4340 MONTHLY WEATHER REVIEW VOLUME 142 Improvements to a Tropical Cyclone Initialization Scheme and Impacts on Forecasts HIEP VAN NGUYEN* AND YI-LENG CHEN Department of Atmospheric Sciences, University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 10 October 2013, in final form 11 August 2014) ABSTRACT This study makes improvements to the tropical cyclone (TC) initialization method introduced by Nguyen and Chen (i.e., the NC2011 scheme). The authors found that prescribing sea level pressure associated with the initial vortex using a modified Fujita formula has very little impact on the vortex structure and intensity during a series of 1-h model integration and relocation. On the other hand, inserting an artificial warm core makes the vortex spin up much faster. When a warm core is inserted during the initial spinup process, the computational time required for model initialization is reduced by 1/2–1/3. Because prescribed sea level pressure is not required to spin up the vortex, information on vortex size, such as radius of maximum wind, is no longer needed. The performance of the improved NC2011 scheme with an initial prescribed warm core during the initial spinup process is tested for typhoons that made landfall over southern China or Vietnam in 2006. Before landfall, these storms were over the open ocean where conventional data were sparse, without special observations. Two sets of model runs, with (NC2011-CTRL) and without (CTRL) vortex initialization, are performed for comparison. The initial and time-dependent boundary conditions are from the NCEP Final Analyses (FNL). There are twelve 48-h simulations in each run set. Results show that the vortex initialization improves TC track and intensity simulations. 1. Introduction bogus data have been known to significantly improve TC track and intensity forecasts (Zou and Xiao 2000; Pu and Tropical cyclone (TC) initialization in numerical Braun 2001; Wu et al. 2006; Zhao et al. 2007; Zhang et al. models is a crucial factor affecting TC track and intensity 2012; and others). A TC initialization technique needs forecasts. Because TCs develop and spend most of their to generate a vortex with a structure similar to the real lifetime over the open ocean before making landfall, TC and remain compatible with the numerical model high-resolution observational data depicting detailed TC employed. structure in the core area are frequently lacking. Usually, Recently, Nguyen and Chen (2011) developed a new a bogus TC vortex is used to improve the TC structure in TC initialization technique (hereafter the NC2011 the model initial conditions. The bogus vortex can be scheme) to generate high-resolution initial conditions constructed with empirical functions of pressure and/or using the Advanced Research version of the Weather winds (Fujita 1952; Holland 1980; Davidson and Weber Research and Forecasting (WRF) Model (ARW; 2000; Davis and Low-Nam 2001; Kwon and Cheong 2010) Skamarock et al. 2005). The NC2011 scheme signifi- or by model integration (Kurihara et al. 1993; Liuetal. cantly improves model initial conditions with better de- 1997). Data assimilation of both observed and vortex piction of TC intensity and structure including minimum sea level pressure (SLP) (Pmin); maximum wind speed * Current affiliation: Institute of Meteorology, Hydrology and (Vmax); and 3D distributions of temperature, pressure, Climate Change, Hanoi, Vietnam. winds, water vapor, as well as the condensate mixing ratio and asymmetric structure of rainbands. Nguyen and Chen (2011) performed seven 48-h simulations for Typhoon Corresponding author address: Yi-Leng Chen, Dept. of Mete- orology, School of Ocean and Earth Science and Technology, Morakot (2009). Another 48-h simulation was initialized University of Hawaii at Manoa, Honolulu, HI 96822. about 36 h before landfall over Taiwan for both Typhoon E-mail: [email protected] Jangmi (2008) and Typhoon Kalmaegi (2008). For all the DOI: 10.1175/MWR-D-13-00326.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 09/27/21 08:05 AM UTC NOVEMBER 2014 N G U Y E N A N D C H E N 4341 TABLE 1. Abbreviations of sensitivity tests and their descriptions. TABLE 2. Abbreviations and descriptions for sensitivity tests with a warm core. Abbreviation Description Cumulus NSLP WRF single-moment 6-class microphysics scheme Abbreviation Microphysics parameterization and Grell–Devenyi cumulus parameterization without prescribed SLP Eta-BM Eta (Rogers Betts–Miller–Janjic WSLP As in NSLP, but SLP is prescribed during cycle run et al. 2001) (Janjic 1994, 2000) for with observed minimum center SLP domain 1 only LSLP As in NSLP, but SLP is prescribed during cycle run Lin-KF Lin et al. (1983) Kain–Fritsch (Kain 2004) with minimum center SLP of 870 hPa for domain 1 only WRF-GD WRF single-moment Grell–Devenyi (Grell and 6-class (Hong et al. Devenyi 2002) for cases considered, using NC2011 to construct the initial 2004; Hong and Lim domain 1 only TC vortex leads to significant improvement in the simu- 2006) lations of track, intensity, radar reflectivity patterns, and Eta-GD Eta Grell–Devenyi for domain TC circulations. Nguyen and Chen (2011) also showed 1 only that for Typhoon Morakot (2009), using the NC2011 initialization scheme results in better prediction of rain- factors affecting the TC initialization process are dis- fall distribution before and after landfall because the cussed in section 3. Some preliminary results on verifi- scheme is able to depict better storm structure. cations of TC intensity and track simulations with the In this study, the impacts of prescribed SLP and pre- improved NC2011 scheme for four landfalling typhoons scribed warm core structure on spinning up a TC vortex in 2006 are discussed in section 4. A summary and dis- in the NC2011 scheme are further investigated. One of cussion are given in section 5. the main challenges in TC research is forecasting the intensity change (Kucas 2010) and improving typhoon 2. Data and model descriptions intensity at the model initial time in high-resolution models can help intensity forecast. In this work, this This research uses version 3.1 of the ARW Model, problem is investigated for typhoons that occurred in which is a nonhydrostatic, three-dimensional primitive a relatively data-sparse region to determine the extent equation model. Two nested domains with horizontal to which large-scale settings affect the TC spinup pro- grids of 18 km (287 3 226) and 6 km (379 3 316), re- cess and subsequent model simulations. Nguyen and spectively, and 38 vertical levels from the surface to the Chen (2011) focused on typhoons that occurred over the 50-hPa level are employed in the model with two-way Taiwan area where the observational network is dense nesting. The full sigma levels are as follows: 1.00, 0.97, with supplementary dropsonde observations over the 0.95, 0.92, 0.89, 0.86, 0.84, 0.81, 0.78, 0.76; 0.73, 0.70, adjacent oceans (Wu et al. 2005). 0.68, 0.65, 0.62, 0.59, 0.57, 0.54, 0.51, 0.49, 0.46, 0.43, The case used in this study is Chanchu (2006), which 0.41, 0.38, 0.35, 0.32, 0.30, 0.27, 0.24, 0.22, 0.19, 0.16, formed as a tropical depression on 9 May 2006 around 0.14, 0.11, 0.08, 0.05, 0.03, and 0.00. The model con- 8.38N, 133.48E. It made landfall over the Philippines on figurations include the following: the WRF single- late 11 May 2006 and weakened. However, after crossing moment 6-class microphysics scheme (WSM6; Hong the Philippines, it intensified as a super typhoon in a data- et al. 2004; Hong and Lim 2006), the Grell–Devenyi sparse region between Vietnam and the Philippines. It cumulus parameterization scheme (Grell and Devenyi made landfall again over the southern China coast on 18 2002) for the 18-km domain only, a Rapid Radiative May 2008. At 0000 UTC 15 May 2006, Typhoon Chanchu Transfer Model scheme (Mlawer et al. 1997)forlong- (2006) had a minimum SLP (Pmin) of ;916 hPa and wave radiation, the Dudhia scheme (Dudhia, 1989)for 2 a maximum wind speed (Vmax) of ;64 m s 1 based on shortwave radiation, the Monin–Obukhov similarity the Joint Typhoon Warning Center (JTWC) best track scheme (Monin and Obukhov 1954) for surface layer data (http://www.usno.navy.mil/JTWC). We investigate physics, and the Yonsei University scheme (Noh et al. the TC vortex associated with Typhoon Chanchu (2006) 2003) for planetary boundary layer physics. The pre- initialized at 0000 UTC 15 May. In addition, three more cipitation schemes used are the same as Wu et al. landfalling typhoons (Durian, Prapiroon, and Xangsane) (2012). The initial and lateral boundary conditions are occurred in the same region during 2006. These storms from the National Centers for Environmental Pre- are used to assess the impact of the new TC initialization diction (NCEP) Final Analyses (FNL) with a 18 hori- technique on the intensity and track simulations. Data zontal resolution. Two separate simulations with and and model descriptions are discussed in section 2.Abrief without (CTRL) the NC2011 scheme are performed for description of the NC2011 scheme and sensitivity tests on comparison. Unauthenticated | Downloaded 09/27/21 08:05 AM UTC 4342 MONTHLY WEATHER REVIEW VOLUME 142 TABLE 3. The name, number of run cases, initial time, minimum central SLP, and maximum temperature anomalies (K) for the four TCs in 2006. Name Initial time Pmin (hPa) dT (K) Chanchu 0000 UTC 14 May 960 4 Chanchu 1200 UTC 14 May 940 6 Chanchu 0000 UTC 15 May 930 7 Chanchu 1200 UTC 15 May 930 7 Chanchu 0000 UTC 16 May 930 7 Durian 0000 UTC 2 Dec 965 4 Durian 1200 UTC 2 Dec 965 4 Durian 0000 UTC 3 Dec 955 5 Prapiroon 0000 UTC 2 Aug 980 3 Prapiroon 1200 UTC 2 Aug 970 3 Xangsane 0000 UTC 29 Sep 960 4 Xangsane 1200 UTC 29 Sep 955 5 integrated vortex are close to the observed values.