1568 WEATHER AND FORECASTING VOLUME 25 NOTES AND CORRESPONDENCE Impact of Resolution Degradation of the Initial Condition on Typhoon Track Forecasts TAKEMASA MIYOSHI Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland TAKUYA KOMORI AND HITOSHI YONEHARA Numerical Prediction Division, Japan Meteorological Agency, Tokyo, Japan RYOTA SAKAI River Office, Osaka Prefectural Government, Osaka, Japan MUNENHIKO YAMAGUCHI RSMAS, University of Miami, Miami, Florida (Manuscript received 17 December 2009, in final form 11 May 2010) ABSTRACT The operational numerical weather prediction (NWP) systems at the Japan Meteorological Agency (JMA) indicated that the typhoon track forecasts made by the control member of the ensemble prediction system (EPS) tended to be worse than those made by the high-resolution global NWP. The control forecast of the EPS with horizontal triangular truncation at 319 wavenumbers and 60 vertical levels (T319/L60 resolution) was initialized by eliminating the higher-wavenumber components of the global analysis at T959/L60 reso- lution. When the data assimilation cycle was performed at the lower T319/L60 resolution, the forecast gave typhoon track forecasts closer to the high-resolution global NWP. Therefore, it stands to reason that the resolution transform of the initial condition must be responsible for the degradation of the typhoon track forecasts at least to considerable extent. To improve the low-resolution forecast, two approaches are tested in this study: 1) applying a smoother spectral truncation for the resolution transform and 2) performing non- cycled lower-resolution data assimilation during preprocessing. Results from the single case study of Typhoon Nuri (2008) indicate almost no impact from the former approach, but a significant positive impact when using the latter approach. The results of this study illuminate the importance of considering a model’s resolving capability during data assimilation. Namely, if the initial conditions contain features caused by unresolved scales, degraded forecasts may result. 1. Introduction EPS has a lower resolution than the global NWP system, which is true for the TEPS at a T319/L60 resolution. The The Japan Meteorological Agency (JMA) began to initial conditions for the control forecast of TEPS are operate its global NWP system at a T959/L60 resolution generated by simply eliminating higher-wavenumber in November 2007 (Nakagawa 2009) and its typhoon components of the high-resolution global analyses. Five ensemble prediction system (TEPS) in 2008 (Yamaguchi ensemble perturbations are generated by the singular and Komori 2009; Yamaguchi et al. 2009). Typically, the vector (SV) method (e.g., Buizza 1994), and the plus– minus pairs are added to the control initial conditions. Thus, the ensemble mean of the 11-member initial con- Corresponding author address: Takemasa Miyoshi, Dept. of At- mospheric and Oceanic Science, University of Maryland, College ditions equals the set of control initial conditions. Park, College Park, MD 20742. Operations of the high-resolution global NWP and the E-mail: [email protected] lower-resolution TEPS in 2008 yielded an interesting DOI: 10.1175/2010WAF2222392.1 Ó 2010 American Meteorological Society Unauthenticated | Downloaded 09/29/21 12:12 AM UTC OCTOBER 2010 N O T E S A N D C O R R E S P O N D E N C E 1569 result in that the typhoon track forecasts by the TEPS TABLE 1. List of operational forecasts and experiments. control member tended to have larger errors than Expt Description Resolution the high-resolution global forecasts. Moreover, the 11-member ensemble mean of TEPS ensemble forecast T959_OPE Operational high-resolution NWP T959/L60 TEPS_CTL Operational TEPS control forecast T319/L60 tracks, which was generally better than the single control T319_CYC T319 data assimilation cycle T319/L60 member, was still worse than the single high-resolution T319_TS1 Smooth resolution transform T319/L60 forecasts. The initial conditions were essentially identi- T319_TS2 Noncycled T319 data assimilation T319/L60 cal; the higher-wavenumber components were simply eliminated. The forecast degradation may have been due to the differences among the forecast models. Namely, the developed and investigated TC bogusing and reposi- T959/L60 model may have been more accurate than the tioning methods: for example, the National Centers for T319/L60 model. However, the preoperational investi- Environmental Prediction (NCEP; Lord 1991; Liu et al. gations in 2007 indicated essentially identical performance 2000), NRL (Goerss and Jeffries 1994), the Met Office between the high- and low-resolution models in the case (Heming et al. 1995), and ECMWF [never used in op- where the initial conditions are generated by each in- erations; Serrano and Unden (1994)]. More recent studies dependent data assimilation cycle at each resolution. include that of Wu et al. (2006), who assimilated bogus Therefore, we expect that the forecast degradation was data into a mesoscale model and found that wind data caused by the resolution transformation of the initial con- played an important role in TC prediction due to the ditions. This idea agrees with a commonly accepted hy- geostrophic adjustment of smaller scales. pothesis that the initial conditions are suitable to the The focus of this study is different from those of the model with which the data assimilation cycle is performed. previous studies. Global NWP began to be useful for TC This was our first experience at JMA with clear forecasts only about a decade ago (e.g., Elsberry 1995), forecast degradation due to the resolution transform of and NCEP started the operation of the first EPS around the initial conditions using high-wavenumber elimina- that time (Toth and Kalnay 1993). Because the global tion. C. Reynolds (2010, personal communication) had EPS began to increase the level of resolution and be- a similar experience with the Navy Operational Global came potentially useful for TC forecasts only very re- Atmospheric Prediction System (NOGAPS) at the Naval cently, the need to initialize TCs in a lower-resolution Research Laboratory (NRL), and R. Buizza (2010, per- EPS using a higher-resolution analysis has emerged very sonal communication) mentioned the experience of the recently. This has not been addressed in the literature to European Centre for Medium-Range Weather Forecasts the best of the authors’ knowledge. (ECMWF) with their EPS in terms of the importance In this note, we report the diagnostics obtained by the of resolution change from the high-resolution analysis. operations of high-resolution global NWP and TEPS in However, to the best of the authors’ knowledge, no study 2008. In addition, we propose and test two approaches to published thus far has brought to light the problem of the improve the low-resolution forecasts at a minimal addi- degradation of tropical cyclone (TC) track forecasts. Al- tional cost. Section 2 describes the error statistics of the though it is not easy to identify the exact mechanism for 2008 operational typhoon predictions, in which the skill track degradation, it would be beneficial to report the of the high-resolution global NWP exceeded that of the facts observed in the operational systems, which may TEPS control forecast. In section 3 we test two possible motivate further scientific investigations in the wider re- approaches to improve the low-resolution forecast using search community. a case study of Typhoon Nuri (the 12th typhoon in 2008). Many studies have been published on the initialization Finally, a summary and discussion are provided in section of TCs, including those discussing JMA’s prior devel- 4. For simplicity, we define the short names of the oper- opments (e.g., Iwasaki et al. 1987; Ueno 1989; Ueno and ational forecasts and our experiments in Table 1. Details Onogi 1991; Ueno 1995). Most of the previous studies of the experiments will be described later in section 3. focused mainly on TC bogusing and repositioning methods Following the Japanese operational convention, in this to better initialize TC vortices that are compatible with note the term typhoon includes tropical cyclones in the a model’s resolution when only relatively poor analyses northwestern Pacific with maximum wind speed of greater are available due to the general lack of observations than 34 kt (17.5 m s21). around TCs over the ocean. For example, Kurihara et al. (1993) proposed a method of initializing a high-resolution 2. Statistical fact in 2008 typhoon forecast TC forecast using a low-resolution analysis by replacing a crudely resolved TC in the large-scale analysis with a In daily weather briefings to monitor typhoons in 2008, high-resolution vortex. Many operational centers have the developers of TEPS noticed that TEPS_CTL tended Unauthenticated | Downloaded 09/29/21 12:12 AM UTC 1570 WEATHER AND FORECASTING VOLUME 25 FIG. 1. Typhoon track forecast errors averaged for all typhoon forecasts in 2008. FIG. 2. The best track (BEST) and forecast tracks (T959_OPE, operational high resolution; TEPS_CTL, TEPS control; T319_CYC, T319 data assimilation cycle; T319_TS1, smooth resolution trans- to have larger typhoon track prediction errors than T959_ form; T319_TS2, noncycled T319 data assimilation) of Typhoon OPE. This has been proven correct by a statistical in- Nuri (2008) initialized at 1200 UTC 19 Aug 2008 up to 54 h later. vestigation of forecast track errors for all typhoons that Dots indicate typhoon center positions at every 12 h defined by the occurred in 2008, where T959_OPE has significantly minimum mean SLP. smaller track errors by at least 30 km for forecast lead times longer than or equal to 36 h (Fig. 1). In the present case of Typhoon Nuri analyzed at A possible explanation of this statistical fact is the 1200 UTC 19 August 2008, essentially no observation difference between the forecast models; that is, the T959 was available in the vicinity of the typhoon. Therefore, model provides a more accurate prediction than the the preprocessing of the operational global data assimi- T319 model.