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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.
Fig8.1.16.3-1: Reliability diagram of tropical cyclone strike probabilities, at day 10, for TCs in existence at T+0, for year July to June (years indicated by colours) showing reasonably good reliability (plots are near the diagonal), but a tendency towards over- confidence (plots to right of diagonal).
Fig8.1.16.3-2: Relative operating characteristics (ROC) diagram for tropical cyclone strike probabilities, at day 10, for TCs in existence at T+0, for year July to June (years indicated by colours) showing high ROC score (plots lie towards left and upper axes) with low False Alarm Rate and high Probability of Detection.
Fig8.1.16.3-3: IFS model forecast location error of tropical cyclones HRES (blue); Ensemble Mean (EM)(Yellow) and ensemble spread (Red dots). The diagram indicates that in 2016 the average location error and spread at:
- Day3: HRES location error about 180km; EM location error about 190km; ensemble spread about 190km.
- Day5: HRES location error about 340km; EM location error about 320km; ensemble spread about 310km.
Fig8.1.16.3-4: An illustration of what can happen when the model resolution is increased. HRES model performance Aug-Nov 2015 using test runs (~9km resolution - Red; ~16km resolution - Blue), both without ocean coupling. On average, ~9km resolution forecast location error is slightly better to about Day5 but marginally worse from Day5 to Day7. However, beyond about Day5 the low sample size makes statistics unreliable and ~9km resolution is unlikely to be significantly different to ~16km resolution. Users should note that this diagram is included only to illustrate that resolution changes have a significant impact. Before ocean-atmosphere coupling was introduced in June 2018 there was a tendency to over-deepen cyclones.
Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
Analysis Aspects
Interpretation of Increments near Tropical Cyclones
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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. The observations marked by increment vectors have, at least partially, been accepted and incline the analysis to better reflect the true structure of the atmosphere.
Fig8.1.16.3-6: Tropical cyclone strike probability for Hurricane Florence from 12UTC 6 Sep 2018 (left). Large upper troposphere increments are evident near the location of the hurricane during data assimilation (centre) and the related modifications to the IFS model atmosphere will have had some impact upon the subsequent forecast run at 00UTC 7 Sep 2018 (right).
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General Forecast Information
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. 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.
The forecast central pressure and track of a tropical storm can show quite large run-to-run variations. The example below shows HRES and ensemble member forecasts from two runs ( data times 12UTC 30 July 2017 and 12UTC 1 August 2017) to illustrate the differences that can occur. In fact neither HRES nor ensemble mean forecasts were correct. The weakening tropical storm actually moved northeast through central Japan. Outlying ensemble members proved the better guidance in each case.
Fig8.1.16.3-7: Strike probabilities for NORU up to 12UTC 8 August 2017 (T+240) based on ensemble and HRES forecast, data time 12UTC 30 July 2017. Note: HRES and Ensemble Control are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
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Extra-tropical 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).
Fig8.1.16.3-9: Strike probabilities for tropical cyclone17W up to 10 August 2017 (T+240) based on ensemble and HRES forecasts, data time 12UTC 1 August 2017. It is notable that the ensemble mean track (dotted line) and the tracks of the majority of ensemble members veer eastwards towards mid-Pacific as extra-tropical transition occurs while HRES (solid line) and a few ensemble members curve the low pressure centre westwards towards Japan.
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It is inadvisable to use strike probability values right at the edge of the domain, because there is no outside-domain detection of cyclones. The reduced resolution that is used (~45km) may be one reason.
Fig8.1.16.3-11: Strike probabilities of NORU up to 17 August 2017 (T+240) based on ensemble and HRES forecast, data time 00UTC 7 August 2017. ensemble 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 ensemble mean (dotted line), and the HRES (solid line), whereas the strike threat should be approximately centred on the ensemble mean track.
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Genesis: Tropical Cyclone Activity charts
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 misidentify 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 result will be spurious probabilities of tropical storm strikes. Future improvements to the technique will will aim to remove this problem.
Potential error in tracking TCs:
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Rarely two Tropical Storms are relatively close the possibility arises that the tracking algorithm may jump from one to another - e.g. if one is decaying. This seems to be a very rare occurrence and which is under investigation, but users should be aware of the potential for this error.
Fig8.1.16.3-12: Precipitation and surface isobaric forecast NW Australia and NE Indian Ocean T+156 VT12UTC 29 Feb 2020. TC Ferdinand is moving slowly off NW Australia and weakening. TC Esther is moving towards the SW and strengthening.
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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.
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