APRIL 2015 L I A N G A N D W U 1307 Sudden Track Changes of Tropical Cyclones in Monsoon Gyres: Full-Physics, Idealized Numerical Experiments* JIA LIANG AND LIGUANG WU Pacific Typhoon Research Center, Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, and State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China (Manuscript received 9 December 2013, in final form 4 December 2014) ABSTRACT Tropical cyclones (TCs) in the eastern semicircle of large-scale monsoon gyres (MGs) were observed to take either a northward (sudden northward and northward without a sharp turn) or a westward TC turn, but only the northward turn was previously simulated in a barotropic model. To understand what controls TC track types in MGs, idealized numerical experiments are performed using the full-physics Weather Research and Forecasting (WRF) Model. These experiments indicate that TCs initially located in the eastern semicircle of MGs can generally take three types of tracks: a sudden northward track, a westward track, and a northward track without a sharp turn. The track types depend upon the TC movement relative to the MG center. In agreement with barotropic simulations, the WRF simulation confirms that approaching and being collo- cated with the MG center is crucial to the occurrence of sudden northward TC track changes and that sudden northward track changes can be generally accounted for by changes in the steering flow. TCs that take westward tracks and northward tracks without a sharp turn do not experience such a coalescence process. Westward TCs move faster than MGs and are then located to the west of the MG center, while TCs move more slowly than MGs and then take a northward track without a sharp turn. This study reveals that the specific TC track in the eastern semicircle of an MG is sensitive to the initial wind profiles of both MGs and TCs, suggesting that improvement in the observation of TC and MG structures is very important for predicting TC track types in MGs. 1. Introduction Wu and Kurihara 1996; Wu and Wang 2001a,b). Wu and Wang (2000) related TC movement to PV tendency Early studies indicated that tropical cyclone (TC) (PVT). In their dynamic framework, the TC is treated as movement results mainly from the advection of TC a positive PV anomaly relative to surrounding flows and relative vorticity by the environmental flow (environ- tends to move toward the area of the maximum of the mental steering) and the beta drift due to the interaction azimuthal wavenumber-1 PVT. The PVT theory has been between the TC circulation and the planetary vorticity applied to understanding contributions of various physi- (Holland 1983; Chan and Williams 1987; Fiorino and cal processes to the observed TC movement (Chan et al. Elsberry 1989; Carr and Elsberry 1990). Since the 1990s, 2002; Wang et al. 2012; Wu et al. 2012; Hsu et al. 2013; the influences of environmental vertical wind shear and Choi et al. 2013). Additionally, Lander and Holland diabatic heating on TC movement were investigated in (1993) and Ritchie and Holland (1993) noted the role of terms of potential vorticity (PV) (Shapiro 1992; Wu and vortices in mesoscale convective systems embedded in Emanuel 1993, 1995a,b; Wang and Holland 1996a,b,c; tropical cyclones as forcing track changes through early studies of binary vortices. However, Wu et al. (2013a) * Earth System Modeling Center Contribution Number 028. recently found that sudden northward track changes of TCs in the western North Pacific (WNP) is still a major challenge in operational TC forecasting, since the track Corresponding author address: Dr. Liguang Wu, Pacific Typhoon forecast error around the turning time is much larger than Research Center, Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science the average forecast error. and Technology, Nanjing 210044, China. TCs over the WNP are usually embedded in the large- E-mail: [email protected] scale summer monsoon circulation (e.g., Lau and Lau DOI: 10.1175/JAS-D-13-0393.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 10/07/21 10:18 AM UTC 1308 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 1990, 1992; Chang et al. 1996; Straub and Kiladis 2003; Ko and Hsu 2006, 2009). Sometimes the low-level summer monsoon circulation evolves into a specific pattern named the monsoon gyre (MG), which can be identified as a low-frequency, nearly circular cyclonic vortex in the low troposphere with a diameter of about 2500 km (Lander 1994; Harr et al. 1996; Wu et al. 2013b). Carr and Elsberry (1995) first suggested that the in- teraction between an MG and a TC led to sudden northward track changes of TCs in the WNP, which are characterized by the rapid slowing of the westward movement and then a substantial northward accelera- 21 tion within a few hours. FIG. 1. Initial radial profiles of tangential wind speed (m s ) for the MG (solid) and the TC (dashed). Using a barotropic model, Carr and Elsberry (1995) revealed that sudden northward track changes usually occur when a TC that is initially in the eastern semicircle observed when TCs are in the eastern semicircle of of an MG approaches toward and is collocated with the MGs. This observational study suggests that it is neces- MG center. They called such a process the coalescence of sary to further investigate the sudden track change. the TC with the MG. They suggested that the b-induced The main objective of this study is to examine the key Rossby wave energy dispersion is enhanced during the factors that affect TC track types in MGs through a se- coalescence, leading to strong ridging in the southeast- ries of idealized numerical experiments. In this study, ern periphery of the coalesced system. The enhanced the full-physics Weather Research and Forecasting southwesterly flows across the TC lead to a sudden (WRF) Model is used to understand the occurrence of northward-turning track. the different track types. The WRF Model has been Liang et al. (2011) numerically simulated two sudden widely used in the study of TC activity and has proven to northward changes in the track of Typhoon Morakot be one of the best models for studies of TCs. More im- (2009) when the typhoon was in the vicinity of Taiwan portantly, as a state-of-the-art atmospheric simulation Island. They found that the sudden track changes were system, the WRF Model can simulate more realistic TC associated with two cyclonic gyres on the quasi-biweekly circulation and its interaction with MGs than a baro- oscillation and Madden–Julian oscillation time scales, tropic model, which may be important for understanding respectively. The observational analysis of real typhoons of the different track types in the observation. by Wu et al. (2011a,b) showed that sudden northward track changes that occurred near centers of MGs were 2. Experimental design associated with the coalescence with the large-scale low-frequency MGs and the enhanced synoptic-scale Numerical experiments in this study are conducted southwesterly flows on the southeastern side of TCs. with the Advanced Research version of WRF (ARW), While these studies generally agree with Carr and Elsberry version 2.2.1, with three two-way interactive domains (1995), Wu et al. (2013a) revealed that both northward- with horizontal resolutions of 27, 9, and 3 km. The out- turning and westward-turning TC tracks can be ermost domain centered at 208N and middle domain TABLE 1. Description of idealized numerical experiments. Experiment Description EXP1 The TC is initially located 400 km east of the MG center on an f plane. EXP2 As in EXP1, but on a spherical surface and without the MG. CTRL The TC is initially located 400 km east of the MG center on a spherical surface. MG-P As in CTRL, but the Gaussian vortex in Mallen et al. (2005) is used to construct the initial MG. 2 MG-intensity As in CTRL, but for MGs with initial maximum tangential wind speeds of 5 and 15 m s 1. MG-RMW As in CTRL, but for MGs with initial radii of maximum tangential wind speeds of 405 and 810 km. TC-location As in CTRL, but TCs are initially located 200, 600, 800, and 1000 km east of the MG center. 2 TC-intensity As in CTRL, but for TCs with initial maximum tangential wind speeds of 20 and 40 m s 1. TC-size As in CTRL, but for TCs with initial sizes of 600 and 1400 km in diameter. TS-S As in CTRL, but the TC has stronger outer strength. TS-W As in CTRL, but the TC has weaker outer strength. Unauthenticated | Downloaded 10/07/21 10:18 AM UTC APRIL 2015 L I A N G A N D W U 1309 FIG. 2. Simulated TC tracks in an MG on the f plane (red) and the spherical surface (black), with closed dots indicating the 12-h positions. The black cross indicates the initial center of the MG. include 335 3 335 grids points (9018 km 3 9018 km) and 703 3 703 grids points (6318 km 3 6318 km), which are sufficiently large to cover the interaction between a TC and an MG and to reduce the lateral boundary influence. The innermost domain with 301 3 301 grids points (900 km 3 900 km) is designed to move with a TC. The model has 41 levels in the vertical from the surface to 50 hPa. To reduce artificial wave reflection into the model interior, open lateral boundary conditions are used in the simulation.