1. Introduction
During the past several decades, much effort has been made to understand the tropical cyclone (TC) motion. The complexity of the TC motion derives from a wide variety of external, internal, and interactive dynamical forcing (Wang et al. 1998; Chan 2005). The most fundamental process of the interactive dynamics is known as the beta drift. This mechanism, which depends on vortex structure and latitude, refers to the northwestward TC motion in the Northern Hemisphere on a beta plane in the absence of an environmental steering flow (Holland 1983; Chan and Williams 1987; Fiorino and Elsberry 1989); this motion is determined by the beta-induced secondary steering flow over the vortex center, referred to as the ventilation flow. The secondary steering flow can be modulated by the internal dynamic factors and the interaction between a TC and external forcing (Wang et al. 1998; Chan 2005).
Sea surface temperature (SST) is an important factor affecting internal and external TC dynamics. The role of SST in the TC genesis and intensity has been widely investigated (Chang 1979; Tuleya and Kurihara 1982; Schade 2000; Michaels et al. 2006). In addition, thermodynamical influences of SST also modulate the TC motion. Wu et al. (2005) investigated how the TC-induced SST anomaly affects the TC motion. They designed symmetric and asymmetric SST anomalies with respect to a TC center to focus on the impacts of air–sea interaction on the TC motion. A large-scale asymmetric SST distribution may also affect the TC motion in a different way. Chang and Madala (1980) showed the influence of various SST distributions with a mean flow on the behavior of a translating TC. They showed that TCs tend to move into regions of warmer SST and a favorable condition for TC deflection is established when the SST gradient is perpendicular to the mean flow. The SST distributions affect the TC motion by altering total surface friction and heat flux exchange. Yun et al. (2012) studied a northeastward-moving TC to demonstrate that the TC motion is sensitive to the SST magnitude and gradient. They suggested that an eastward SST increase produces a greater eastward TC deflection than does a meridional SST gradient or variation of SST magnitude.
While most studies used idealized numerical experiments to understand the dynamics of the TC motion, real TCs experience further complicated environments which result in intricate interactions. Although an attempt has been made to understand the effects of SST gradient on Typhoon Maemi (2003) in a realistic environment (Yun et al. 2012), a single northeastward-moving TC case may be insufficient to verify and generalize the results. It is well known that TCs exhibit a wide variety of motions according to such environmental conditions as baroclinicity, vertical wind shear, SST gradients, and the position of the subtropical ridge. For example, as a TC moves to a higher latitude, the energy source of the cyclones comes gradually more from baroclinic processes than latent heat release (Jones et al. 2003). Further, Holland and Wang (1995) showed that potential for recurvature depends on the initial location of a TC relative to an idealized subtropical ridge with a midlatitude westerly trough. The environmental wind field observed in the northwest region of a TC is also important for determining the TC recurvature (Hodanish and Gray 1993). A comparison between straight-moving and recurving TCs, therefore, is critical for understanding the physical processes that contribute to various TC motions in nature. In the present study, Super Typhoons Ewiniar (2006) and Maemi (2003) are chosen respectively as a representative of straight-moving and recurving TCs.
Baroclinic TC motion is controlled by various physical processes such as asymmetric flow within the vortex core region, vertical shear, vortex structure, and diabatic heating (DH) (Wang and Holland 1996a,b,c). A potential vorticity tendency (PVT) diagnostic approach is useful in understanding the physical mechanisms of baroclinic and diabatic TC motion (Wu and Wang 2000, 2001; Chan et al. 2002; Wong and Chan 2006). These studies suggested that TCs tend to move into the region of the maximum wavenumber-1 (WN1) PVT. We, therefore, adopt the PVT approach to understand the role of asymmetric SST distributions, which are responsible for various TC motions in realistic environments.
The objective of this study is to investigate the effects of asymmetric SST distributions upon straight-moving and recurving TC motions in real environments using the PVT approach. Experiments with asymmetric SST distributions are performed with the Weather Research and Forecasting Model (WRF), version 3.2. Section 2 describes the model experiments, real TC cases, and methodology used in this study. How an asymmetric SST distribution affects the straight-moving and recurving TC motions is examined in sections 3 and 4, respectively. Section 5 contains our major findings and discussion.
2. Model experiment and methodology
a. Model experiment
WRF version 3.2 is used to simulate Super Typhoons Ewiniar (2006) and Maemi (2003); details of these TCs are included in the following subsection. The model domain consists of 240 × 240 grid points with a uniform horizontal resolution of 12 km and 27 vertical levels with a top at 50 hPa. Each experiment is conducted as a 72-h integration with a 60-s time step. The Kain–Fritsch scheme (Kain and Fritsch 1993) is used for cumulus parameterization. Details of the model physics are documented in Yun et al. (2012). The National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis 6-hourly data with a 1.0° × 1.0° resolution are used as initial and boundary conditions. The best track dataset is obtained from the Joint Typhoon Warning Center (JTWC).
A simple bogussing scheme used in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5; Davis and Low-Nam 2001) is employed to insert a bogus vortex into a background field for improving simulation performance. Briefly, the bogussing procedure consists of identifying an initial TC vortex as compared with the best track data, and removing the vortex from the first guess field. Finally, the bogus vortex generated by using the simple Rankine vortex (Davis and Low-Nam 2001) replaces the removed vortex in the initial guess field. To generate the vortex, which is an improvement over the initial NCEP vortex, we choose 300 km (400 km) as the radius of Typhoon Ewiniar (Typhoon Maemi) with a radius of maximum wind of 27 780 m (46 300 m) and a maximum wind speed of 60 m s−1 (45 m s−1) based on the observation records.
A baseline control (CTL) experiment (Figs. 3a and 7a) is set with an initial SST condition obtained from the FNL data. SST distributions for the experiments are either zonal or meridional (as shown in Figs. 3b–d and 7b–d). For example, SST increases southward to study the effects of meridional asymmetry of SST on the TC motion in our southern warming (SW) experiment. Other experiments in this study are labeled as western warming (WW) and eastern warming (EW; see Table 1). The SST gradient is approximately 1°C (480 km)−1. To minimize the discrepancy of the initial TC intensity among the experiments, SST at the initial location of a TC is set to be similar to that of CTL. The TC center is determined by the minimum sea level pressure. Experimental designs are identical for both the typhoons.
An experimental design of the control run and runs with asymmetric SST distributions.
b. Case selection: Typhoon Ewiniar (2006) and Typhoon Maemi (2003)
A major problem in TC forecasting is the determination of the recurvature. Different TC motions could be expected, depending on the SST distribution and environmental flow. A comparison of straight-moving and recurving TCs is thus useful for understanding the physical processes contributing to diverse TC motions in nature. To determine the influence of SST distribution on TC passing through the western North Pacific and affecting the Korean Peninsula, two TCs are considered in this study: Typhoon Ewiniar (2006), which destroyed more than 600 homes through severe flooding and claimed 62 lives in Korea; and Typhoon Maemi (2003), the strongest typhoon to hit the Korean peninsula in nearly 100 years of the recorded history (Ye 2004).
Typhoon Ewiniar exhibits a straight northward path during the 3-day integration in the present study (Fig. 1). The JTWC defines the tropical cyclone recurvature as “the turning of a tropical cyclone from an initial path west and poleward to east and poleward.” As shown in Fig. 2, Typhoon Maemi is a typical recurving TC developed in September. Westward-moving TC is normally referred to as straight-moving TC. In this study, however, northward-moving Typhoon Ewiniar is defined as the straight-moving TC to differentiate the two different TCs in terms of their recurvature. Accordingly, these storms are selected as respective representatives of straight-moving and recurving TCs for this study. The western North Pacific subtropical high is a crucial component in the East Asian weather system (Yun et al. 2008; Lee et al. 2013). Previous studies investigated interactions between the synoptic-scale circulations (westerly trough and subtropical ridge) and TCs with respect to the recurvature process (Hodanish and Gray 1993; Holland and Wang 1995). Eastward-retreating subtropical ridge and an approaching westerly trough are dominant environmental features during the recurvature (Li and Chan 1999). A comparison of the geopotential heights at 500 hPa between Typhoon Ewiniar (Fig. 1) and Typhoon Maemi (Fig. 2) demonstrates that the straight-moving and recurving TC motions mainly result from different large-scale circulations. Thus, responses and their physical mechanisms are expected to vary for various asymmetric SST distributions between the two cases.
The best track of straight-moving Typhoon Ewiniar (black solid line) obtained from JTWC and 500-hPa geopotential height (gray solid line, m) from FNL data at 0000 UTC 9 Jul 2006. The 3-day integration is conducted from 0000 UTC 7 Jul to 0000 UTC 10 Jul 2006.
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
As in Fig. 1, but for recurving Typhoon Maemi and 500-hPa geopotential height (m) at 1800 UTC 10 Sep 2003 (before recurvature). The 3-day integration is conducted from 0000 UTC 9 Sep to 0000 UTC 12 Sep 2003.
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
Yun et al. (2012) focused on the motion of Typhoon Maemi for the period of 0000 UTC 11 September–0000 UTC 13 September, during which it showed a straight northeastward motion. To focus on the recurvature process, however, we investigate the following three consecutive 12-h periods during the 72-h integration interval: before (from 1200 UTC 10 September to 0000 UTC 11 September), during (from 0000 UTC 11 September to 1200 UTC 11 September), and after (from 1200 UTC 11 September to 0000 UTC 12 September) recurvature. In comparison, the following two consecutive 12-h periods are investigated for Typhoon Ewiniar—1200 UTC 8 July–0000 UTC 9 July and 0000 UTC 9 July–1200 UTC 9 July—during the 72-h integration interval. The period of 1200 UTC 9 July–0000 UTC 10 July is not considered in this study to exclude the land influence; TCs could drift toward the land associated with roughness length over land (Wong and Chan 2006). To remove temporal fluctuations and focus on the tendency of the TC motion, a 12- or 24-h composite analysis is employed.
c. Potential vorticity tendency framework
The PVT diagnostic approach is useful in understanding the physical mechanisms of baroclinic and diabatic TC motion. Previous research has demonstrated that TCs tend to migrate to the region of maximum WN1 PVT (Wu and Wang 2000, 2001; Chan et al. 2002; Wong and Chan 2006; Yun et al. 2012).
In this study, vertically averaged PVT is used because environmental flow can vary significantly with height. According to Wong and Chan (2006), the PVT analysis is conducted in the lower layer (LL: 0.9 ≥ σ ≥ 0.55; ~914–578 hPa) and upper layer (UL: 0.55 ≥ σ ≥ 0.25; ~578–290 hPa). A baroclinic vortex moves approximately with the environmental flow in the low- to midtropospheric layer (Holland and Wang 1995; Wang and Holland 1996b,c; Wang et al. 1998). As demonstrated by Yun et al. (2012), the WN1 component of the PVT based on a LL average is fairly consistent with the movement of a TC. However, changes in environmental flow associated with the recurvature are more evident in upper-tropospheric flow than in lower-tropospheric flow (George and Gray 1977; Hodanish and Gray 1993; Li and Chan 1999). In addition, asymmetric flow which affects TC motion depends on large-scale circulation (Chan and Cheung 1998). Therefore, different vertical averages are used for the straight-moving typhoon Ewiniar (LL) and recurving typhoon Maemi (UL).
The PVT arises from the horizontal and vertical PV advections, DH, and friction (the appendix). In this study, friction is assumed negligible since the vertical average above the boundary layer (1.0 ≥ σ ≥ 0.9; ~1010–914 hPa) is used. Wu and Wang (2000) suggested that the contributions of various physical processes could be determined by investigating their effects on the WN1 component of PVT. Adopting this point of view, a tendency of the TC motion can be attributed to the most dominant term if the most dominant physical process for PVT is determined. We follow this concept to examine the physical mechanisms of both straight-moving and recurving TCs. Total PVT is primarily controlled by three components: horizontal advection (HA), vertical advection (VA), and DH. The HA term includes beta-induced circulation, environmental steering flow, and heating-induced steering flow. A dominant term is chosen among the three by considering both the magnitude and location of maximum total PVT.
3. Effects of asymmetric SST distribution for a straight-moving case (Ewiniar 2006)
To understand the effects of asymmetric SST distribution under a less-complicated environment, a straight-moving TC is first investigated. As shown in Fig. 3a, the track of the TC in CTL is similar to the best track in Fig. 1, which shows a northward TC translation. During the mature stage (from 0000 UTC 7 July to 0000 UTC 10 July), the TC track in the SW experiment (Fig. 3b) is similar to that of CTL, which has a similar SST distribution to SW but a steeper SST gradient. Asymmetric SST distributions in the zonal direction shift the TC westward or eastward (Figs. 3c,d). It is noted that a northward-moving TC deflects toward the region of warmer SST. Larger deflections occur in WW and EW than in SW, which indicates that SST gradient perpendicular to TC translation offers more favorable conditions for TC deflection. This result is consistent with earlier studies (Chang and Madala 1980; Yun et al. 2012). Chang and Madala (1980) suggested that TCs tend to move toward the regions of warmer SST when the SST gradient is perpendicular to the idealized easterly current (i.e., a westward-moving TC) due to the asymmetries associated with heat exchange, enhanced evaporation, and friction. Yun et al. (2012) showed that an eastward SST increase, which is the same as EW in the present study, produced a larger eastward deflection for a northeastward-moving TC, owing to the southwestward tilt of the vortex axis and the resulting vertical southeasterly wind shear. However, a significant track change was realized only in an eastward SST increase. This discrepancy may arise from dominant baroclinic processes in higher latitudes. Jones et al. (2003) suggested that the main energy source for the cyclone is baroclinic processes rather than latent heat release during a TC translation to higher latitudes. Thus, a northeastward-moving TC in higher latitude is mainly affected by the vertical wind shear, in contrast to Ewiniar in the present study.
The simulated tracks from (a) CTL (black solid line), (b) SW, (c) WW, and (d) EW (gray solid lines) of Typhoon Ewiniar. The initial forecast time is at 0000 UTC 7 Jul 2006 (0 h). The SST distributions (gray dotted line) are asymmetric in zonal and meridional directions for the each experiment. The contour interval is 1.0 (°C).
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
To understand the physics of the SST distribution-induced TC motion, we compare both the PVT analysis and the asymmetric flow in LL among WW, CTL, and EW, which reveal northwestward, northward, and northeastward motion, respectively. As shown in Fig. 4, TC heading direction is largely consistent with the location of maximum total PVT averaged over a 24-h period. The location of maximum HA corresponds reasonably to that of the total PVT (Figs. 4a–f). For example, the maximum total PVT in EW is located northeastward and agrees well with the TC heading direction. The location and magnitude of the maximum HA are more consistent with those of the total PVT than VA or DH (Figs. 4c,f,i,l). The VA and DH terms, which have maxima in opposite locations, tend to cancel each other. We, therefore, infer that the contribution of HA to a TC motion dominates. The contribution of HA becomes more significant as a TC moves to higher latitudes in all the experiments (figure not shown). Chan et al. (2002) found that the main contributor to the total PVT is HA for straight-moving TCs.
Wavenumber-1 components of (a)–(c) total potential vorticity tendency, (d)–(f) horizontal advection, (g)–(i) vertical advection, and (j)–(l) diabatic heating, which are 24-h time composites of lower-level (0.9 ≥ σ ≥ 0.55) averages during 36–60 h in (left) WW, (middle) CTL, and (right) EW of Typhoon Ewiniar. Positive values are shaded. The contour interval is 3.0 in (a)–(c) and (j)–(l), 4.0 in (d)–(f), 1.0 in (g), 0.5 in (h), and 2.0 in (i) [106 potential vorticity unit (PVU, where 1 PVU = 10−6 K m2 kg−1 s−1)].
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
The HA term consists of both the advection of symmetric PV by asymmetric flow, which includes environmental steering flow and beta-induced circulation (i.e., ventilation flow) and the advection of asymmetric PV by symmetric flow (Chan et al. 2002). The steering flow defined over the inner core includes environmental steering flow and secondary steering flow generated by internal dynamics and interaction between a TC and external forcing (Wang et al. 1998). In the present study, the effect of steering flow is investigated by examining asymmetric flow induced by both large-scale circulation and asymmetric SST forcing. An asymmetric flow was obtained by subtracting a symmetric wind field from a total wind field (Wong and Chan 2006; Yun et al. 2012).
Southwesterly asymmetric flow over the TC core region in EW (Figs. 5e,f) effectively provides a better condition for eastward deflection compared to CTL (Figs. 5c,d). In WW (Figs. 5a,b), a westward deflection occurs related to the southerly or southeasterly over the TC center. These results can be inferred from the PVT analysis, which reveals that the TC motion can be determined by the location of maximum HA. In addition, these results also support previous studies in that the environmental steering is well estimated by the PVT approach (Wu and Wang 2000; Yun et al. 2012). The horizontal advection of PV implicitly includes the influence of the heating-induced asymmetric flows (Wang and Holland 1996b; Wu and Wang 2001). Therefore, the location of maximum HA faithfully depicts the TC motion with various SST distributions.
The 12-h averages of lower-level asymmetric flow in the TC core region over two consecutive periods (36–48 and 48–60 h) in (a),(b) WW, (c),(d) CTL, and (e),(f) EW of Typhoon Ewiniar. The wind speed is shaded according to the scale at the bottom of the figure.
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
A comparison of PVT and asymmetric flow over the TC center between CTL and SW exhibits little difference despite the difference in SST gradient (Figs. 3a,b). As shown in Figs. 6a,b, total PVT and HA in SW are almost the same as those in CTL in magnitude and location (Figs. 4b,e). The VA and DH terms in SW (Figs. 6c,d) are comparable in magnitude with those in CTL (Figs. 4h,k), although the maximum is rotated more counterclockwise; this does not make a much difference in the TC motion between CTL and SW since VA and DH tend to cancel each other. The southerly asymmetric flow over the TC center in SW is shown, which is similar to that of CTL (Figs. 5c,d and 6e,f). This seems to imply that a north–south SST gradient plays an insignificant role in the TC motion.
Wavenumber-1 components of (a) total potential vorticity tendency, (b) horizontal advection, (c) vertical advection, and (d) diabatic heating, which are 24-h time composite of lower-level (0.9 ≥ σ ≥ 0.55) averages during 36–60 h in SW of Typhoon Ewiniar. Positive values are shaded. The contour interval is 3.0 in (a) and (d), 4.0 in (b), and 0.5 in (c) (106 PVU). (e),(f) The 12-h averages of lower-level asymmetric flow in the TC core region over two consecutive periods (36–48 and 48–60 h). The wind speed is shaded according to the scale at the bottom of the figure.
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
Although the location of maximum HA does not precisely align with the TC heading direction, the former agrees approximately with the tendency of a TC motion. The magnitude of both VA and DH in EW is greater than that in WW (Figs. 4g,i,j,l). This result may imply that a warmer SST over the eastern side of the ocean presents a more favorable condition for TC intensification than does a warmer SST over the western side. Chang and Madala (1980) suggested that TC intensification in the Northern Hemisphere is more likely to occur when warmer SSTs appear to the right of the TC heading direction than to the left. This result is partially caused by the enhanced DH associated with relatively strong wind on the right-hand side of the TC heading direction. The effects of asymmetric SST distribution on the recurving TC motion are explained in the following section.
4. SST–TC motion relationship under the recurvature environment (Maemi 2003)
A similar approach is used to study Typhoon Maemi. The simulated TC track in CTL (Fig. 7a) shows a fair agreement with the best track data (Fig. 2). The TC moves northwestward before recurvature (36–48 h) and begins to recurve at 48 h (0000 UTC 11 September). During recurvature (48–60 h), the TC moves northward before showing a northeastward translation after recurvature (60–72 h). The point of initial recurvature is defined as the beginning of recurvature (Hodanish and Gray 1993). In the present study, the recurvature point is 0000 UTC 11 September. A comparison between CTL (Fig. 7a) and SW (Fig. 7b) reveals an insignificant difference in the TC motion, which is analogous to the straight-moving TC. This result indicates that a north–south SST gradient has an insignificant role in the TC motion. Unlike the straight-moving TC, WW and EW reveal different TC translation speeds (Figs. 7c,d) after the recurvature point. A TC tends to accelerate after a sudden track change (i.e., recurvature) over the western North Pacific under the steering flow associated with the subtropical high (Wu et al. 2011). Compared with the TC motion in CTL, WW and EW show northeastward acceleration and deceleration, respectively. This result differs from that of Yun et al. (2012), in which a west–east zonal SST gradient offered a favorable condition for a large eastward deflection. Therefore, relative configuration of TC heading direction and SST distribution could play a significant role in determining the TC deflecting direction as well as the translation speed in a zonally asymmetric SST distribution.
The simulated tracks from (a) CTL (black solid line), (b) SW, (c) WW, and (d) EW (gray solid lines) of Typhoon Maemi. The initial forecast time is at 0000 UTC 9 Sep 2003 (0 h). The SST distributions (gray dotted line) are asymmetric in zonal and meridional directions for each experiment. The contour interval is 1.0 (°C).
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
To understand the physics of the SST distribution-induced TC motion under the recurvature environment, we analyze the PVT in UL before (36–48 h), during (48–60 h), and after (60–72 h) recurvature. The HA term after recurvature agrees well with the total PVT in magnitude and location in all the experiments (Figs. 8c,f and 9c,f). Although the region of maximum total PVT after recurvature coincides with the northeastward-moving TC direction of movement, those before and during recurvature do not align with the northwestward- and northward-moving TC, but are located to the east of the TC core in WW and EW (Figs. 8a–c and 9a–c); this is consistent with CTL and SW (figure not shown). Chan (1984) reported that the maximum relative vorticity tendency of a recurving TC rotates to its future direction before recurvature. In this study, the location of maximum total PVT before and during recurvature corresponds to that after recurvature. The location of maximum total PVT and HA cannot depict the TC heading direction before and during recurvature since complex nonlinear interaction can occur between the TC and environmental flow such as a subtropical ridge and westerly trough; Chan and Cheung (1998) demonstrated that interaction between environment and TC circulation developed wavenumber-2 flow about 36 h prior to recurvature, and wavenumber-1 flow becomes dominant again after recurvature.
The 12-h time composite of the wavenumber-1 components of (a)–(c) total potential vorticity tendency and (d)–(f) a dominant term mainly contributing to total potential vorticity tendency in upper level (0.55 ≥ σ ≥ 0.25) over three consecutive periods divided into (left) before, (middle) during, and (right) after recurvature in WW of Typhoon Maemi. Positive values are shaded. The contour interval is 10.0 (106 PVU).
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
The 12-h time composite of wavenumber-1 components of (a)–(c) total potential vorticity tendency and (d)–(f) a dominant term mainly contributing to total potential vorticity tendency in upper level (0.55 ≥ σ ≥ 0.25) over three consecutive periods divided into (left) before, (middle) during, and (right) after recurvature in EW of Typhoon Maemi. Positive values are shaded. The contour interval is 2.0 in (a), 3.0 in (b), and 5.0 in (c)–(f) (106 PVU).
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
Despite the fact that the region of maximum total PVT reveals the future TC moving direction before and during recurvature, it is meaningful to investigate the dominant term constituting total PVT; relative importance of various physical processes can be determined by investigating their contributions to the WN1 component of PVT (Wu and Wang 2000). Based on this point of view, we examine the dominant term contributing to total PVT in order to understand the most dominant physical process associated with the two different TC motions in the zonally asymmetric SST distributions. Note that HA is a major contributor to total PVT in WW (Fig. 8), whereas DH is a dominant term in EW (Fig. 9) before and during recurvature. In connection with DH, a strong southerly vertical wind shear is seen compared to a weak zonal vertical wind shear over the TC center in EW (Fig. 10). DH is determined by the vertical and horizontal variations of heating, as well as vertical wind shear (Wu and Wang 2000; Chan et al. 2002). Vertical tilt of a TC induced by external forcing alters advection of PV through a vertical coupling. Wang and Holland (1996c) determined that the vertical interaction between upper- and lower-level PV associated with DH and the development of convective asymmetries within the TC core region can modulate the TC motion. It is noted that westerly and southerly vertical wind shears are dominant in WW and EW, respectively (Fig. 10). As shown in Fig. 11, the location of maximum WN1 component of vertical wind in UL indicates upward and downward motions over the region of downshear left and upshear right, respectively. This result is consistent with previous studies (Wang and Holland 1996c; Bender 1997; Frank and Ritchie 2001; Corbosiero and Molinari 2002), which reveal that an upward (downward) motion is enhanced to the downshear left (upshear right) of the TC center due to the TC's response to imbalances caused by the vertical wind shear. The potential temperature anomaly associated with the downward motion can affect mainly the location and magnitude of maximum DH. Jones (1995) explained that warm (cold) anomaly develops in the descent (ascent) region through vertical advection when PV is tilted by vertical shear, which shows a physical relationship between vertical circulation and potential temperature anomaly. Thus, the southerly vertical wind shear under the recurvature condition may play a significant role in the vertical circulation and the resulting potential temperature anomalies, which affect the location and magnitude of maximum DH (Figs. 9d,e).
The 12-h averaged (a)–(c) zonal and (d)–(f) meridional winds averaged within 100 km from the TC center over three consecutive periods (before, during, and after recurvature) in CTL, SW, WW, and EW of Typhoon Maemi.
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
The 12-h time composite of wavenumber-1 components of vertical wind in upper level (0.55 ≥ σ ≥ 0.25) over three consecutive periods divided into (left) before, (middle) during, and (right) after recurvature in (a)–(c) WW and (d)–(f) EW of Typhoon Maemi. Positive values are shaded. The contour interval is 0.5 in (b),(c) and 0.3 in (a),(d),(e),(f) (m s−1).
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
As shown in Fig. 2, the TC shows a rapid northward track change, and then its northeastward translation accelerates. Holland and Wang (1995) suggested that as a TC approaches a subtropical ridge, an anticyclonic gyre develops poleward of the ridge axis and is advected eastward by the westerly flow. As shown in Fig. 12, the asymmetric flow over the TC core region changes from southeasterly to southerly, and then southwesterly throughout the recurvature process. These asymmetric flows correlate well with the recurving TC motion, which is characterized as a northwestward movement (before recurvature), a turn toward north (during recurvature), and an acceleration of northeastward movement (after recurvature).
The 12-h averaged asymmetric flow in upper level over three consecutive periods (before, during, and after recurvature) in (left) WW and (right) EW of Typhoon Maemi. The wind speed is shaded according to the scale at the bottom of the figure.
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
Comparing the fast-moving TC in WW with the slow-moving TC in EW, a significant difference is revealed in the magnitude of asymmetric flow after recurvature (Figs. 12e,f). Wu and Wang (2001) demonstrated that the advection of symmetric PV by heating-induced asymmetric flow could affect the TC motion. Thus, a nonlinear interaction between heating-induced asymmetric flow and environmental steering flow can affect the TC motion through the resulting horizontal advection of PV. The southwesterly flow becomes a strong and deep layer in the low- to upper troposphere after recurvature (Figs. 10c,f). This steering flow effectively produces an acceleration of northeastward movement in WW, whereas the southwesterly flow in EW does not strengthen after recurvature (Figs. 10c,f), which is an unfavorable condition for acceleration. The asymmetric flow over the TC core in UL is enhanced throughout the recurvature process, particularly after recurvature, in WW (Figs. 12a,c,e). The enhanced synoptic-scale wind strongly shifts Typhoon Maemi northward, placing the TC under the southwesterly steering flow (Wu et al. 2011). Because of the steering flow associated with the subtropical high, the TC showed a sudden track change and northeastward acceleration. Large-scale circulations at 500 hPa in WW and EW are shown in Fig. 13. A relatively weak steering flow is seen in EW compared to WW because of the interference by a cyclonic vortex over the region of subtropical high. The counterclockwise flow of the cyclone offsets the southerly or southwesterly steering flow, which provides a favorable condition for recurvature and acceleration. In other words, the environmental steering effect does not sufficiently influence the TC motion in EW, particularly after recurvature. The TC therefore shows a slow motion compared to other experiments.
Large-scale flow (m s−1) at 500 hPa in (a) WW and (b) EW of Typhoon Maemi after recurvature (1800 UTC 11 Sep 2003). The gray contour indicates geopotential height (m).
Citation: Monthly Weather Review 141, 11; 10.1175/MWR-D-12-00207.1
5. Summary and discussion
The effects of asymmetric SST distributions on the tracks of the straight-moving Typhoon Ewiniar (2006) and recurving Typhoon Maemi (2003) have been examined through the PVT analysis with WRF. Different TC motions could be expected, depending on both the configuration of TC heading direction and asymmetric SST distribution and the interaction with environmental flows. A comparison of the straight-moving and recurving TCs is thus useful for understanding the physical processes contributing to various TC motions in nature. Baroclinic vortices move approximately with the environmental flow in the low- to midtropospheric layer (Holland and Wang 1995; Wang and Holland 1996b,c; Wang et al. 1998). Changes in environmental flow associated with the recurvature are more related to the upper-tropospheric flow than the lower-tropospheric flow (George and Gray 1977; Hodanish and Gray 1993; Li and Chan 1999). Different vertical averages are thus used to investigate the TC motion for the straight-moving Typhoon Ewiniar in LL and recurving Typhoon Maemi in UL.
Zonally asymmetric SST distributions result in different TC translating directions and speeds depending on the interaction between the TC and environmental current associated with asymmetric SST distribution. A north–south SST gradient has an insignificant role in the TC motion. It is noted in the zonally asymmetric SST distribution that the straight-moving (i.e., northward moving) TC is deflected toward the region of warmer SST. A larger deflection occurs in the zonal direction than in the meridional direction. This result indicates that a SST gradient perpendicular to the TC translating direction offers a more favorable condition for a TC deflection toward the region of warmer SST (Chang and Madala 1980; Yun et al. 2012). A contribution of HA including asymmetric flows induced by asymmetric forcing dominates the deflection. The location and magnitude of maximum HA are generally consistent with those of total PVT compared to VA or DH. Southeasterly and southwesterly asymmetric flow over the TC core region in WW and EW effectively provides a larger deflection compared to CTL, respectively. Unlike the straight-moving TC, the recurving TC reveals northeastward acceleration (deceleration) after the recurvature point in WW (EW). The location of maximum total PVT before and during recurvature reveals the future TC moving direction, whereas it agrees well with the northeastward-moving TC direction after recurvature. While HA is a major contributor to total PVT in WW, DH is a dominant term in EW before and during recurvature. In connection with DH, a strong southerly vertical wind shear is shown compared to a weak zonally vertical wind shear over the TC center in EW. The southerly vertical wind shear under the recurvature condition may play a significant role in the vertical circulation and the resulting potential temperature anomalies, which affect the location and magnitude of maximum DH. The enhanced southwesterly flow effectively produces an acceleration of northeastward movement in WW after recurvature, whereas the environmental steering effect does not sufficiently affect the TC motion in EW, particularly after recurvature. A configuration of TC heading direction and SST distribution, therefore, may be crucial in determining not only the TC deflecting direction but also the translation speed when SST distribution is zonally asymmetric. The main results are summarized in Table 2.
Summary of the results from runs with zonally asymmetric SST distributions for the straight-moving Typhoon Ewiniar and recurving Typhoon Maemi. Mean distance from the CTL and mean translation speed are averages during the 3-day integration.
Even though we suggest a possible role of vertical wind shear in the location and magnitude of maximum DH under the recurvature condition, the physical process can be much more complicated depending on the vertical structure and intensity of TC and its environment (Wang and Holland 1996c; Chan 2005). Although a lot of effort has been put into understanding the role of vertical wind shear in the TC dynamics over the last several decades (Jones 1995; DeMaria 1996; Wang and Holland 1996c; Wang and Wu 2004), how it influences the TC dynamics is not thoroughly understood. More study is needed in this regard.
Observations from satellites, aircraft reconnaissance, etc., have been utilized by many of the recent data assimilation efforts to improve the performance of TC forecasts (Zhang and Pu 2010; Singh et al. 2011; Liu and Xie 2012; Uhlhorn and Nolan 2012). Therefore, modeling of more realistic TC cases should be carried out in future employing data assimilation to relax uncertainties coming from the idealized SST distribution. Such simulations will show more realistic performance of the model and reveal relative importance of bogussing scheme and SST distribution in determining the TC track.
Acknowledgments
This work was supported by a GRL grant of the National Research Foundation (NRF) funded by the Korean Government (MEST 2011-0021927).This research was also a part of the project titled “Construction of Ocean Research Stations and their Application Studies” funded by the Ministry of Land, Transport and Maritime Affairs, South Korea. The work of JCLC was supported by the Research Grants Council of the Hong Kong Special Administrative Region Grant CityU 100210. The authors thank Prof. Bin Wang for his insightful comments. Valuable comments and suggestions from the anonymous reviewers improved this manuscript.
APPENDIX
Potential Vorticity Tendency Approach
Wu and Wang (2000) provided a general dynamic framework for the study of baroclinic TC motion with the potential vorticity tendency (PVT) approach. They demonstrated that a baroclinic TC migrates to the region in which the azimuthal wavenumber-1 (WN1) component of PVT reaches a maximum. On the basis of this finding, we identified the contributions of various physical processes to the TC motion. Chan et al. (2002) validated the PVT framework by analyzing different observational datasets.
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