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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.

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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.  

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.

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Fig8.1.16.3-10: The same case as in Fig8.1.16.3-8.  Top: Probability from ensemble 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 ensemble for the 10m wind (kt) and MSLP (hPa) at tropical cyclone 17W centre.  Tropical cyclone 17W is considered to have become extra-tropical by Day9 and central pressure and wind information is discontinued at that time.  HRES central pressure and winds diverge from the ensemble beyond Day5 and forecasts cease as winds fall below the threshold for a tropical cyclone to exist and the depression effectively transitions to become extra-tropical (or even non-existent) sooner than it appears to in most (~80%, see top row) of the ensemble members. 

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.

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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.  

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.

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Fig8.1.16.3-13: 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 ensemble mean and ensemble probabilities do not suffer from the same problem.  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.

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• See ECMWF Newsletter, Summer 2020 issue for further explanatory information regarding Tropical Cyclone Wind Forecasts.
• Read more about forecasting tropical cyclones in the medium-range.
  
• Watch a webinar on tropical products and skill (30sec delay before start)
• Read more about tropical cyclone products in a Newsletter feature.
• Read more about tropical cyclone predictions at ECMWF in a Newsletter feature.

(FUG Associated associated with Cy50r1)