OCTOBER 2016 H E M I N G 1433 Met Office Unified Model Tropical Cyclone Performance Following Major Changes to the Initialization Scheme and a Model Upgrade JULIAN T. HEMING Met Office, Exeter, Devon, United Kingdom (Manuscript received 1 March 2016, in final form 27 June 2016) ABSTRACT The Met Office has used various schemes to initialize tropical cyclones (TCs) in its numerical weather pre- diction models since the 1980s. The scheme introduced in 1994 was particularly successful in reducing track forecast errors in the model. Following modifications in 2007 the scheme was still beneficial, although to a lesser degree than before. In 2012 a new trial was conducted that showed that the scheme now had a detrimental impact on TC track forecasts. As a consequence of this, the scheme was switched off. The Met Office Unified Model (MetUM) underwent a major upgrade in 2014 including a new dynamical core, changes to the model physics, an increase in horizontal resolution, and changes to satellite data usage. An evaluation of the impact of this change on TC forecasts found a positive impact both on track and particularly intensity forecasts. Following implementation of the new model formulation in 2014, a new scheme for initialization of TCs in the MetUM was developed that involved the assimilation of central pressure estimates from TC warning centers. A trial showed that this had a positive impact on both track and intensity predictions from the model. Operational results from the MetUM in 2014 and 2015 showed that the combined impact of the model upgrade and new TC initialization scheme was a dramatic cut in both TC track forecast errors and intensity forecast bias. 1. Introduction In 2007, a complete reevaluation of the initialization scheme was undertaken to assess whether it was still There is a long history of initializing tropical cyclones proving beneficial to forecasts of TCs from the MetUM. (TCs) in the Met Office Unified Model (MetUM) in In the years since the scheme was first introduced there order to improve the model’s representation of TCs in had been many improvements in model formulation, both the analysis and the forecast. In the late 1980s and increases in model resolution, and the introduction of early 1990s forecasters had a tool available that inserted new observational data, particularly from satellites. so-called bogus observations of central pressure, sur- These changes were likely to have diminished the need rounded by four values of wind speed and direction at for artificial initialization of TCs. In the event, this the surface and three lower-tropospheric levels. In 1994, evaluation first found that the initialization scheme was this was superseded by a new initialization scheme that still reducing TC track forecast errors by an average of involved the insertion of bogus observations of wind 12.2%. Furthermore, a modification to the scheme that speed and direction at the surface and three lower- reduced the horizontal spread of ‘‘bogus’’ observations tropospheric levels. This technique proved extremely generated for small TCs resulted in a further reduction successful and reduced TC track forecast errors by 34% in TC track forecast errors of 4.7% (Heming 2009). This on average in trials (Heming et al. 1995). The following configuration of the initialization scheme will be known year the MetUM produced better guidance for TC track as the 2007 scheme hereafter. A diagrammatic repre- prediction to the National Hurricane Center than any sentation of the scheme is shown in Fig. 1. After a few other numerical weather prediction model for the ex- years in operation a further evaluation of the 2007 tremely active Atlantic hurricane season of that year scheme was undertaken to ensure it was still providing (Gross 1996; Heming and Radford 1998). benefit to the model forecasts. Section 2 of this paper presents the results of this evaluation. Corresponding author address: Julian T. Heming, Met Office, A major change to the MetUM was implemented in FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom. 2014, which was the culmination of many years’ work E-mail: julian.heming@metoffice.gov.uk (Met Office 2014; Walters et al. 2016, manuscript submitted DOI: 10.1175/WAF-D-16-0040.1 Unauthenticated | Downloaded 09/25/21 08:16 PM UTC 1434 WEATHER AND FORECASTING VOLUME 31 FIG. 1. The configuration of the 2007 scheme. to Geosci. Model Dev.). This included changes to the periods covered a significant amount of TC activity in model dynamics, physics, horizontal resolution, and both the Northern and Southern Hemispheres. The first satellite data usage. Early trials indicated that this period was from 23 August to 19 September 2010, the change would have a significant impact on TC intensity second from 20 August to 15 October 2011, and the third predictions. Section 3 of this paper evaluates the impact from 19 January to 18 March 2012. In total there were 57 of the model change on TC predictions and explains how TCs during these three periods comprising 17 in the this resulted in the development of a completely new western North Pacific, 10 in the eastern North Pacific, 19 form of TC initialization in the MetUM using estimates in the Atlantic, 4 in the western south Indian Ocean of central pressure from TC warning centers (discussed (west of 908E), and 7 in the eastern south Indian Ocean in section 4). and South Pacific. A sizeable number of forecasts were verified at each forecast lead time: for example, 460 at 24 h, 243 at 96 h, and 87 at 168 h. The control and trial 2. Evaluation of the 2007 scheme used the configuration of the MetUM operational from July 2011 to January 2012 (known as OS27), which a. Trial configuration and results had a horizontal grid spacing of approximately 25 km at In 2012 experiments were undertaken to assess the midlatitudes and 70 vertical levels. The control in- impact on TC forecasts of transplanting analysis fields cluded use of the 2007 scheme to initialize TCs, as was from another model into the MetUM. The European done in the operational model at the time, whereas in Centre for Medium-Range Forecasts (ECMWF) model the trial this scheme was switched off. Table 1 shows was chosen since it had performed better than the the various verification scores for TC track prediction MetUM for TC track prediction in the previous few for the control and trial. Scores were calculated at years. The results indicated that the performance of the 12-hourly forecast intervals, but only the 24-hourly forecast was very sensitive to the lower-tropospheric values are shown in Table 1. Details of the TC tracking winds in the analysis (Heming 2012). Given this result, it method and verification scores can be found in Heming was decided to undertake another evaluation of the 2007 (2016). scheme to ensure it was still providing benefit to MetUM For TC track the results show that despite much larger forecasts of TCs since the scheme primarily adjusted the analysis errors in the trial, which might be expected model’s lower-tropospheric wind fields. Standard con- because of the removal of the initialization, track fore- trol runs of approximately 4–8 weeks are available to cast errors were lower at all lead times. When averaged model developers at the Met Office. The 2007 scheme over all forecasts from 12 to 168 h at 12-hourly intervals, was evaluated by using three control runs available at the trial track forecast errors were 8.4% lower. The re- the time for periods during 2010, 2011, and 2012 and duction in track errors up to 96 h was significant beyond running trial forecasts without the 2007 scheme. These the 1% level. The reduction in errors at longer lead Unauthenticated | Downloaded 09/25/21 08:16 PM UTC OCTOBER 2016 H E M I N G 1435 TABLE 1. 2007 scheme control versus trial results. Mean track forecast statistics, where boldface indicates the better score. The 12-hourly statistics are calculated, but only 24-hourly results are shown. The t values and significance levels (%) of track error differences are shown. The trial had the 2007 scheme switched off. 0 h 24 h 48 h 72 h 96 h 120 h 144 h 168 h No. of cases 571 460 386 310 243 182 132 87 Control track errors superior 346 198 159 130 103 76 47 28 Trial track errors superior 191 254 219 179 140 104 85 59 Track errors equal 34 8 8 1 0 2 0 0 Control track error (km) 33 103 184 285 403 557 723 871 Trial track error (km) 47 92 160 258 372 523 665 797 Reduction in track error of trial relative to 240.8 11.4 11.9 9.8 5.8 3.8 6.6 6.0 control (%) t value of track error differences 8.103 23.585 24.421 22.908 22.414 21.628 21.599 21.503 Significance level of track error differences (%) ,0.1 ,0.1 ,0.1 0.2 0.8 5.3 5.6 6.8 Control track skill score (%) — 41 51 49 — — — — Trial track skill score (%) — 48 58 55 ———— times was significant beyond the 7% level. Model skill in forecast tracks for individual forecasts for Hurricanes predicting the track of TCs against climatology and Igor, Katia, and Maria. For Hurricane Igor the control persistence (CLIPER; Neumann 1972) was calculated persisted with a westward track for too long and was for the first 72 h of the forecast. The trial skill scores were also a little too fast. The trial was slower and turned on average 6.0% higher than the control results.