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General Forecast Information

CONTROL-10 versus ENS

One of the consequences of the high spatial resolution of HRE is that, compared to the ENS, it responds more readily to the changes in the evolving structure of the modelled atmosphere.  This can be especially evident when there is extensive active convection, and the mean sea level pressure in the vicinity can often be forecast to fall, sometimes steeply, with a consequent increase in the low altitude wind circulation (for developing tropical storms). The track of these low pressure centres can vary significantly from run to run according to variations in the low pressure structure. Users should always compare HRES forecasts with corresponding ENS member forecasts.  Nevertheless, HRES should generally be considered as a more probable member of the ensemble, particularly with regard to central pressure, and should not be discarded just because of major differences in intensification rates or in the subsequent low track.  Indeed HRES can often give early indication of the development of small or vigorous circulations and the associated strong and potentially dangerous winds.

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Fig8.1.10.3.5: Tropical cyclone strike probability for Hurricane Florence (left, date and time given above).  HRES forecast values are shown by a black line in the graphs on the right of the charts.  Plots for the central pressure HRES (black line) are lower than the lowest values from ENS (box and whisker plot) with winds correspondingly greater than indicated by ENS.  This can be attributed, at least in part, to the finer resolution of HRES leading to more detailed (perhaps more active) forecast convection.  It is noticeable the effect stops or is reversed as the hurricane is forecast to make landfall on Friday 14 Sep, significantly reducing convection.  Winds decrease due to the weakening of the hurricane and also because of the increased surface friction over land which may be better modelled by HRES.  Similar effects can be seen in the forecast pressures and winds associated with Typhoon Mangkhut (right).

Forecast Consistency

Tropical storms are highly energetic and small differences between the analyses and background fields in sequential runs, or small perturbations developed by the IFS models in the circulating flow, can have a major impact on the forecast evolution.  This is especially true with the higher resolution of the HRES which can show major differences in depth and track of the the tropical storm, particularly later in the forecast, when compared with previous HRES runs or ENS.   It is important therefore the circulating flow, can have a major impact on the forecast evolution.  It is important to monitor the progress of forecasts of tropical storms and assess critically changes over a series of runs.  Some possible problems are illustrated below.

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Extratropical transition of tropical storms is, in general, difficult for NWP models to deal with.  NWP models often show large run-to-run variability in forecast track, movement and depth of a tropical depression as it moves into the mid-latitudes.  Small differences between the analyses and background fields in sequential runs, or small perturbations developed by NWP models in the circulating flow, can have a major impact on the forecast evolution.   The higher resolution of the HRES can induce spurious developments not indicated by lower resolution models (e.g. ENS).  This  This is especially so with energetic systems, such as well developed tropical storms, and can make some forecast aspects unsafe later in the forecast period (say beyond Day5).

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Although often very useful it should be remembered that these are an experimental product based upon identification of the warm-core circulation during the forecast period.  The technique can mis-identify as a tropical cyclone a high-latitude circulation containing something of a warm core (e.g. a well occluded frontal depression with cooler air encircling some warm, moist air remnants near the centre). The  The result will be spurious probabilities of tropical storm strikes.  Future improvements to the technique will will aim to remove this problem.

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Once official reports signify the existence of a tropical cyclone, it is automatically tracked.  The tracking algorithm uses HRES output and is based on using the extrapolation of past movement and the mid-tropospheric steering flow to obtain a first-guess position 6 hours into the future.  The actual forecast position is then determined by searching for mean sea level pressure (MSLP) and 850hPa vorticity extremes around the first-guess position.  In some circumstances the thickness maximum, the central MSLP, surface winds, and the orography are also considered in the evaluation.  This process repeats at 6 hour intervals through the forecast until either the tropical cyclone dies or the end of day10 is reached.

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