MAY 1998 HEMING AND RADFORD 1323 The Performance of the United Kingdom Meteorological Of®ce Global Model in Predicting the Tracks of Atlantic Tropical Cyclones in 1995 J. T. HEMING AND A. M. RADFORD Meteorological Of®ce, Bracknell, Berkshire, United Kingdom (Manuscript received 3 September 1996, in ®nal form 6 May 1997) ABSTRACT In 1994 the United Kingdom Meteorological Of®ce (UKMO) introduced a new method for intitializing tropical cyclones (TCs) in its global model by use of ``bogus'' data. This has resulted in a major reduction in TC forecast errors and has made the model far more competitive in the ®eld of TC forecasting. The unusually active Atlantic TC season of 1995 produced a stern test of the qualities of the model forecasts. A full assessment of UKMO forecasts shows that the mean forecast errors for TCs in the Atlantic in 1995 were lower than mean errors for the UKMO model in any previous year. Errors were also lower than those of the European Centre for Medium- Range Weather Forecasts model at all forecast ranges shorter than 120 h. In addition, assessment carried out at the National Hurricane Center (NHC), Miami, found that UKMO forecasts were better than all forms of guidance available to NHC, although they were not available to NHC in real time. 1. Introduction The tangential wind speed at each point on the mesh is derived from a combination of TC advisory message in- The current con®guration of the United Kingdom Me- formation and real observations. An asymmetry to the teorological Of®ce (UKMO) global numerical weather wind ¯ow is achieved by imposing a steering ¯ow (past prediction (NWP) model has a horizontal grid spacing 6-h movement vector) on the tangential wind values. This of 139 km 3 93 km at the equator and has 19 levels is the best approximation of asymmetry given the data in the vertical (Cullen 1993). This resolution is not ®ne available in real time. This formulation is shown diagram- enough to represent the detailed structure of a tropical matically in Fig. 1 and is described in more detail by cyclone (TC) vortex. However, the position of a TC and Heming et al. (1995). In tests, this new technique produced the interaction of the environmental ¯ow upon it can track forecast error reductions of between 25% and 46%. be modeled with suf®cient accuracy so as to produce a A diagnostic study by Chan and Kwok (1996) found that useful NWP forecast of the track of the TC. these improvements resulted from a combination of better Since most TCs form in data-sparse oceanic areas, re- initial positioning and representation of the radial structure liance cannot be placed on conventional observations to and asymmetry of the TC. de®ne the structure of the TC in the NWP model. Hence, Following implementation of the new intialization for some years the UKMO has used ``bogus'' observations technique in 1994, there were two more Atlantic TCs to de®ne the location and structure of the TC in the model. before the end of the season. UKMO forecasts for both These are derived from information found in TC advisory Hurricanes Florence and Gordon were very good with messages issued by TC warning centers. The earliest meth- mean skill scores against climatology and persistence od of bogussing used (Radford 1994) was relatively sim- (CLIPER) forecast methods for the unusual track of plistic and, despite producing an improvement in TC track Hurricane Gordon of between 51% and 67% (see section forecasts when compared with forecasts using no bogus 3 for a de®nition of skill score). observations (Heming 1993), also produced a systematic This paper will examine the performance of the UKMO bias in forecasts (Heming et al. 1995). In late 1994 a new, global model in predicting the tracks of all 19 tropical more sophisticated technique for initializing TCs in the storms of the 1995 Atlantic season (see Fig. 2) and will model was implemented. This involves creating a mesh compare the results with other NWP models and forecast of bogus wind data at the surface and three upper levels. guidance and with UKMO results from previous years. 2. Comparison with other models Corresponding author address: Julian T. Heming, Forecasting Sys- a. Models available to the National Hurricane Center tems Branch, Meteorological Of®ce, London Rd., Bracknell, Berk- shire RG12 2SZ, United Kingdom. UKMO forecasts were not available to the National E-mail: [email protected] Hurricane Center (NHC), Miami, in real time during the q 1998 American Meteorological Society Unauthenticated | Downloaded 09/30/21 05:00 PM UTC 1324 MONTHLY WEATHER REVIEW VOLUME 126 UKMO TC veri®cation scheme Radford (1996) com- pared all TC forecasts in 1995 for the UKMO and ECMWF global models and found that UKMO forecast track errors were smaller at all forecast ranges shorter than 120 h. The results for the Atlantic basin alone are similar to those for all TC basins in 1995. These are shown graphically in Figs. 4 and 5 (see section 3 for de®nitions of forecast errors and skill scores). UKMO analyses were much better due to the use of the bo- gussing technique described above (ECMWF do not use an arti®cial TC initialization technique). UKMO T 1 24 forecasts showed 40% more skill over CLIPER than the corresponding ECMWF forecasts, but by T 1 120 the two models showed very similar results. This dif- ference in error growth rate can probably be attributed to the models' formulation rather than any difference in analysis systems. 3. Assessment of individual storm tracks The UKMO TC veri®cation method identi®es a TC center by locating a maximum in 850-hPa relative vor- ticity; this is in accordance with WMO recommenda- tions. A forecast is only veri®ed if the TC was classi®ed as a tropical storm (winds greater than 34 kt) both at the data time and validity time of the forecast. Full FIG. 1. Formulation of the new UKMO TC bogus scheme. details of the veri®cation technique can be found in Heming (1994). The forecast error (FE) is de®ned as the straight-line distance between the observed and fore- 1995 Atlantic season. However, following the end of cast positions of the TC. Forecast skill is a comparison the season, NHC performed an assessment of all the between the model FE and the FE as computed by CLI- guidance available to NHC and compared it with the PER (Neumann 1972). In this paper skill scores are of®cial NHC forecasts and UKMO forecasts (Gross de®ned as (CLIPER error 2 model error)/(CLIPER er- 1996, personal communication). Figure 3 shows mean ror) expressed as a percentage. A positive skill score forecast errors (in kilometers) for all 10 forms of guid- indicates the model was more skilful than CLIPER. Ta- ance for all storms during the season. The graph indi- ble 1 lists all 19 Atlantic TCs in 1995 in order of strength cates that at all forecast times beyond T 1 12, the as de®ned by the observed maximum sustained wind. UKMO track forecast errors were the smallest, beating Tables 2 and 3 show mean FEs and skill scores for each even the high-resolution Geophysical Fluid Dynamics of these TCs in strength order and the mean ®gures for Laboratory model (Kurihara et al. 1993; Bender et al. the period 1988±94. Nineteen eighty-eight was the ®rst 1993). Mean UKMO forecast errors for T 1 24, T 1 year that TC tracks were veri®ed at the UKMO using 48, and T 1 72 were some 12% below the of®cial NHC the current method. The forecasts for each TC are dis- forecast. The storms where UKMO forecasts were par- cussed below in order of observed maximum wind ticularly good compared to other guidance were Mari- strength of the storm. lyn, Opal, and Roxanne (see individual track veri®cation Hurricane Opal was the strongest storm of the year in section 3). and was one of the three best forecast storms by the UKMO model when compared to other models available b. Comparison with the European Centre for to NHC (see section 2a). The FEs and skill scores were, Medium-Range Weather Forecasts (ECMWF) however, far from the best of all storms and early fore- model casts failed to predict the acceleration of the storm to the northeast as it raced toward the Gulf Coast. However, Comparisons made in recent years by the Japanese the timing of the sudden turn was well predicted as, Meteorological Agency (JMA) have con®rmed eventually, was the landfall of this destructive hurricane. ECMWF forecasts of TCs in the western North Paci®c Hurricane Luis was one of the most noteworthy hur- as the most accurate when compared with UKMO and ricanes of the year as it devastated parts of the northeast JMA forecasts until 1994 when UKMO forecasts started Caribbean before turning north and northeastward and to show an improvement (Sato 1996, personal com- transforming into a very strong midlatitude depression. munication). In a separate assessment made using the UKMO FEs and skill scores for Hurricane Luis were Unauthenticated | Downloaded 09/30/21 05:00 PM UTC MAY 1998 HEMING AND RADFORD 1325 FIG. 2. Observed tracks of tropical cyclones in the North Atlantic in 1995. FIG. 3. Mean tropical cyclone forecast track errors for models available to NHC for the 1995 North Atlantic season. Unauthenticated | Downloaded 09/30/21 05:00 PM UTC 1326 MONTHLY WEATHER REVIEW VOLUME 126 FIG. 4. Mean tropical cyclone forecast track errors for UKMO and ECMWF for the 1995 North Atlantic season.
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