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Once official reports signify the existence of a tropical cyclone, it is automatically tracked.  The tracking algorithm uses CONTROL-10/ 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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ECMWF ensemble forecast with data time 00UTC 08 March 2017.  Time series (Fig8.1.10.1.2C) are plotted for the ensemble mean (black dotted) and CONTROL-10/ HRES (black solid); this convention is is also used on the map track plot (Fig8.10.1.2B).  On the individual trajectories plot in Fig8.10.1.2A the CONTROL-10/ HRES track is shown instead of the ensemble mean.

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Fig8.1.10.1.2B(Centre):  Strike probability chart. Colours show the proportion of members that predict that the centre of Tropical Cyclone 07S will pass within a 120km radius at any time during the next five days.  Ensemble mean (dotted line) and CONTROL-10/ HRES (solid line).

Fig8.1.10.1.2C(Right):  Top: Ensemble probabilities (not including CONTROL-10/ HRES) for the intensity of this tropical cyclone 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 the full ensemble (in box and whisker format), the ensemble mean (dotted line) and CONTROL-10/ HRES (solid line) for the maximum 10m mean wind speed (kts) associated with tropical cyclone 07S, and MSLP (hPa) at the Tropical Cyclone 07S pressure centre.  Note the categories used on the top panel are derived on the basis of the variable shown on the middle panel.

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Fig8.1.10.1.4: ecChart showing the ensemble member tracks of TC03S up to 1 February 2017 (T+72)  and the associated probabilities of significant wave height >4m based on ensemble members (probability given by colour in the scale below the chart).  TC tracks from the ensemble forecast sequence are shown as thin lines (fading for older times) and CONTROL-10/ HRES  forecast sequence as the black line.    Data time of forecast, 12UTC 29 January 2017.

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Fig8.1.10.1.5A(Right): Locations of TC Ferdinand from CONTROL-10/ 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 forecast values of ensemble mean and ensemble probabilities do not suffer from the same problem.

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