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Sudden Track Changes of Tropical Cyclones in 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 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 (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 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

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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.

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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. In the model, the generally used physics options are adopted. The WRF single-moment 3-class (WSM3) simple ice microphysics scheme (Dudhia 1989)isused in the outermost domain, and the Lin et al. (1983) mi- 21 crophysics scheme suitable for high-resolution simu- FIG. 3. The simulated wind field (vectors, m s ) and speed 21 21 lations is used in the middle and innermost domains. greater than 10 m s (shaded, m s ) at 36 h at (a) 200 and (b) 850 hPa in the CTRL experiment. Because convective schemes are usually used in do- mains with a horizontal resolution lower than 10 km, the Kain–Fritch convective scheme (Kain and Fritch The model is initialized with a large, weak MG cen- 1993) is only used in the outermost domain. Moreover, tered at 208N and a small, intense TC over an open the Yonsei University PBL scheme (Noh et al. 2003), ocean with a constant sea surface temperature (SST) of the Dudhia shortwave parameterization (Dudhia 298C. The MG and the TC are initially axisymmetric 1989), and the Rapid Radiative Transfer Model vortices with the radial profile of tangential wind used in (RRTM) longwave parameterization (Mlawer et al. Wang (2007) (Fig. 1). Several different profiles for 1997) are used in all three model domains. All experi- the MG and TC are also used for sensitivity experi- ments are integrated up to 120 h. ments. The idealized MG (TC) has an initial maximum

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FIG. 4. The simulated TC track with closed dots indicating 12-h positions in the CTRL experiment. The inset shows the track 21 change from 72 to 90 h, with 3-h positions indicated with closed FIG. 5. Time series of the (a) zonal (m s ) and (b) meridional 2 dots. The black cross is the initial center of the MG. (m s 1) components of TC translation speed estimated with 2-h difference of TC center positions (solid) and of the steering flow (dashed) in the CTRL experiment. The vertical lines indicate 2 tangential wind of 10 (30) m s 1 at the radius of northward turning times, and the arrows show the trends in speeds. 594 (81) km from the gyre (TC) center, gradually de- creasing in the vertical to zero at 100 hPa, with an initial In this study, the TC center is defined as that which size of 3000 (1000) km in diameter (where the vortex wind maximizes the symmetric tangential wind, and a varia- speed becomes zero). The initial vertical profiles of the tional approach is used to locate the TC center through environmental relative humidity and temperature are maximizing the azimuthal-mean tangential wind speed based on the August average over 58–258N, 1258–1508E (Wu et al. 2006). during 2000–11 from the Modern-Era Retrospective Analysis for Research and Applications (MERRA). Two idealized experiments are conducted to validate the realism of the previous highly idealized numerical framework with the WRF integration (EXP1 and EXP2, Table 1). The EXP1 is run on an f plane. With the ab- sence of the planetary vorticity gradient, the TC is simply steered by the flow of the MG and generally takes a cyclonic track (Fig. 2). It is well known that the TC movement is also associated with the beta drift (Holland 1983; Fiorino and Elsberry 1989; Carr and Elsberry 1990; Wang et al. 1997). Because of the beta effect, a pair of counterrotating asymmetric circulations with an an- ticyclone to the northeast and a cyclone to the southwest (beta gyres) is induced in the Northern Hemisphere. The near-uniform asymmetric flow between the two gyres is called the ventilation flow, which steers a TC northwestward (b drift).The experiment EXP2 is de- signed as a typical b-drift case, in which the TC moves northwestward with a mean translation speed of about 2 2.2 m s 1, in agreement with previous studies (Chan and Williams 1987; Wang and Li 1992; Carr and Elsberry FIG. 6. Contributions of multilevel-averaged horizontal advec- 1995). The results of these two experiments enhance our tion (HA, red) and diabatic heating (DH, blue) on the TC trans- confidence in conducting the following numerical lation speed (C, black) at (a) 72, (b) 78, (c) 84, and (d) 90 h in the experiments. CTRL experiment.

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3. TC track changes in the control experiment In the control experiment (CTRL), the TC is initially located 400 km to the east of the MG center. During the first 36-h integration, the initially axisymmetric MG gradually evolves into an asymmetric structure, which is generally similar to the composite structure of the ob- servation in Wu et al. (2013b). Figure 3 shows the sim- ulated 36-h 200- and 850-hPa wind fields. Because of the b-induced Rossby wave energy dispersion (Carr and Elsberry 1995; Wu et al. 2013b; Liang et al. 2014), the low-level southwesterly are enhanced in the eastern semicircle of the MG with a maximum of about 2 15 m s 1 (Fig. 3b). The upper-level MG circulation be- comes an with very strong northerly flows in the eastern periphery (Fig. 3a). Based on the definition for sudden northward track changes in Wu et al. (2013a) that a track direction change exceeds 408 (378) during the 12 (6)-h period, the simulated track change in the CTRL experiment rep- resents a sudden northward TC track change. The 12-h track change occurs between 75 and 87 h (Fig. 4). The cyclonic flow associated with the MG leads to the cy- clonic steering of the TC during the first 75 h, and then the TC turns slightly southward and makes a northward turn of about 768 around 84 h. During 75–87 h, the westward component of the TC translation speed esti- mated from the 2-h differences of the simulated TC 2 centers rapidly decreases to 1.5 m s 1 at the turning time; 2 subsequently, a northward acceleration of nearly 3 m s 1 occurs (Fig. 5). The simulated rapid slowing of the westward movement and then a substantial northward acceleration within a few hours are in agreement with observations and previous numerical simulations (Carr and Elsberry 1995; Liang et al. 2011; Wu et al. 2011a,b, 2013a). After the sudden northward turn, the TC maintains a north-northeastward track. a. Relationship of the sudden TC track change with the steering flow TC movement is primarily steered by the asymmetric flow across the TC center (steering flow), which consists of the large-scale environmental flow and the ventilation flow that results from the interaction between the gra- dient of Earth’s vorticity and the TC circulation (Holland 1983; Fiorino and Elsberry 1989). Wu et al. (2011b) and Wu et al. (2013a) showed that the observed 21 sudden northward track change can be accounted for by FIG. 7. Simulated 700-hPa asymmetric winds (vectors, m s )and 2 2 the change of steering flows on the synoptic time scale. speeds greater than 4 m s 1 (shaded, m s 1) in the 9-km domain in the In this study, the steering flow is calculated as the mean CTRL experiment at (a) 72, (b) 84, and (c) 90 h. Red and black thick vector averaged in a radius distance of 500 km from arrows respectively indicate the TC-movement vector and the average asymmetric wind vector within 500 km of the TC center. Black circles the TC center between 950 and 250 hPa. Figure 5 are 300 and 500 km away from TC centers, which are always at (0, 0). shows that the calculated steering is consistent with the

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FIG. 8. Wavenumber-1 asymmetric components from simulated 700-hPa asymmetric wind fields in the 9-km do- 2 2 main (black vectors, m s 1) and estimated translation speeds (red vectors, m s 1) of the simulated TC in the CTRL experiment: (a) 42, (b) 66, (c) 75, (d) 78, (f) 81, and (g) 84 h. The dashed lines with arrows outline TC beta gyres. Letters A and C indicate the and cyclones, respectively.

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FIG. 10. The radius–time cross section of the azimuthal-mean asymmetric kinetic energy (shaded and contoured, with the in- terval of 2 J).

TC translation. Further calculation indicates that the calculated steering is not sensitive to the selection of the radius when it is between 400 and 700 km. The difference in the zonal speed between the TC move- ment and the steering flow after 60 h may be due to diabatic heating (Wu and Wang 2000, 2001a,b; Chan et al. 2002; Wang et al. 2012). Thus, the role of the di- abatic heating on sudden track changes is also exam- ined by using the PVT diagnostic approach advanced by Wu and Wang (2000) (Fig. 6). The PV horizontal advection (mainly from steering) plays the most im- portant role in sudden northward track changes, while the relatively small contribution of diabatic heating leads to the eastward movement, which slows down the westward motion of the TC. Therefore the sudden northward track change of the diabatic baroclinic TC in this study is primarily caused by the change of the steering flow. Sudden track changes are also shown in low-level 21 FIG. 9. The 700-hPa environmental winds (vectors, m s ) and asymmetric wind fields associated with the TC, which 2 speeds (shaded, m s 1) in the outermost domain in the CTRL ex- are generally represented by remaining wind fields after periment at (a) 66, (b) 78, and (c) 84 h, with black dots and typhoon removing the azimuthal average centered in the TC. marks indicating initial MG positions and TC centers, respectively. Figure 7 shows the evolution of 700-hPa asymmetric The red circles are 500 km away from TC centers. The blue dashed lines roughly outline MGs. wind fields within 500 km from the TC center from 72 to 90 h. At 72 h (Fig. 7a), easterly winds greater than

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21 FIG. 11. Simulated 700-hPa winds (vectors, m s ) and relative 2 2 2 2 vorticity less than 1 3 10 5 s 1(shaded, 10 5 s 1) in the 9-km domain in the CTRL experiment at (a) 72 and (b) 87 h. Black circles are 300 and 500 km away from TC centers, which are always at (0, 0).

2 4ms 1 occupy the region within 300 km from the TC center, corresponding to the westward movement of the TC. During 72–84 h (Figs. 7b), the TC structure gradu- 21 ally becomes more symmetric with mean asymmetric FIG. 12. Simulated 700-hPa winds (vectors, m s ) and speeds 2 2 winds less than 1 m s 1, accompanying the slowdown of (shaded, m s 1) in the 9-km domain in the CTRL experiment at the westward TC. In the next 6 h (Fig. 7c), strong (a) 96, (b) 108, and (c) 120 h. Black thick arrows indicate the TC- 2 southerly winds greater than 4 m s 1 rapidly spread in movement vectors. Black circle is 350 km away from the TC center, which is always at (0, 0). the eastern and southern quadrant of the TC, leading to enhanced southerly winds in the southeastern periphery

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beta gyres. To demonstrate the change of the orienta- tion of the beta gyres, Fig. 8 shows the evolution of the beta gyres during 42–84 h in terms of the wavenumber-1 asymmetric component of 700-hPa wind fields. The mean wavenumber-1 asymmetric component of 700-hPa averaged wind fields over the area within 1000 km of TC center has been removed in this figure to make beta gyres clearer. We can see that the beta gyres rotate cy- clonically because of the influence of the MG. At 42 h (Fig. 8a), the anticyclonic (cyclonic) gyre is located to the northeast (southwest) of the TC center. The south- easterly ventilation flow between beta gyres corre- sponds to the northwestward movement of the TC. At 66 h, the beta gyres rotate cyclonically, leading to a northeasterly ventilation flow and southwestward movement of the TC (Fig. 8b). The pair of asymmetric gyres continues to rotate cyclonically until the anticy- clonic (cyclonic) gyre is located to the northwest (southeast) (Figs. 8c–f). In the presence of the MG, it is clear that the cyclonic rotation of the beta gyres changes the direction of the ventilation flow and reduces the northwestward movement of the TC during 75–81 h. By 84 h (Figs. 8g), the ventilation flow between the beta gyres is generally northerly. It is suggested that the cy- clonic rotation of the beta gyres causes the initial northwestward-ventilation-flow turn to the south and then reduces the westward translation of the TC before the northward turn. c. Coalescence of the TC with the monsoon gyre The cyclonic rotation of the beta gyres leads to the coalescence of the TC with the MG. Figure 9 shows the 700-hPa environmental wind field in the outermost do- main during the sudden TC track change, obtained from the filtering technique developed by Kurihara et al.

21 (1993, 1995). It is used twice to better remove the TC FIG. 13. Initial radial profiles of (a) tangential wind speed (m s ) 2 2 and (b) relative vorticity (10 5 s 1) of MGs and (c) simulated TC circulation. At 66 h (Fig. 9a), the TC is located in the tracks with an interval of 12 h in the CTRL (black) and MG-P (red) northern portion of the MG, about 400 km away from experiments. the MG center. Then it gradually moves into the MG interior, and the TC center coincides with the MG center by 84 h (Figs. 9b,c). Note that the MG moves north- 2 of the TC, in agreement with the sudden northward turn. westward at a mean translation speed of about 2.7 m s 1 Note that the direction of the TC movement slightly in the CTRL experiment. It is thus evident that the co- deviates from the 700-hPa mean asymmetric wind vec- alescence happens when the MG catches up with the tor. However, Fig. 7 suggests that the TC movement is slowing TC, suggesting that the TC movement relative basically consistent with the evolution of low-level TC to the MG center is key to the coalescence. asymmetric wind patterns, including the weakening of The coalescence causes the symmetrization of the TC the asymmetric winds and subsequently suddenly en- structure, corresponding to the decrease of the TC hanced southerly winds. asymmetric kinetic energy. Figure 10 shows the evolu- tion of the azimuthal-mean asymmetric kinetic energy b. Beta gyres associated with sudden track changes during 36–120 h. During 72–84 h, the area of the The MG can affect the TC motion by environmental azimuthal-mean asymmetric kinetic energy greater than steering and changing the strength and orientation of 10 J contracts within 100 km of the TC center, indicating

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FIG. 14. As in Fig. 8, but for the MG-P experiment at (a) 42, (b) 66, (c) 75, and (d) 84 h. that the TC is experiencing a symmetrization process. 4. Sensitivity to initial structural parameters of the After 84 h, the azimuthal-mean asymmetric kinetic monsoon gyre energy rapidly increases in the outer region, in agreement with the enhancement of asymmetric Based on the analysis in section 3, the rotation of beta southerly winds shown in Fig. 7c. It is found that the gyres is crucial for the coalescence, which is caused by enhanced asymmetric winds are associated with the the sheared environmental flow associated with the increase of the negative relative vorticity in the east- MG. Wang et al. (1997) found that the cyclonic rotation ern quadrant of the TC outside 500 km of the TC angle of TC beta drift is sensitive to the magnitude of center, similar to the energy dispersion shown by Carr cyclonic sheared environmental flows: the larger the and Elsberry (1995) (Fig. 11). The enhanced periph- cyclonic environmental shears, the stronger the west- eral southerly winds produce a southerly asymmetric ward deflection of beta drift in the Northern Hemi- flow across the center of the TC to steer the TC sphere. Here, we examine the TC movement when the northward. MG is initialized with a Gaussian vortex (MG-P; Table 1; After the sudden northward turn, southwesterly winds Mallen et al. 2005). The Gaussian vortex can be used are enhanced along the southeastern periphery of the as an approximation to the broad vorticity distribution TC (Figs. 12a,b). By 120 h, strong winds mainly appear in observed in the weak vortex (Reasor and Montgomery the eastern side of the TC (Fig. 12c). The MG and TC 2001; Reasor et al. 2004). The Gaussian MG with become a combined system after the northward turn. stronger tangential winds has much weaker cyclonic

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21 21 21 FIG. 15. Simulated 700-hPa winds (vectors, m s ) and speeds greater than 15 m s (shaded, m s ) in the 9-km domain in the MG-P experiment at (a) 60, (b) 72, (c) 84, and (d) 96 h. Black circles are 500 km away from TC centers. vorticity in the 400–600-km band from the center than the beta gyres do not rotate farther. The persistent the MG in the CTRL experiment (Figs. 13a,b). easterly ventilation flow in the MG-P experiment ac- Figure 13c shows the simulated TC track in the sensi- celerates the westward movement of the TC. As shown tivity experiment. The simulated TC track turns west- in Fig. 15, the TC that is initially embedded in the MG ward around 77 h and maintains a southwestward track gradually moves northwestward and is located to the in the last 43 h. Prior to the turning time, the TC takes north of the MG center by 72 h (Fig. 15b). Because of a general northwestward track and then, following the easterly steering flow, the TC moves to the a westward shift of about 328 within 6 h, shows a mod- northwest of the MG center. It is clear that the co- erate speed change, in agreement with features of ob- alescence process that occurs in the CTRL experiment served westward-turning TC track cases in Wu et al. cannot be observed in this experiment by 84 h (2013a). (Figs. 15c,d). The northeasterly flow in the north- We further examine the evolution of the beta gyres western periphery of the MG steers the TC south- in the MG-P experiment (Fig. 14). Before 66 h westward (Fig. 15d). The sustained westward acceleration (Figs. 14a,b), the beta gyres rotate cyclonically with of the TC in the MG-P experiment leads to the TC the easterly ventilation flow, which is similar to that in moving into the western side of the MG before 77 h the CTRL experiment. But in the next 18 h (Figs. 14c,d), (Fig. 16), suggesting that the westward-turning TC track

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FIG. 16. Time series of the zonal component of TC translation speeds estimated with 12-h difference of TC center positions in the CTRL (black) and MG-P (red) experiments, respectively. The black and red dots indicate the northward and westward turning times in the CTRL and MG-P experiments, respectively. associated with the MG is due to the relatively faster TC movement. In addition, we also examine the sensitivity of the intensity and the radius of the maximum tangential wind of the MG and find that these two parameters have little effect on the sudden northward track changes (Fig. 17). The MG with the weaker intensity or larger radius of the maximum tangential wind makes a later sudden northward turn and a slower northward acceleration.

5. Sensitivity to initial structural parameters of the TC Based on Carr and Elsberry (1995), the sensitivity to the initial structure of the TC on sudden track changes are examined by organizing three experiment sets (TC-location, TC-intensity, and TC-size; Table 1). Set TC-location only varies the initial distance of the TC to the MG center. Sets TC-intensity and TC-size vary the FIG. 17. Simulated TC tracks with an interval of 12 h for sensi- initial maximum tangential wind speed and the initial tivity experiments associated with the initial (a) MG-intensity and size in diameter of the TC, respectively. The simulated (b) MG-RMW. The simulated TC track for the CTRL is marked in black. The insets in (a) and (b) show the initial profile of the tan- TC tracks from these three experiment sets are shown in 2 gential wind speed (m s 1) for MGs in numerical experiments. Fig. 18. All the simulated TCs experience the northward turn, although with different turning angles. We find that the turning angle is more sensitive to the initial location simulated TC tracks are shown in Fig. 19b. The TC with of the TC: the farther the TC is initially located east of stronger outer winds experiences a less-sharp turning the MG center, the smaller its northward turning angle angle, indicating that the turning angle is also sensitive (Fig. 18a). to the outer strength of the initial TC. It is found that the To examine the influence of the initial TC outer pro- less-sharp turning angle is associated with the reduction file, two additional sensitivity experiments are con- of the westward translation speed in the TC-S experi- ducted (TC-S and TC-W; Table 1). Compared to the ment (Fig. 20). Since 36 h, the TC maintains a slower CTRL experiment, the outer strength of the TC is westward translation than TCs in the CTRL and TC-W stronger in TC-S and weaker in TC-W (Fig. 19a); experiments. As a result, the TC is gradually located east

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21 FIG. 19. (a) Initial radial profiles of tangential wind speed (m s ) for TCs and (b) simulated TC tracks with an interval of 12 h in the CTRL (black), TC-W (red), and TC-S (blue) experiment, respectively.

of the MG center and steered by the southerly and southeasterly winds in the eastern portion of the MG. It is suggested that the relatively faster movement of the MG leads to the northward TC track without a sharp turn.

6. Summary TCs that are initially located in the eastern semicircle of a large-scale MG over the WNP occasionally expe- rience a sudden northward track change. Following the FIG. 18. Simulated TC tracks with an interval of 12 h for sensi- pioneering study of Carr and Elsberry (1995) with tivity experiments associated with the initial (a) TC-location, a barotropic numerical model, idealized numerical ex- (b) TC-intensity, and (c) TC-size. The simulated TC track for the periments are performed with the Advanced Research CTRL is marked in black. The insets in (b) and (c) show the initial 2 profile of the tangential wind speed (m s 1) for TCs in numerical WRF Model with a focus on what controls the track experiments. types of these TCs. In this study, the WRF Model is initialized with an intense TC-like vortex that is

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move northwestward, its circulation cyclonically ro- tates the beta gyres of the TC, reducing the north- westward movement of the TC. As a result, the coalescence process happens. As the TC center is collocated with the MG center, the energy dispersion associated with the symmetrized TC circulation is enhanced, and the negative relative vorticity rapidly strengthens southerly winds in the eastern periphery of the TC. The resulting steering makes the TC take a sudden northward track. TCs that take westward-turning tracks and north- ward tracks without a sharp turn do not experience such a coalescence process. Westward-turning TCs FIG. 20. Time series of zonal component of TC translation speeds move faster than MGs and are then located to the estimated with 12-h difference of TC center positions in the CTRL (black solid), TC-W (red, long dashed), and TC-S (blue, short west of the MG center, while TCs move more slowly dashed) experiments, respectively. Three dots with different colors than MGs and then take a northward track without indicate northward turning times. a sharp turn. The sensitivity experiments are con- ducted to examine the influences of initial MG and TC structures. In the numerical experiment with weak embedded in a large-scale MG. These numerical ex- relative vorticity of the MG, it is found that the MG periments indicate that TCs that are initially located in cannot rotate beta gyres of the TC to such a degree the eastern semicircle of MGs can generally take three that the TC center can be collocated with the MG types of tracks—a sudden northward track, a westward- center, thus eventually leading to a southwestward turning track, and a northward track without TC track. We also find that the northward turning a sharp turn—and that the different track types de- angle is sensitive to the initial location and the outer pend on the TC movement relative to the MG center strength of the TC. The TC with strong outer strength (Fig. 21). experiences a less-sharp turning angle. Its slow In agreement with the barotropic simulations in translation makes the TC remain behind the MG Carr and Elsberry (1995), the WRF simulation con- center; thus, it is steered by the southerly and south- firms that the TC coalescence with the MG (ap- easterly winds to the east of the MG center. We sug- proaching and collocating with the MG center) is gest that improvement in the observation of TC and a key process to the occurrence of sudden northward MG structures is very important for successfully TC track changes. While the b effect makes the MG predicting TC track types in MGs.

FIG. 21. Schematic diagrams of three track types in the MG. The blue and red circles with arrows denote the MG and the TC circulation, respectively.

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Acknowledgments. This research was jointly supported Kain, J. S., and J. M. Fritch, 1993: Convective parameterization for by the National Basic Research Program of China mesoscale models: The Kain–Fritch scheme. The Representa- (2013CB430103, 2015CB452803), the National Natural tion of Cumulus Convection in Numerical Models, Meteor. Monogr., No. 46, Amer. Meteor. Soc., 165–170. Science Foundation of China (41275093), the project of Ko, K.-C., and H.-H. Hsu, 2006: Sub-monthly circulation features the specially appointed professorship of Jiangsu Prov- associated with tropical cyclone tracks over the East Asian ince, the Natural Science Foundation for Higher Educa- monsoon area during July–August season. J. Meteor. Soc. tion Institutions in Jiangsu Province (14KJB170014), the Japan, 84, 871–889, doi:10.2151/jmsj.84.871. pre-research program of the Nanjing University of In- ——, and ——, 2009: ISO modulation on the sub-monthly wave 3 pattern and the recurving tropical cyclones in the tropical formation Science and Technology (2013 001), the western North Pacific. J. Climate, 22, 582–599, doi:10.1175/ project of the startup foundation for scientific research of 2008JCLI2282.1. the Nanjing University of Information Science and Kurihara, Y., M. A. Bender, and R. J. Ross, 1993: An initialization Technology (20123037), and the open project of the scheme of hurricane models by vortex specification. Mon. Wea. , State Key Laboratory of Severe Weather, Chinese Rev., 121, 2030–2045, doi:10.1175/1520-0493(1993)121 2030: AISOHM.2.0.CO;2. Academy of Meteorological Sciences (2013LASW-A13). ——, ——, R. E. Tuleya, and R. J. Ross, 1995: Improvements in the GFDL hurricane prediction system. Mon. Wea. Rev., 123, 2791–2801, doi:10.1175/1520-0493(1995)123,2791: REFERENCES IITGHP.2.0.CO;2. Carr, L. E., III, and R. L. Elsberry, 1990: Observational evi- Lander, M. A., 1994: Description of a monsoon gyre and its effects dence for predictions of tropical cyclone propagation rel- on the tropical cyclones in the western North Pacific during ative to environmental steering. J. Atmos. Sci., 47, 542–546, August 1991. Wea. Forecasting, 9, 640–654, doi:10.1175/ , . doi:10.1175/1520-0469(1990)047,0542:OEFPOT.2.0.CO;2. 1520-0434(1994)009 0640:DOAMGA 2.0.CO;2. ——, and ——, 1995: Monsoonal interactions leading to sudden ——, and G. J. Holland, 1993: On the interaction of tropical- tropical cyclone track changes. Mon. Wea. Rev., 123, 265–290, cyclone-scale vortices. I: Observations. Quart. J. Roy. Meteor. doi:10.1175/1520-0493(1995)123,0265:MILTST.2.0.CO;2. Soc., 119, 1347–1361, doi:10.1002/qj.49711951406. Chan, J. C. L., and R. T. Williams, 1987: Analytical and numerical Lau, K.-H., and N.-C. Lau, 1990: Observed structure and propa- studies of the beta-effect in tropical cyclone motion. Part I: gation characteristics of tropical summertime synoptic scale Zero mean flow. J. Atmos. Sci., 44, 1257–1265, doi:10.1175/ disturbances. Mon. Wea. Rev., 118, 1888–1913, doi:10.1175/ , . 1520-0469(1987)044,1257:AANSOT.2.0.CO;2. 1520-0493(1990)118 1888:OSAPCO 2.0.CO;2. ——, F. M. F. Ko, and Y. M. Lei, 2002: Relationship between po- ——, and ——, 1992: The energetics and propagation dynamics of tential vorticity tendency and tropical cyclone motion. J. Atmos. tropical summertime synoptic-scale disturbances. Mon. Wea. , Sci., 59, 1317–1336, doi:10.1175/1520-0469(2002)059,1317: Rev., 120, 2523–2539, doi:10.1175/1520-0493(1992)120 2523: . RBPVTA.2.0.CO;2. TEAPDO 2.0.CO;2. Chang, C.-P., J. M. Chen, P. A. Harr, and L. E. Carr, 1996: Liang, J., L. Wu, X. Ge, and C.-C. Wu, 2011: Monsoonal influence Northwestward-propagating wave-like patterns over the on Typhoon Morakot (2009). Part II: Numerical study. J. At- tropical western North Pacific during summer. Mon. Wea. mos. Sci., 68, 2222–2235, doi:10.1175/2011JAS3731.1. Rev., 124, 2245–2266, doi:10.1175/1520-0493(1996)124,2245: ——, ——, and H. Zong, 2014: Idealized numerical simulations of NPWPOT.2.0.CO;2. tropical cyclone formation associated with monsoon gyres. Choi, Y., K.-S. Yun, K.-J. Ha, K.-Y. Kim, S.-J. Yoon, and J. C. L. Adv. Atmos. Sci., 31, 305–315, doi:10.1007/s00376-013-2282-1. Chan, 2013: Effects of asymmetric SST distribution on Lin, Y.-L., R. D. Farley, and H. D. Orville, 1983: Bulk parameteri- straight-moving Typhoon Ewiniar (2006) and recurving Ty- zation of the snow field in a cloud model. J. Climate Appl. phoon Maemi (2003). Mon. Wea. Rev., 141, 3950–3967, Meteor., 22, 1065–1092, doi:10.1175/1520-0450(1983)022,1065: doi:10.1175/MWR-D-12-00207.1. BPOTSF.2.0.CO;2. Dudhia, J., 1989: Numerical study of convection observed Mallen, K. J., M. T. Montgomery, and B. Wang, 2005: Reexamining during the Winter Monsoon Experiment using a mesoscale the near-core radial structure of the tropical cyclone primary two-dimensional model. J. Atmos. Sci., 46, 3077–3107, circulation: Implications for vortex resiliency. J. Atmos. Sci., doi:10.1175/1520-0469(1989)046,3077:NSOCOD.2.0.CO;2. 62, 408–425, doi:10.1175/JAS-3377.1. Fiorino, M., and R. L. Elsberry, 1989: Some aspects of vortex structure Mlawer, E. J., S. J. Taubman, P. D. Brown, M. J. Iacono, and S. A. related to tropical cyclone motion. J. Atmos. Sci., 46, 975–990, Clough, 1997: Radiative transfer for inhomogeneous atmospheres: doi:10.1175/1520-0469(1989)046,0975:SAOVSR.2.0.CO;2. RRTM, a validated correlated-k model for the longwave. Harr, P. A., R. L. Elsberry, and J. C. L. Chan, 1996: Transformation J. Geophys. Res., 102, 16 663–16 682, doi:10.1029/97JD00237. of a large monsoon depression to a tropical storm during Noh, Y., W. G. Cheon, S. Y. Hong, and S. Raasch, 2003: Im- TCM-93. Mon. Wea. Rev., 124, 2625–2643, doi:10.1175/ provement of the K-profile model for the planetary boundary 1520-0493(1996)124,2625:TOALMD.2.0.CO;2. layer based on large eddy simulation data. Bound.-Layer Holland, G. J., 1983: Tropical cyclone motion: Environmental Meteor., 107, 401–427, doi:10.1023/A:1022146015946. interaction plus a beta effect. J. Atmos. Sci., 40, 328–342, Reasor, P. D., and M. T. Montgomery, 2001: Three-dimensional doi:10.1175/1520-0469(1983)040,0328:TCMEIP.2.0.CO;2. alignment and corotation of weak, TC-like vortices via linear Hsu, L.-H., H.-C. Kuo, and R. G. Fovell, 2013: On the geographic vortex Rossby waves. J. Atmos. Sci., 58, 2306–2330, asymmetry of typhoon translation speed across the mountain- doi:10.1175/1520-0469(2001)058,2306:TDAACO.2.0.CO;2. ous island of Taiwan. J. Atmos. Sci., 70, 1006–1022, doi:10.1175/ ——, ——, and L. D. Grasso, 2004: A new look at the problem of JAS-D-12-0173.1. tropical cyclones in vertical shear flow: Vortex resiliency.

Unauthenticated | Downloaded 10/07/21 10:18 AM UTC 1322 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

J. Atmos. Sci., 61, 3–22, doi:10.1175/1520-0469(2004)061,0003: Wea. Rev., 123, 69–92, doi:10.1175/1520-0493(1995)123,0069: ANLATP.2.0.CO;2. PVDOHM.2.0.CO;2. Ritchie, E. A., and G. J. Holland, 1993: On the interaction of ——, and ——, 1995b: Potential vorticity diagnostics of hurricane tropical-cyclone-scale vortices. II: Discrete vortex patches. movement. Part II: Tropical Storm Ana (1991) and Hurricane Quart. J. Roy. Meteor. Soc., 119, 1363–1379, doi:10.1002/ Andrew (1992). Mon. Wea. Rev., 123, 93–109, doi:10.1175/ qj.49711951407. 1520-0493(1995)123,0093:PVDOHM.2.0.CO;2. Shapiro, L. J., 1992: Hurricane vortex motion and evolution in ——, and Y. Kurihara, 1996: A numerical study of the feedback a three-layer model. J. Atmos. Sci., 49, 140–153, doi:10.1175/ mechanisms of hurricane–environment interaction on hurricane 1520-0469(1992)049,0140:HVMAEI.2.0.CO;2. movement from the potential vorticity perspective. J. Atmos. Straub, K. H., and G. N. Kiladis, 2003: Interactions between the Sci., 53, 2264–2282, doi:10.1175/1520-0469(1996)053,2264: boreal summer intraseasonal oscillation and higher- ANSOTF.2.0.CO;2. frequency tropical wave activity. Mon. Wea. Rev., 131, 945– ——, S.-G. Chen, C.-C. Yang, P.-H. Lin, and S. D. Aberson, 2012: 960, doi:10.1175/1520-0493(2003)131,0945:IBTBSI.2.0.CO;2. Potential vorticity diagnosis of the factors affecting the track Wang, B., and X. Li, 1992: The beta drift of three-dimensional of Typhoon Sinlaku (2008) and the impact from dropwind- vortices: A numerical study. Mon. Wea. Rev., 120, 579–593, sonde data during T-PARC. Mon. Wea. Rev., 140, 2670–2688, doi:10.1175/1520-0493(1992)120,0579:TBDOTD.2.0.CO;2. doi:10.1175/MWR-D-11-00229.1. ——, ——, and L. Wu, 1997: Direction of drift Wu, L., and B. Wang, 2000: A potential vorticity tendency di- in horizontally sheared flows. J. Atmos. Sci., 54, 1462–1471, agnostic approach for tropical cyclone motion. Mon. Wea. doi:10.1175/1520-0469(1997)054,1462:DOHBDI.2.0.CO;2. Rev., 128, 1899–1911, doi:10.1175/1520-0493(2000)128,1899: Wang, C.-C., H.-C. Kuo, Y.-H. Chen, H.-L. Huang, C.-H. Chung, APVTDA.2.0.CO;2. and K. Tsuboki, 2012: Effects of asymmetric latent heating ——, and ——, 2001a: Movement and vertical coupling of adia- on typhoon movement crossing Taiwan: The case of Morakot batic baroclinic tropical cyclones. J. Atmos. Sci., 58, 1801–1814, (2009) with extreme rainfall. J. Atmos. Sci., 69, 3172–3196, doi:10.1175/1520-0469(2001)058,1801:MAVCOA.2.0.CO;2. doi:10.1175/JAS-D-11-0346.1. ——, and ——, 2001b: Effects of convective heating on movement and Wang, Y., 2007: A multiply nested, movable mesh, fully com- vertical coupling of tropical cyclones: A numerical study. J. At- pressible, nonhydrostatic tropical cyclone model—TCM4: mos. Sci., 58, 3639–3649, doi:10.1175/1520-0469(2001)058,3639: Model description and development of asymmetries without EOCHOM.2.0.CO;2. explicit asymmetric forcing. Meteor. Atmos. Phys., 97, 93–116, ——, S. A. Braun, J. Halverson, and G. Heymsfield, 2006: A nu- doi:10.1007/s00703-006-0246-z. merical study of (2001). Part I: Model verifi- ——, and G. J. Holland, 1996a: The beta drift of baroclinic cation and storm evolution. J. Atmos. Sci., 63, 65–86, vortices. Part I: Adiabatic vortices. J. Atmos. Sci., 53, 411– doi:10.1175/JAS3597.1. 427, doi:10.1175/1520-0469(1996)053,0411:TBDOBV.2.0.CO;2. ——, J. Liang, and C.-C. Wu, 2011a: Monsoonal influence on Ty- ——, and ——, 1996b: The beta drift of baroclinic vortices. Part II: phoon Morakot (2009). Part I: Observational analysis. J. At- Diabatic vortices. J. Atmos. Sci., 53, 3737–3756, doi:10.1175/ mos. Sci., 68, 2208–2221, doi:10.1175/2011JAS3730.1. 1520-0469(1996)053,3737:TBDOBV.2.0.CO;2. ——, H. Zong, and J. Liang, 2011b: Observational analysis of ——, and ——, 1996c: Tropical cyclone motion and evolution in sudden tropical cyclone track changes in the vicinity of the vertical shear. J. Atmos. Sci., 53, 3313–3332, doi:10.1175/ East China Sea. J. Atmos. Sci., 68, 3012–3031, doi:10.1175/ 1520-0469(1996)053,3313:TCMAEI.2.0.CO;2. 2010JAS3559.1. Wu, C.-C., and K. A. Emanuel, 1993: Interaction of a baroclinic vortex ——, Z. Ni, J. Duan, and H. Zong, 2013a: Sudden tropical cyclone track with background shear: Application to hurricane movement. changes over the western North Pacific: A composite study. Mon. J. Atmos. Sci., 50, 62–76, doi:10.1175/1520-0469(1993)050,0062: Wea. Rev., 141, 2597–2610, doi:10.1175/MWR-D-12-00224.1. IOABVW.2.0.CO;2. ——, H. Zong, and J. Liang, 2013b: Observational analysis of ——, and ——, 1995a: Potential vorticity diagnostics of hurricane tropical cyclone formation associated with monsoon gyres. movement. Part I: A case study of (1991). Mon. J. Atmos. Sci., 70, 1023–1034, doi:10.1175/JAS-D-12-0117.1.

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