A Dynamical Initialization Scheme for Real-Time Forecasts of Tropical Cyclones Using the WRF Model*

964 MONTHLY WEATHER REVIEW VOLUME 141 A Dynamical Initialization Scheme for Real-Time Forecasts of Tropical Cyclones Using the WRF Model* DONG-HYUN CHA AND YUQING WANG International Pacific Research Center and Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 8 March 2012, in final form 27 September 2012) ABSTRACT To improve the initial conditions of tropical cyclone (TC) forecast models, a dynamical initialization (DI) scheme using cycle runs is developed and implemented into a real-time forecast system for northwest Pacific TCs based on the Weather Research and Forecasting (WRF) Model. In this scheme, cycle runs with a 6-h window before the initial forecast time are repeatedly conducted to spin up the axisymmetric component of the TC vortex until the model TC intensity is comparable to the observed. This is followed by a 72-h forecast using the Global Forecast System (GFS) prediction as lateral boundary conditions. In the DI scheme, the spectral nudging technique is employed during each cycle run to reduce bias in the large-scale environmental field, and the relocation method is applied after the last cycle run to reduce the initial position error. To demonstrate the effectiveness of the proposed DI scheme, 69 forecast experiments with and without the DI are conducted for 13 TCs over the northwest Pacific in 2010 and 2011. The DI shows positive effects on both track and intensity forecasts of TCs, although its overall skill depends strongly on the performance of the GFS forecasts. Compared to the forecasts without the DI, on average, forecasts with the DI reduce the position and intensity errors by 10% and 30%, respectively. The results demonstrate that the proposed DI scheme im- proves the initial TC vortex structure and intensity and provides warm physics spinup, producing initial states consistent with the forecast model, thus achieving improved track and intensity forecasts. 1. Introduction to uncertainties in model physics and initial conditions. One way to reduce TC forecast errors is to improve TC Tropical cyclones (TC) are severe weather systems forecast models. There have been a number of efforts that cause human fatalities and property damage in their in recent years to improve these models in terms of affected areas. An accurate prediction of the motion physical processes related to surface flux under TC and intensity of a TC is critical to preparedness and conditions (Emanuel 2003; Donelan et al. 2004; Moon evacuation for areas that could potentially be hit by an et al. 2004; Zeng et al. 2010) and convective parame- intense TC. Dynamical prediction by numerical models terization (Ma and Tan 2009), ocean feedback processes is a major objective approach to TC forecasting in most (Emanuel et al. 2004; Davis et al. 2008), and increased major TC centers. Considerable progress has been made model resolution (Chen et al. 2007; Davis et al. 2010). in numerical TC forecasts because of the advancements Another way to reduce TC forecast errors is to im- in observations and the rapid increase in computing re- prove the initial condition of the forecast model. An sources. However, numerical TC forecasts still suffer accurate initial condition, particularly for the initial TC from considerable errors in both track and intensity due structure and intensity, is very important for improving TC forecasts. Uncertainties in initial conditions are un- avoidable due to imperfect observations and analysis * School of Ocean and Earth Science and Technology Publication Number 8767 and International Pacific Research Center Publication methods used. In particular, initial conditions for TC Number 921. forecasts usually have large errors because most TCs occur over open oceans where observations are grossly insufficient, particularly for defining the precise location Corresponding author address: Dr. Yuqing Wang, IPRC/SOEST, Post 409G, University of Hawaii at Manoa, 1680 East–West Rd., and inner-core structure. Although observing platforms Honolulu, HI 96822. for TCs, including satellites and radar, have been sig- E-mail: [email protected] nificantly advanced in the past decade or so, they still DOI: 10.1175/MWR-D-12-00077.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 09/26/21 06:11 PM UTC MARCH 2013 C H A A N D W A N G 965 have limitations in fully resolving the three-dimensional scale environment in which the TC is embedded. How- structure of a TC. Therefore, various initialization methods ever, to ensure a convergence to the observed TC intensity, have been developed to obtain improved initial condi- they have to specify the surface pressure field for the tions for TC forecast models. axisymmetric TC vortex during the cycle runs. The One TC initialization method involves the use of a specification of the axisymmetric vortex structure is synthetic or bogus vortex. In this method the bogus artificial because no such data are available from ob- vortex, which is generated by analytic empirical func- servations. In addition, this DI scheme spins up the dy- tions for surface pressure and wind, is used to replace the namical fields of the TC vortex only because all model vortex in the global (or regional) analysis. Using this physics are still a cold start for the forecast run. method, however, it is difficult to generate the asym- A real-time TC forecasting system has been opera- metric structure of the TC vortex associated with the TC tional at the International Pacific Research Center motion. This method might also result in physical and (IPRC) since the 2011 TC season using the Advanced dynamical inconsistencies between the initial condition Research Weather Research and Forecasting (WRF) and the forecast model. Nonetheless, a number of studies Model. To improve TC forecast skill, we recently devel- have shown that this method can reasonably reproduce oped a new DI scheme and implemented it into the real- TC features with improved track and intensity forecasts time TC forecasting system. This new DI scheme is based (Ueno 1989; Leslie and Holland 1995; Wang 1998; Ma on cycle runs as used in Nguyen and Chen (2011) but et al. 2007; Kwon and Cheong 2010). Another approach a 6-h window before the initial forecast time is used for to TC initialization is the bogus data assimilation (BDA), the cycle runs and also a large-scale spectral nudging which uses variational data assimilation with synthetic technique is utilized. We will show that there are several observations of a TC vortex that closely matches the additional advantages to using this new scheme. The ob- observed TC intensity and structure (George and Jeffries jectives of this paper are to introduce the IPRC real-time 1994; Zou and Xiao 2000; Davidson and Weber 2000; Pu TC forecasting system, introduce a new DI scheme, and and Braun 2001; Zhang et al. 2007; Wang et al. 2008). investigate the effect of the scheme on TC forecasts over Another TC initialization method is dynamical ini- the northwest Pacific. The real-time forecasting system tialization (DI) by forecast model integration (Kurihara and a new DI scheme are introduced in section 2. Model et al. 1993; Bender et al. 1993; Peng et al. 1993; Nguyen setup and experimental design are described in section 3. and Chen 2011). This method has the advantage that the The results of the forecast experiments for TCs, which initial TC vortex generated by the DI is consistent with occurred in the 2010 and 2011 TC seasons, are verified in the dynamics and physics of the forecast model, while it section 4 and two case studies are discussed in section 5. requires additional model integration for the TC to in- Main conclusions are drawn in the last section. tensify in the model. Kurihara et al. (1993) proposed a DI method, where an axisymmetric vortex component 2. The real-time TC forecasting system and a new was generated by the integration of an axisymmetric dynamical initialization scheme version of the TC forecast model. The asymmetric vor- a. The IPRC real-time forecasting system tex component was constructed by integrating the non- for northwest Pacific TCs divergent barotropic vorticity equation model on a beta plane using the initial conditions from the constructed The IPRC real-time TC forecasting system was built symmetric vortex flow. A similar DI scheme is currently based on the WRF Model (Skamarock et al. 2005) and used in the U.S. Navy regional coupled model for TC operated since 2011 TC season (see online at http://iprc. prediction (Hendricks et al. 2011) but the vortex is spun soest.hawaii.edu/users/dhcha). In the initial version of up in a different three-dimensional TC model of Wang the real-time forecasting system, a bogus vortex with (2001) under idealized conditions. empirical idealized vortex specification (Wang 1998) Recently, Nguyen and Chen (2011) developed a TC was used for TC initialization and the model was run at initialization scheme where a TC vortex was spun up the horizontal resolution of 15 km, which was relatively through 1-h cycle runs from the initial forecast time and coarse due to the limitation of computing resources at tested the scheme for a TC case. In their work, the initial that time. Despite the better skill in the intensity fore- condition generated from the cycle runs is consistent casts compared with the global model forecast, the real- with the dynamics and physics of the forecast model time forecasting system still had low skills in track and because the model used in the cycle runs and that in the intensity forecasts due to the low model resolution and forecast run are identical. The most important advan- the simple TC initialization. tage of this scheme is that the TC vortex, generated Recently, we improved the real-time forecasting system through the cycle runs, is adapted to the actual large- by implementing a new DI scheme (see section 2b) and Unauthenticated | Downloaded 09/26/21 06:11 PM UTC 966 MONTHLY WEATHER REVIEW VOLUME 141 FIG.

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