A new scheme for the initialisation of tropical cyclones in the UK Meteorological Office global model

Abstract

AbstractFor several years the UK Met. Office has initialised tropical cyclones (TCs) in its global model by use of ‘bogus’ observations representing a simple symmetric vortex. Although this has yielded acceptable results, in terms of the verification of TC tracks in the short and medium range, it was felt certain improvements could be made. A new scheme has been developed which addresses two major deficiencies of the old scheme; the representation of the radial structure of winds and the asymmetry of the TC vortex. Preliminary tests of the new scheme resulted in the reduction of three‐day track forecast errors by 52%. Subsequently, a 19‐day trial of the new scheme running in parallel with the old scheme was undertaken. There were 12 active TCs during this period. The improvement in the track forecast verification statistics during the trial was striking. Analysis errors were cut by 65% (to 39 km); 72‐hour forecast errors fell from 528 km to 394 km and 120‐hour errors fell from 818 km to 440 km. The model's skill over a standard climatological/persistence forecast also improved dramatically. The new scheme was introduced operationally on 25 October 1994. Since then, mean forecast errors have been considerably lower than mean statistics from recent years.