Evaluation of and improvements to the Met Office tropical cyclone initialisation scheme

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AbstractIn 1994 a new method for the initialisation of tropical cyclones (TCs) was implemented in the Met Office Unified Model (MetUM). In trials this resulted in a 34% drop in track forecast errors. TC track forecast errors in the MetUM have continued to show a modest downward trend since then. However, the considerable increase in satellite data available to numerical models and improvements in data assimilation techniques are likely to have reduced the model's dependency on the initialisation technique to produce a good analysis and forecast of TCs.The global configuration of the MetUM was rerun for a period of a month during August and September 2006 with TC initialisation switched off. The results show that the initialisation technique still produces a mean decrease in track forecast error of 12.2%. In addition, the detection rate of TCs 96 h into the forecast dropped from 97 to 75% and the mean forecast intensity of TCs dropped by 20.6% when the initialisation technique was not used.A subsequent trial was undertaken of a change to the technique to reduce the areal coverage of the ‘bogus’ data produced for all but the largest TCs. This change was proposed in order to address a perceived weakness in the original formulation of the initialisation technique. The revised formulation produced a further reduction in track forecast errors of 4.7% for the August–September 2006 trial period. As a consequence of these good results, the revised formulation became operational in the MetUM in November 2007. © Crown Copyright 2009. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd