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Considerations when using Tropical Cyclone products
Overall Performance
In general, analysis of IFS model performance suggests:
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For tropical cyclone motion, in addition to a known slow bias, the largest errors usually arise in cases when the tropical cyclone is gaining latitude. Forecasts of both the position and intensity are also dependent on the initial motion, structure, and intensity of the tropical cyclone.
Characteristics regarding Forecast Movement
It has been found that the errors in forecast position and movement of tropical storms are:
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- moderate or strong - the tropical storm moves realistically.
- weak - the tropical storm moves slowly and more erratically, and especially so if a deep tropical storm.
Forecast Error Statistics for Pre-existing Tropical Cyclones
The figures below provide verification examples, over multiple or single cases, illustrating typical IFS model characteristics, be they strengths or weaknesses. Key points for the forecast user are highlighted in the captions.
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Before ocean-atmosphere coupling was introduced in June 2018 there was a tendency to over-deepen cyclones.
Interpretation of Increments near Tropical Cyclones
Large vector increments suggest a sensitivity to observations and hence the possibility of fast growing errors. Wind increments also impact upon the thermal structure of the model atmosphere. Users should consider the potential for increased uncertainty in the subsequent evolution.
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Users should consider the potential for increased uncertainty in the forecast (perhaps beyond that shown by the ENS) when large increments are indicated.
Impact of Scatterometer Winds on Tropical Cyclone Analyses
Scatterometer winds are only incorporated into analyses at low resolution. Moreover correct interpretation of wind direction ambiguities relies upon a correct background wind pattern derived from earlier forecast runs which of course may differ from reality. In consequence an erroneous forecast can potentially be "reinforced" if incorrect wind directions are assigned. In turn this can degrade the analysis quality for tropical storms (or conceivably also intense depressions). Viewed separately, scatterometer winds can give the user an indication of the true severity of winds associated with low pressure features (but note that there is an upper limit for their utility).
General Forecast Information
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. 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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- The earlier forecast has a 30% threat to coastal central eastern Japan and Tokyo, 10%-20% threat to the islands south of Kyushu, but little threat to the Sea of Japan.
- The later forecast has <10% threat to eastern Japan and Tokyo but 30%-40% threat to the Sea of Japan (and 40%-50% in the south), and 70%-80% threat to the islands south of Kyushu.
Extratropical Transition
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. 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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Fig8.1.10.3.7B(Right): The same case as in Fig8.1.10.3.7A. Top: Probability from ENS members for tropical cyclone 17W to fall into each of the 5 tropical cyclone intensity categories shown at 6hr intervals to 10 days. Centre and Bottom: Lagrangian meteograms of distribution of the ENS for the 10m wind (kt) and MSLP (hPa) at tropical cyclone 17W centre. Tropical cyclone 17W is considered to have become extratropical by Day9 and central pressure and wind information is discontinued at that time. HRES central pressure and winds diverge from the ENS beyond Day5 and forecasts cease as winds fall below the threshold for a tropical cyclone to exist and the depression effectively transitions to become extratropical (or even non-existent) sooner than it appears to in most (~80%, see top row) of the ENS members.
Misleading Indications of Threat areas.
The strike threat areas sometimes appear shifted away from the track of the centre shown by the ENS mean (dotted line) and the HRES (solid line). This appears to be a plotting problem and can occasionally be very misleading. The problem will be rectified in future updates.
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Fig8.1.10.3.8: Strike probabilities of NORU up to 17 August 2017 (T+240) based on ENS and HRES forecast, data time 00UTC 7 August 2017. ENS mean track (dotted line) and HRES track (solid line). Crosses mark previous positions of NORU. The strike threat area is shifted to the northwest of the forecast track of the centre shown by the ENS mean (dotted line), and the HRES (solid line), whereas the strike threat should be approximately centred on the ENS mean track.
Genesis: Tropical Cyclone Activity charts
Potential error in tracking TCs:
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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Fig8.1.10.3.10: Locations of TC Ferdinand from HRES forecast DT 00UTC 23 Feb 2020 (solid line on chart). At 12UTC 29 Feb 2020 the algorithm incorrectly identifies the location of Ferdinand at the location of TC Esther and subsequently follows the movement and developments of TC Esther. The sudden change in the HRES forecast values of central pressure (much lower) and 10m wind (much higher) are shown on the respective graphs - these correspond to TC Esther. On this occasion the forecast values of ENS mean and ENS probabilities do not suffer from the same problem.
Ocean Waves in association with Tropical Cyclone forecasts
A limit is placed on the roughness length scale parameter in order to avoid the effect of too much drag from the sea surface in the lower atmosphere and enable more realistic (stronger) winds to be forecast in the vicinity of relatively intense tropical cyclones. It should also be noted that tropical cyclone development with strengthened winds has only a limited effect upon the size and character of waves developed by the Wave model. The model change to limit the roughness length scale at very high speeds was introduced in cycle 47r1 in June 2020. See Section on Waves near tropical storms.
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Note: IFS products on these pages regarding tropical cyclones are generated automatically without any editing by forecast experts. RSMCs (Regional Specialized Meteorological Centres) have ultimate responsibility for official forecasts of tropical cyclones within their respective regions (ECMWF is one of a number of centres that provide data to them). Up-to-date information is available by direct access to official RSMC forecasts through the WMO Severe Weather Information Centre. For up-to-date forecast information for their own local area users should refer to forecasts from their own National Meteorological Service.
Additional Sources of Information
(Note: In older material there may be references to issues that have subsequently been addressed)
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