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In general, analysis of IFS model performance suggests:

• CONTROL-10/HRES HRES (and CTRL) errors in forecast positions of tropical cyclones are on average less than 200 km at Day3 and less than 350 km at Day5.
• Broadly, the IFS models tend on average to move tropical cyclones rather too slowly (by about 1-2 km h^-1).
• Strike probabilities seem to be a little high.  This applies to TCs in existence at T+0, but in in view of some conflicting results this topic is currently undergoing further investigation (Nov 18).  ECMWF plans to start verifying the 'activity' product soon.
• The IFS models handle rapid intensification of tropical cyclones rather poorly.  It is quite common to see an under-estimate of the speed of intensification during periods of intensification.
• CONTROL-10/HRES HRES (and CTRL) errors in central pressure are erratic.  They have a relatively small average bias though tend to over-deepen the central pressure.  The ENS Control run is generally not deep enough by around 15 hPa.   
• There is on average an under-estimation of maximum winds in the circulation, particular during periods of rapid intensification.
• Results are mixed regarding genesis of tropical cyclones:

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Fig8.1.10.3.2: IFS model forecast location error of tropical cyclones (CONTROL-10/HRES HRES (and CTRL)) - blue; Ensemble Mean (EM) - Yellow)  and ENS spread (Red dots).  The diagram indicates that in 2016 the average location error and spread at:

• Day3: CONTROL-10/HRES HRES (and CTRL) location error about 180km; EM location error about 190km; ENS spread about 190km.
• Day5: CONTROL-10/HRES HRES (and CTRL) location error about 340km; EM location error about 320km; ENS spread about 310km.

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Fig8.1.10.3.3: An illustration of what can happen when the model resolution is increased.  CONTROL-10/HRES HRES (and CTRL) 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 to illustrate that  resolution changes have a significant impact.

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• the threat area in eastern USA is broadened.
• there is less chance that the hurricane track will curve northwards off the eastern seaboard.
• the southern flank of the threat extends southward into Florida.
• CONTROL-10/HRES ( and ENS mean) make more progress westwards into USA.

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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 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:

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.

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.

Fig8.1.10.3.9: 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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