Impact of Resolution Degradation of the Initial Condition on Typhoon Track Forecasts

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NOTES AND CORRESPONDENCE

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 resolution. 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 noncycled 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

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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. However, the preoperational experiments lation system, being aware of the existence of the typhoon of TEPS in 2007, whose initial conditions were provided from the real-time Dvorak analysis of RSMC Tokyo by a T319 data assimilation cycle, did not indicate such Typhoon Center, generated bogus data for assimilation degradation of typhoon track forecasts. This motivated around the typhoon. The bogus data are generated based a T319 data assimilation cycle experiment in 2008, which on the Dvorak analysis and model’s grid spacing, retain- is denoted as T319_CYC in Table 1. The typhoon track ing the asymmetric typhoon structure in the first guess errors of T319_CYC are compared with those of TEPS_ (Ueno 1995). In the present case, no notable analysis in- CTL and T959_OPE for all typhoons in 2008. The re- crement was found around Typhoon Nuri; that is, the sults indicate that T319_CYC significantly outperforms location and strength of the typhoon in the first guess TEPS_CTL, although it does slightly worse than T959_ were very close to those of the bogus typhoon. OPE. Therefore, the resolution transform from the T959 Although operating an independent data assimilation initial conditions to T319 must be responsible for the cycle for T319_CYC would improve the TEPS signifi- forecast degradation, at least to some extent. cantly, we aim to avoid this approach due to the high computational cost. Here, we introduce two alternative methods for improving the T319 forecast at minimal 3. Case study of Typhoon Nuri (2008) additional cost when the T959 analysis cycle preexists. The NWP forecast for Typhoon Nuri (2008), initial- The first approach, called T319_TS1 in Table 1, is to ized at 1200 UTC 19 August 2008, is a typical case indi- apply a smooth spectral truncation in transforming the cating that T959_OPE outperforms TEPS_CTL. The T959 analysis to the T319 resolution. In TEPS_CTL, best track, analyzed by the Regional Specialized Meteo- zero power is forced to the T959 analysis for wave- rological Center (RSMC) Tokyo-Typhoon Center, is west- numbers greater than 319, but all power remains for northwestward, which is better captured by T959_OPE wavenumbers smaller than 320 (Fig. 3a). A smoother (Fig. 2). TEPS_CTL indicates a stronger northward ten- spectral truncation is chosen, in which the power goes to dency, which would lead to a false alert for Taiwan. The zero at 320 wavenumbers linearly from 106 wavenumbers T319_CYC experiment provides a significantly better (Fig. 3b). The results indicate that T319_TS1 has almost track forecast than TEPS_CTL, although it is slightly no impact on the track forecast (Fig. 2), although the ini- worse than T959_OPE. This agrees with the statistics tial conditions become generally smoother in the global shown in section 2. domain.

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FIG. 3. A schematic diagram showing the spectral truncation in the resolution transform from T959 to T319 for the cases of (a) TEPS_CTL and (b) T319_SP1.

The second approach, called T319_TS2, is to perform FIG. 4. A schematic diagram showing the T319_TS2 experiment a ‘‘noncycled’’ T319 data assimilation, as illustrated in with noncycled T319 data assimilation. ANL, DA, and FCST Fig. 4. Here, the T319 data assimilation is not cycled, represent the analysis, data assimilation, and forecast, respectively. since it is initiated from the existing T959 analysis for the high-resolution deterministic NWP forecast. The T959 guess using the same model as the forecast model. This analysis is degraded to the T319 resolution by simply single case study indicates the potential of the two ap- eliminating the high-wavenumber components in the proaches, although the level of statistical performance same way as TEPS_CTL. Then, a T319 forecast is made over many cases is not yet clear. to generate the first guess for the data assimilation at T319 resolution. Without the cycle process, performing 4. Summary and discussion data assimilation once is much less expensive since it is computed only when typhoons exist and TEPS is acti- The T319/L60 control forecasts of the operational vated. The results indicate that T319_TS2 significantly TEPS in 2008 indicated larger typhoon track errors than outperforms TEPS_CTL and is almost identical to T319_ those of the operational T959/L60 global forecasts. The CYC for the initial 36 h (Fig. 2). T319/L60 initial conditions were generated by simply The major difference in the initial conditions between eliminating higher-wavenumber components of the op- TEPS_CTL and T319_CYC is found near the typhoon erational T959/L60 analysis. Since the independent T319/ center. The central pressure is deepest for T959_OPE L60 data assimilation cycle improved the T319/L60 and shallowest for T319_CYC (Fig. 5). TEPS_CTL and typhoon track forecasts significantly, the spectral trun- T319_TS1 are relatively similar to T959_OPE, whereas cation of the resolution change must be responsible for T319_TS2 is closer to T319_CYC. We find basically the the forecast degradation, at least to some extent. same tendency in the wind field. Obviously, the T959 Two approaches for improving the T319/L60 forecasts model generated the sharper typhoon structure than the without keeping the independent data assimilation cycle T319 model. After the resolution degradation, TEPS_ were proposed in this study: 1) applying a smoother CTL captured the sharp structure near the typhoon spectral truncation for the resolution transform and 2) center, as did T319_TS1 with the smoother spectral performing noncycled lower-resolution data assimila- truncation. Namely, the T959 model generated the ty- tion during preprocessing. According to the case study phoon core structure that could be represented well by of Typhoon Nuri (2008), the former approach had es- up to about 150 wavenumbers, so that the smoother sentially no impact, whereas the latter approach im- spectral truncation did not change it significantly. How- proved the track forecast significantly. ever, T319_CYC generated the shallower typhoon struc- In general, it is very difficult to identify what differ- ture, which was a natural consequence of the T319 model. ences in the initial conditions would have caused the We expect that the T319 model does not have the sharp forecast track difference. Although we saw in Fig. 5 that typhoon structure on its solution attractor, even though it the shallower typhoon existed on the solution attractor of can technically be represented by the T319 resolution. the T319 model and was preferable for lower-resolution Since T319_TS2’s first guess was generated by the T319 forecasts, the exact source of the forecast improvement model, the typhoon structure was spun up to some ex- remains unknown. tent to provide a shallower structure, closer to that of Applying the smoother spectral truncation in this study T319_CYC. Hence, it was essential to generate the first apparently made the global fields smoother in general,

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the forecast. For example, the T319 resolution (not the model processes) could technically resolve the deep typhoon structure in Fig. 5 (b), but this deep structure was generated by the T959 model. This suggests that the best initial conditions for a given model should be generated by data assimilation using the same model at the same resolution as the forecast system. Since many operational NWP centers operate EPSs with a lower-resolution model initialized by a higher- resolution analysis, the issue raised by this study may be important for improving operational EPSs in general. Although this study focused on only typhoon track forecasts, the conclusion may also be valid for other high- impact weather phenomena that are sensitive to a model’s resolving capability. When the deterministic NWP system adopts an even higher-resolution model and the resolution difference from the EPS becomes larger, which was actually the case at JMA, the issue raised by this study may become more important. In many studies, it is common to use operational analyses to initialize research models for real case studies. However, it is worthwhile to keep in mind that the forecasts could be signiﬁcantly degraded simply because the analysis is generated by data assimilation at a different resolution, sometimes even with a different model. This suggests the possible utility of data assimilation methods without using operational analysis.

Acknowledgments. The authors thank three anony- mous reviewers for their constructive comments, which improved the paper. The authors are grateful to W. Hogsett of the National Hurricane Center, C. Reynolds of NRL, R. Buizza of ECMWF, and D. Kleist of NCEP for their useful comments. This research was partly sup- FIG. 5. (a) A mean SLP field (hPa) from the T319_CYC analysis, and (b) the associated vertical cross sections at 124.38E in five ported by the Office of Naval Research (ONR Grant different analyses, all valid at 1200 UTC 19 Aug 2008. The mini- N000141010149) under the National Oceanographic mum pressure of the best-track data is 955 hPa at 18.08N, 124.38E. Partnership Program (NOPP).

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