139_smaller.pdf
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.
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The figures above shows the return period of the event from wind gust observations (left, limited number of stations) and HRES forecast compared to ERA Interim climatology (right). The two estimates agrees that the 10 year return period was exceeded along the North Sea coast.
On the 24 October, UKMO issued an amber warning (see figure above) for southern England for wind speeds. They classifed the potential impact as the highest category and the likelihood of the event as 2 out of 4. In their warning matrix it resulted in an amber warning.
2. Description of the event
Sequence is the Met Office surface analysis, with fronts, covering the period when the maximum gusts occurred . It shows the storm to be a small and rather innocuous looking feature! Note also that some of the charts - eg when the low was over Sweden - show a Keyser-Shapiro type structure, with a bent back cold front rather than an occlusion - implying a warm core.
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This figure shows the same as the figure above but zoomed in over Denmark.
3. Mesoscale Structure of the Storm
Animation - IR |
 Reading temp/humidity trace |
Animation - Reading SJ phase | 
Animation - Reading WCB phase
 Animation - time of Reading max SJ gust
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Part of Sting Jet conceptual model |
Damage Swathe conceptual model |
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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)
4.0 Data assimilation
The figures above shows MSLP forecast valid 28 October 00 UTC. The black contours shows forecasts from 27 Oct 00 UTC (left), 27 Oct 12 UTC (mid) and 28 Oct 00 UTC (right). The red contours are forecasts initialised 12 hours earlier. The figures therefore illustrates the shift between two consecutive forecasts. (The black contours become red in consecutive plot.) For the 27 Oct 00 UTC, the cyclone was shifted to the east and had not developed as much as in the previous forecast. For the forecast from 12 UTC (mid), the cyclone is even more shifted to east. For the last plot, where we compare the analysis from 28 Oct 00 UTC with the forecast from 12 hours earlier, the cyclone is shifted somewhat back towards west, although not a deep as in 26 Oct 12 UTC (red contour in the right plot). These shifts in the forecast had a big impact on the forecasts for the storm (see Section 4.2).
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The figures above shows the same plots but fore the e-suite. The there seems to be more consistency between the forecasts.
4.1 HRES
4.1.1 O-Suite
Sequence of forecasts below (left and second left columns) shows 48h maximum gusts from successive HRES forecasts from DT 00UTC 21st through to 00UTC 27th for VT period 06UTC 27th to 06UTC 29th (on 12up image left click, then right click -> view image to get full resolution). Note how the early forecasts jumped around, as one might expect. Later on very extreme gusts were forecast for areas SW of the UK, and as the event approached this zone of extreme gusts migrated its way eastwards, which is perhaps not what one would want/expect. End of period mslp is also shown.
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Forecasts |
Forecasts |
Model-based verification
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Model-based verification
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4.1.2 O-Suite vs E-suite
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.
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The figure above shows forecasts initialised 25 Oct 0 UTC (+84h).
4.1.3 Mesoscale Structure
Mean wind speeds and wind gusts are not just dependant on low depth. The multiple possible scenarios provided by HRES and ENS, in O- and E-suites, have shown considerable sensitivity, in different parts of the storm - that make the difference between a normal windstorm, and a once-in-a-lifetime event. Pinning this down is the real challenge. The following plots illustrate some of the issues - they have been extracted from animations in Ervin's daily reports, linked to above.
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Four supplementary short range forecasts runs were performed to see if any conclusions can be drawn about the gust differences between the E-suite and O-suite: (i) a copy of the E-suite run from 00 UTC on the 28th of october, (ii) a copy of the O-suite run from 00 UTC on the 28th of october, (iii) a run with the E-suite branch initialized from the analysis at 00 UTC on the 28th of october produced by the O-suite, (iv) a run with the O-suite branch initialized from the analysis at 00 UTC on the 28th of october produced by the E-suite. The differences in wind gusts at a short range (steps 3 and 4) between these runs suggest that the differences between the E-suite and the O-suite discussed above are partly due to the differences in the initial conditions. It is therefore difficult to attribute the changes in wind gusts discussed above to the changes to one of the parameterizations or to the data assimilation system. Another aspect examined in these four runs was the contribution of each of the three terms entering the gust parameterization to the differences in wind gusts between the E-suite and the O-suite. It appears that the terms (a) and (b) have relatively similar contributions to these differences, which is expected given they both depend on the mean wind. The convective term contributes on average less than the other terms, but can locally contribute more (up to 5-6m/s in this case), for e.g. in regions where there was no convection in the O-suite and there is convection in the E-suite, or the other way around.
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.
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With regard to ensemble spread, from a well-behaved ensemble, one would expect that, between consecutive runs, the cdf curves tend to become steeper, and gradually move less and less in the lateral direction as the event approaches. There are undoubtedly complicating factors for wind strength when one is close to the low track (which can for example give bimodal distributions = cdf jumps) but if one puts that aspect to one side it would seem that these EPS runs, in E and O-suites, are not behaving quite as they should. The inter-quartile range, which can be exactly inferred from these plots (and likewise the standard deviation which can be usefully estimated) appear not to be reducing with lead-time, and there are also notable lateral jumps in the profiles at shorter leads. A characteristic of a well-behaved ensemble is that measures of spread, for a given validity time, will reduce as the lead time reduces, i.e. for later and later forecasts.
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.
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The figures above show forecasts from 22 October 00 UTC (144 to 168 hour forecasts).
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).
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- The difference in wind speeds between e-suite and o-suite. This has been further evaluated with the ENS reforecasts. Comparing the 99th percentile of the wind speeds and wind gusts for October (in total 1920 forecasts), we see some signs of lower extreme winds in the e-suite. However, the magnitude of the difference is small (about 1 m/s0 and it is hard to verify whether the change is good or bad.
7. Additional material
Tendency plot for a few analysis cycles can be found here: http://intra.ecmwf.int/plots/d/inspect/_dir_dir_38R2_STORMS/dir_38R2_STORMS/Analysis/analysis_tendencies!38R2_20131027-20131027!mean!u!StormLocation!850!/