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As regards the source of the large gust differences between E- and O-suites, these must relate to the gust parametrisation scheme, which contains 3 terms, a) the 10m mean wind, b) a turbulence factor, related to stability and c) a convective gust adjustment. The latter is only activated if the deep convection parametrisation is active (though this does not have to be surface triggered). It would seem that contributions from (c) or more likely (b) are making the E-suite gusts less. Vertical diffusion has changed in the E-suite and this may be relevant; this is now under investigation within Physical Aspects.
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: (a) a copy of the esuite run from 00 UTC on the 28th of october, (b) a copy of the osuite run from 00 UTC on the 28th of october, (c) a run with the esuite branch initialized from the analysis at 00 UTC on the 28th of october produced by the esuite, (d) a run with the osuite branch initialized from the analysis at 00 UTC on the 28th of october produced by the osuite. The differences in wind gusts at a short range (steps 3 and 4) between the 4 runs suggest that the differences between the esuite and the osuite 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 the four runs was the contribution of each of the three terms entering the gust parameterization to the differences in gusts between the esuite and the osuite. It appears that 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), i.e. in regions where there was no convection in the other run.
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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