468 MONTHLY WEATHER REVIEW VOLUME 141

The Effects of Topography on the Evolution of Saomai (2006) under the Influence of Tropical Storm Bopha (2006)

WOOK JANG AND HYE-YEONG CHUN Department of Atmospheric Sciences, Yonsei University, Seoul, South

(Manuscript received 21 September 2011, in final form 21 August 2012)

ABSTRACT

The effects of topography on the evolution of Typhoon Saomai (2006) are investigated by conducting a series of numerical simulations with the Weather Research and Forecasting (WRF) Model using 100%, 75%, 50%, and 25% of terrain heights of the Central Mountain Range (CMR) in . Differences in the track and intensity of Typhoon Saomai between the experiments are strongly related to those of Tropical Storm Bopha, which passed Taiwan earlier than the typhoon. In the sensitivity experiments, the higher CMR drifts Bopha more southward, which results in the weakening of Bopha by prohibiting the interaction between the CMR and Bopha, and the flows induced by Bopha force Saomai to propagate along a more southerly track. The higher CMR weakens the easterly flow in the lower troposphere and suppresses the northerly flow in the upper troposphere to the west of Saomai. The resultant weak vertical wind shear keeps warm air near the typhoon center in the upper troposphere, which promotes the intensification of the typhoon. To examine the direct effects of topography on the track and intensity of Saomai, additional simulations involving the removal of Bopha from the initial condition with 100% and 50% of CMR are conducted. The results without Bopha showed that Saomai moves more southward at a slower speed and with greater intensity, due to the stronger northerly wind to the west of Saomai, which was not canceled out by the southerly wind to the east of Bopha, and there is no significant difference in the tracks or intensity with respect to the mountain heights.

1. Introduction topography induced by the approaching typhoon. Yeh and Elsberry (1993a,b) demonstrated that the track As a typhoon approaches a mountain range, its track, deflections attributable to topography depend on the intensity, and precipitation distribution can be influ- structure, intensity, and transition speed of enced by topography. The effects of topography on ty- and that track discontinuity is related to the formation of phoons are observed on islands located in the path of a secondary vortex on the lee side of mountains. Lin et al. typhoons, such as Taiwan and Luzon in the northwestern (1999) explained the southward track deflection by to- Pacific Ocean. The interaction between a typhoon cir- pography in terms of the potential vorticity generation on culation and terrain is highly complex; therefore, accurate the lee side of mountains, which results from the adiabatic prediction of typhoons under the influence of topogra- warming associated with severe downslope windstorms. phy remains a challenge (Wu and Kuo 1999). Lin et al. (2005) proposed six nondimensional parame- Previous numerical modeling studies based on ideal- ters for diagnosing typhoon continuity and identified ized topography (e.g., Chang 1982; Yeh and Elsberry favorable conditions for track deflection. The influences 1993a,b; Lin et al. 1999, 2005) have focused on the of terrain on precipitation (e.g., Wu et al. 2002) and mechanisms of track deflection when typhoons pass over track deflection (e.g., Lin et al. 2006) have also been mountain ranges. Chang (1982) showed that the typhoon investigated using real-case simulations. Wu et al. (2002) center moves cyclonically when passing over mountain showed that the strength of the lifting motion caused by ranges, due to the cyclonic circulation around the the Central Mountain Range (CMR) in Taiwan is an important factor in determining the amount of pre- cipitation associated with Typhoon Herb (1996). By Corresponding author address: Prof. Hye-Yeong Chun, De- partment of Atmospheric Sciences, Yonsei University, 262 comparing simulation results for Supertyphoon Bilis Seongsanno, Seodaemun-ku, Seoul 120-749, . (2000) and Typhoon Toraji (2001), Lin et al. (2006) E-mail: [email protected] demonstrated that weaker and slower typhoons will

DOI: 10.1175/MWR-D-11-00241.1

Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/05/21 10:42 PM UTC FEBRUARY 2013 J A N G A N D C H U N 469 tend to have a more discontinuous track when they cross In the present study, the orographic effects on the the CMR. evolution of Typhoon Saomai are investigated by con- If there is more than one storm near a mountain area, ducting a series of numerical experiments using a me- the effects of topography on the storms become much soscale model using 100%, 75%, 50%, and 25% of the more complicated due to the interactions between the terrain heights of Taiwan. In section 2, a brief overview storms under differing topographical influences, which of Typhoon Saomai is given. In section 3, the experi- affect their movements. In several previous studies, mental design is described. In section 4, the results of the pioneered by Fujiwhara (1921), concerning the in- numerical simulations conducted with different moun- teraction of two vortices, changes in the movements and tain heights are discussed. Special emphasis is given to intensities of two vortices have been investigated using the influence of mountain height on Tropical Storm idealized vortices under various circumstances (e.g., Bopha, which passed Taiwan prior to Typhoon Saomai, Chang 1983; Dritschel and Waugh 1992; Wang and and its impact on the track and intensity of Typhoon Holland 1995; Shin et al. 2006). These studies mainly Saomai. A summary and conclusions are presented in focused on the on the influence of the critical distance the final section. between two vortices in relation to the merger or mutual cyclonic circulation. Chang (1983) demonstrated the 2. An overview of Typhoon Saomai (2006) Fujiwhara effect using a three-dimensional tropical cy- clone model without background wind. Dritschel and The Joint Typhoon Warning Center (JTWC) first Waugh (1992) categorized the interaction regimes of two identified a tropical depression near the Caroline Islands vortices with an identical vortex strength based on the in the western Pacific Ocean on 4 August 2006, and the nondimensional distance between the two vortices and the Meteorological Agency (JMA) designated it as ratio of the two vortex radii. Wang and Holland (1995) a typhoon at 0600 UTC 7 August. Typhoon Saomai found two critical separation distances—specifically, a mu- moved northwestward continuously without significant tual approach separation and a mutual merger separation— track deflection until its on the southeastern for storms simulated by a three-dimensional primitive coast of on 10 August (Fig. 1a). The typhoon equation model. Shin et al. (2006) demonstrated that the rapidly intensified and propagated, reaching its peak critical separation distance of binary vortices is slightly intensity at 1200 UTC 9 August with a minimum sea less than twice the radius at which the relative vorticity level pressure of 925 hPa and a maximum wind speed of 2 of one vortex becomes zero. In studies of real-storm cases, 54 m s 1 (Fig. 1b). Tropical Storm Bopha (2006) was the observed movements of binary tropical cyclones (TCs) located over Taiwan when Typhoon Saomai approached have been explained by their interactions (e.g., Carr et al. Taiwan. Bopha formed as a tropical depression on 1997; Kuo et al. 2000; Wu et al. 2003; C. C. Yang et al. 5 August over the northwestern Pacific Ocean and pro- 2008). Carr et al. (1997) proposed four conceptual models pagated westward toward Taiwan. Bopha passed over the (direct, semidirect, indirect, and reverse-oriented monsoon south of Taiwan from 1800 UTC 8 August–0000 UTC trough formation) of track-altering binary tropical cy- 9 August (Fig. 1a) with a minimum sea level pressure of 2 clones that occurred in the Pacific Ocean. Kuo et al. (2000) 992 hPa and a maximum wind speed of 21 m s 1 (Fig. 1b). classified the interaction regimes between Typhoons Zeb Figure 2 shows the observed features of Typhoon and Alex (1998) based on Dritschel and Waugh’s (1992) Saomai and Tropical Storm Bopha at 1800 UTC 8 August theory. Wu et al. (2003) and C. C. Yang et al. (2008) used 2006. The infrared image over the northwestern Pacific potential vorticity diagnosis to explain the motions of Ocean is from the Multifunctional Transport Satellite-1R typhoons due to interaction of binary typhoons. (MTSAT-1R; Fig. 2a), and the sea level pressure (Fig. In 2006, 15 typhoons formed in the northwestern Pa- 2b), horizontal wind vector at 850 hPa (Fig. 2c), and the cific Ocean, and 5 of them developed into category-5 geopotential height along with the horizontal wind typhoons based on the Saffir–Simpson hurricane scale vector at 500 hPa (Fig. 2d) are generated from the Eu- (Simpson 1974). Of the five strongest typhoons, Typhoon ropean Centre for Medium-Range Weather Forecasts Saomai (2006) made the closest approach to Taiwan (ECMWF) analysis data at a horizontal resolution of during its peak intensity. Investigations of Typhoon 0.258 latitude 3 0.258 longitude (Figs. 2b–d). In the Saomai (2006) have been conducted by Zhao et al. (2008) MTSAT-1R image (Fig. 2a), the larger convective sys- and Kim and Chun (2010). Using a single Doppler radar tem centered near 238N, 1308E is Typhoon Saomai, and observation, Zhao et al. (2008) investigated the change in the smaller convective system centered near 238N, 1228E the concentric eyewall of the typhoon near landfall. Kim is Tropical Storm Bopha. Typhoon Saomai strengthens and Chun (2010) examined the characteristics of the from this time (1800 UTC 8 August), as shown in Fig. 1b, stratospheric gravity waves generated by the typhoon. and Bopha has already made landfall in Taiwan. In the

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FIG. 1. (a) The tracks and (b) minimum sea level pressures, and maximum wind speeds of Typhoon Saomai (crisscross) and Tropical Storm Bopha (circle) by the JMA observation. The maximum wind speed of Bopha after 0000 UTC 9 Aug was not officially documented because of its weakening.

ECMWF analysis data, Bopha is present in the south- unique case that can be used to investigate the impact of eastern part of Taiwan, approximately 730 km west of topography on a typhoon by considering two vortices Saomai (Fig. 2b), and its secondary low is present to the with different sizes and strengths that approached the west of the CMR. At 850 hPa (Fig. 2c), the strength of same topography with a time lag. the flow between the two circulations becomes weak because the northerlies induced by Saomai are largely 3. Experimental design cancelled out by the southerlies induced by Bopha. The two lows associated with Saomai and Bopha are clearly The Weather Research and Forecasting (WRF) Model, shown up to 500 hPa (Fig. 2d) without significant verti- version 2.2 (Skamarock et al. 2005), is used in the present cal tilt. A short-wave trough between Saomai and Bopha study. The outmost domain (hereafter D1), centered at is embedded in the prevailing easterlies. The strong 208N, 128.08E, has 187 3 187 grid points with a horizontal southeasterlies in the northern part of Saomai flow to- grid spacing of 27 km. Two nested subdomains (hereafter ward the southeastern coast of China, and strong D2 and D3) with a grid spacing of 9 and 3 km have 253 3 northerlies exist in the west of Taiwan. 253 and 397 3 397 horizontal grid points, respectively. To As will be shown later, the influence of topography on investigate the structure of the typhoon in detail, the in- Typhoon Saomai is strongly related to the influence of nermost domain is designed to follow the vortex center. topography on Tropical Storm Bopha, given that the The typhoon center, which is determined by the mini- flows near Saomai when it approaches Taiwan are mum geopotential height at 500 hPa, is always located at largely influenced by those associated with Bopha, the center of D3; D2 also changes with time in accordance which had already passed over Taiwan. This is a rather with the movement of D3. The topography and three

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FIG. 2. (a) The observed brightness temperature from the MTSAT-1R and (b) the sea level pressure, (c) horizontal wind vector at 850 hPa, and (d) geopotential height (contour) with the horizontal wind vector at 500 hPa at 1800 UTC 8 Aug from the analysis data of the ECMWF. The contour intervals in (b) and (d) are 2 hPa and 10 m, respectively. model domains are shown in Fig. 3. Note that the to- maximum height of the CMR in D3 is 1980 m in the pography in the moving domains of D2 and D3 is in- current vortex-following simulations, whereas the maxi- terpolated from the D1-resolution topography in the mum height of the CMR in a regular nonmoving domain current version of WRF, and as a result, the topography with a 3-km horizontal resolution is 3194 m. The model may be underestimated in D2 and D3. For example, the includes 77 vertical layers from the surface to 5 hPa. The

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four numerical simulations are conducted with 100%, 75%, 50%, and 25% of the terrain height of Taiwan; these simulations are referred to as CTL, H75, H50, and H25 simulations, respectively. All model parameters are identical to those used in the CTL simulation, except for the percent-adjusted terrain elevations.

4. Topographic effects on Tropical Storm Bopha and its influence on Typhoon Saomai a. Effects of topography on the evolution of Tropical Storm Bopha As shown in Fig. 2, Tropical Storm Bopha passed Taiwan before Typhoon Saomai passed north of Taiwan. That is, two vortices of different strengths and sizes co- existed side by side near Taiwan for a period of time and passed westward one after another. Therefore, to un- derstand the influence of topography on Typhoon Sao- mai, it is essential to consider the influence of topography on Bopha. Near Taiwan, the environmental flow of FIG. 3. The mountain heights (shading) and three model do- Saomai could have been influenced strongly by the cir- mains with 27- (D1), 9- (D2), and 3-km (D3) grid spacings at 1800 culation associated with Bopha, which made landfall in UTC 9 Aug when Typhoon Saomai is located in the northeastern part of Taiwan. Note that D2 and D3 are following Saomai. Taiwan. To examine the effects of the topography on the evolution of the two vortices, we show in Fig. 4 the sea vertical grid spacing increases with the height, about level pressure at selected times from 1200 UTC 8 August 200 m in the boundary layer below 2 km height and to 0000 UTC 10 August. The main differences between about 400 and 1000 m in the troposphere and lower the simulations shown in Fig. 4 are the location and in- stratosphere, respectively. A damping layer of 5 km is tensity of Tropical Storm Bopha for whole period and included at the uppermost part of the model domain. those of the Typhoon Saomai after 0000 UTC 9 August. The numerical model is integrated for 60 h from 0600 In the CTL simulation, Bopha is located to the southeast UTC 8 August–1800 UTC 10 August 2006, in a two-way of Taiwan at 1200 UTC 8 August. Over time, the storm interactive nested grid system. The integration of D3 moves to the southwest of Taiwan without significant begins at 1200 UTC 8 August, 6 h after the integration in intensification because the blocking effect of the CMR is the outer domains is initiated. strong enough to deflect Bopha. Compared with the For the initial and boundary conditions, we use the CTL simulation, the H75 simulation shows that the in- ECMWF analysis data with a horizontal resolution of tensity of Bopha increases slightly but with almost no 0.258 latitude 3 0.258 longitude. A Geophysical Fluid change in the location of the vortex center. Similar to the Dynamics Laboratory (GFDL)-type typhoon initializa- CTL simulation, the storm does not pass the CMR but tion algorithm (Kwon et al. 2002) is employed to provide moves along the southern coast of Taiwan. In the H50 proper initial conditions. The maximum wind speed and simulation, Bopha is more intense as it passes directly radius of maximum wind used for the bogussing algorithm over southern Taiwan and it is deflected southwestward 2 are 21.75 m s 1 and 82.5 km, respectively. The subgrid- after passing the CMR. The results of the H25 simula- scale physical processes used in the present simula- tion are generally similar to those of the H50 simulation, tions include the WRF single-moment 6-class (WSM6) except that the storm center is located over Taiwan, microphysics scheme (Hong and Lim 2006), the Mellor– even at 0000 UTC 10 August, and the intensity of the Yamada–Janjic (MYJ) PBL scheme (Mellor and Yamada storm is stronger. As a result, the CMR pushes out Bopha 1982; Janjic 2002), and the Kain–Fritsch cumulus pa- to the south, and Bopha has a better chance to interact rameterization scheme (Kain and Fritsch 1993). The with the topography with a decreased terrain height. cumulus parameterization scheme is applied only to D1 The southward-shifted tracks of Typhoon Saomai with and D2. To examine the effects of the topography of the higher elevations of the CMR are partly due to the CMR on the track and intensity of Typhoon Saomai, southward shift of Bopha that occurs with higher elevations

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FIG. 4. The sea level pressure from 1200 UTC 8 Aug to 0000 UTC 10 Aug at intervals of 12 h from the (a) CTL, (b) H75, (c) H50, and (d) H25 simulations. The contour interval is 4 hPa. The results are obtained from D1.

of the CMR. The southward shift of Bopha with higher tropical cyclones tend to have a discontinuous (contin- elevations of the CMR can be explained in terms of the uous) track due to the increase (decrease) in the vortex Froude number (Fr) as a measure of the blocking blocking effect when Fr is smaller (larger) than 1.5. The effect of the topography when a en- calculated Fr from the present CTL, H75, H50, and H25 counters a north–south-oriented topography, as sug- simulations at 0900 UTC 8 August (not shown), when gested by Lin et al. (2005). The vortex Froude number is Bopha was located east of Taiwan but had not passed the defined as Vmax/Nh, where Vmax, N, and h are the max- CMR in all of the simulations, are 0.78, 1.12, 1.88, and imum tangential wind, the Brunt–Va¨isa¨la¨ frequency, 4.16, respectively. This demonstrates that the southward and the mountain height, respectively. By organizing movements of Bopha in the CTL and H75 simulations previous studies about the track deflection of tropical (Fig. 4) stem from the increase in the blocking effect. cyclones traversing the CMR, Lin et al. showed that The influence of development of Bopha on the track,

Unauthenticated | Downloaded 10/05/21 10:42 PM UTC 474 MONTHLY WEATHER REVIEW VOLUME 141 intensity, and structure of Saomai will be examined in the intensity of the typhoon increases monotonically detail in the following section. with mountain height, even after 1200 UTC 10 August, The left panel of each simulation in Fig. 5a is the when the typhoon has passed over Taiwan. Based on the moisture flux divergence ($ rqyV; where r is air den- minimum sea level pressure time series (Fig. 6b), the sity, qy is water vapor mixing ratio, and V is horizontal typhoon intensity in the CTL simulation is the best wind vector) (shading) superimposed on the horizontal among the four simulations, especially in the rapidly wind vector at 850 hPa near Taiwan at 1200 UTC developing and mature stages of typhoon evolution after 8 August, and right panel of each simulation in Fig. 5a is 0600 UTC 9 August at which differences in the intensity the vertical cross section of cloud property (cloud water in the four simulations become significant. As will be mixing ratio plus ice mixing ratio) superimposed on the detailed later, the typhoon cannot develop fully in the zonal wind (denoted by vector form) along the line reduced-topography simulations (H50 and H25) after shown in the left panel. At 1200 UTC 8 August, Bopha is 0600 UTC 9 August. This underdevelopment likely re- located southeast of Taiwan in the CTL and H75 simu- sults from the circulations induced by Tropical Storm lations and it makes a landfall in the H50 and H25 Bopha, which influence the development and movement simulations. In the CTL simulation, the moisture flux of the typhoon. Although Saomai passed north of Tai- converges in the upstream region of the CMR due to the wan rather than made landfall on Taiwan, there was easterlies from the circulation of Bopha and weak cy- likely a direct effect of topography on Saomai consid- clonic circulation in the lee side of the CMR appears. In ering its passage that was very close to Taiwan. To ex- a cross section, dense cloud is developed between 700 amine this possibility, we performed additional numerical and 500 hPa by the influence of the CMR. Results from simulations with 100% and 50% mountain heights, iden- the H75 simulation are generally similar to those from tical to CTL and H50, respectively, except for removing the CTL simulation, except that the cloud extends up to Bopha from the simulations. This will be discussed in 200 hPa as Bopha approached CMR more closely than section 4c. the CTL simulation. In the H50 and H25 simulations, Figure 7 shows the temporal variation of the envi- stronger moisture flux convergence is formed as Bopha ronmental steering flow near the typhoon center for the makes landfall in Taiwan. The cloud above the CMR four simulations. Following M. J. Yang et al. (2008), the extends up to 100 hPa due to the orographic uplift, and environmental steering flow is calculated by averaging the strong outflow above 300 hPa expands to about the horizontal wind in a domain with a radius of 360 km 200 km horizontally. At 1200 UTC 9 August (Fig. 5b), from the typhoon center. In the CTL simulation, east- Bopha is located south of Taiwan for all simulations, and erlies are dominant near the typhoon center from the strength of Bopha and associated convective activity near lower troposphere to the lower stratosphere throughout Taiwan are significantly different depending on the rel- the period shown in Fig. 4, and easterlies below 600 hPa 2 ative distance between CMR and Bopha. Similarly with increase to more than 9 m s 1 after 0600 UTC 9 August. the results at 1200 UTC 8 August, strong divergences at The H75 simulation shows no significant difference from the upper troposphere with tall clouds are only pre- the CTL simulation before 1200 UTC 9 August. After sented in the H50 and H25 simulations at 1200 UTC that time, however, northerlies are created at 150 hPa, 9 August as well. These strong divergences are related and the easterlies above 100 hPa become weaker until with the formation of anticyclonic circulation in the 0000 UTC 10 August in the H75 simulation. The tem- upper troposphere, which will be discussed in section 4b. poral and vertical changes in the environmental steering flow in the H50 and H25 simulations are comparable to b. Changes in track, intensity, and structure of Saomai and generally greater than those in the CTL and H75 under the influence of Bopha simulations. The northerlies between 300 and 150 hPa Figure 6 shows the tracks (Fig. 6a) and minimum sea and the southeasterlies below 700 hPa after 1200 UTC level pressures (Fig. 6b) of Typhoon Saomai from the 9 August are enhanced in the H50 and H25 simulations, four simulations with different mountain heights along with greater magnitudes being observed in the H25 with the observed data from the JMA, as shown in Fig. 1. simulation. The changes in environmental steering flow Compared with the JMA best track, the simulated tracks attributable to mountain height are strongly related to are generally slower, especially after 0000 UTC 9 August. the development of Tropical Storm Bopha (as will be The difference in the track between each simulation is discussed in the explanation of Fig. 9). noticeable after 1200 UTC 9 August when Typhoon Figure 8 shows a zonally oriented cross section of the Saomai approaches Taiwan and the tracks simulated simulated reflectivity through the typhoon center. In all with a higher topography shift southward for most of the simulations, the convection to the right of the typhoon simulation period as shown in Fig. 4. Unlike the track, center developed to 16 km and the convection to the left

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FIG. 5. (left) Moisture flux divergence (shading) and horizontal wind vector at 850 hPa near Taiwan and (right) the vertical cross section of cloud property (cloud water mixing ratio plus ice mixing ratio) superimposed on the zonal wind vector along the line shown in the left panel at (a) 1200 UTC 8 Aug and (b) 1200 UTC 9 Aug for the (top left) CTL, (top right) H75, (bottom left) H50, and (bottom right) H25 simulations. The results are obtained from D1.

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FIG. 6. (a) Simulated typhoon tracks in the CTL (circle), H75 (square), H50 (triangle), and H25 (circle with dot) simulations. The cross symbols indicate the best track of JMA from 1200 UTC 8 Aug to 0600 UTC 10 Aug 2006. Every symbol is marked with an interval of 6 h. (b) Time series of the minimum sea level pressure from the JMA best track (crisscross) and the results in the CTL (solid line), H75 (dotted line), H50 (dotted–dashed line), and H25 (dashed line) simulations. The results are obtained from D3. of the center is relatively low and weak. In the CTL from that of Ramsay and Leslie (2008), who showed that simulation, a concentric eyewall is formed with a clear the topography increases eyewall asymmetry by strength- as its intensity increases after 0000 UTC 9 August. ening the convection above the topography. In the present The typhoon structure in the H75 simulation is gener- study, there is no evidence of convection strengthening ally similar to that in the CTL simulation, except that above the topography for Saomai. For the Bopha case, the symmetry near the eyewall decreases slightly after however, the convection is strengthened over the to- 1800 UTC 9 August in the H75 simulation. The asymmetric pography, as shown in Fig. 5. structure is more pronounced in the reduced-topography The influence of the mountain height on the circula- simulations. In the H50 and 25 simulations, small-scale tion induced by Bopha can be seen clearly in Fig. 9, develop to the west of the typhoon center which shows the horizontal wind vector at 200 hPa from rather than strong convection in the eyewall region. the four simulations at 1200 UTC 9 August when the Figure 8 clearly demonstrates that the eyewall symmetry typhoon is located in the northeastern part of Taiwan. increases with terrain height. This result differs somewhat The CTL simulation presents a cyclonic circulation

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FIG. 7. The vertical profiles of environmental steering flow in the vicinity of the typhoon center from 1200 UTC 8 Aug to 0600 UTC 10 Aug at intervals of 6 h from the (a) CTL, (b) H75, (c) H50, and (d) H25 simulations. The environmental steering flows are calculated by averaging the horizontal wind vectors within a radius of 360 km from the typhoon center. The results are obtained from D3. induced by Saomai and an anticyclonic circulation in- circulation in the southwest of Taiwan. In the H75 sim- duced by a convective system (not shown) in the ulation, the anticyclonic flow between Taiwan and China southern part of Kyushu, Japan. The circulation induced associated with Bopha becomes more visible compared by Bopha is presented only as weak anticyclonic to that presented in the CTL simulation. Because of the

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FIG. 8. Vertical cross sections of the radar reflectivity along the zonal distance passing the typhoon center from 1800 UTC 8 Aug to 0000 UTC 10 Aug at intervals of 6 h from the (a) CTL, (b) H75, (c) H50, and (d) H25 simulations. The results are obtained from D3.

enhanced anticyclonic circulation north of Taiwan, strong (Figs. 10a,b) and at 300 hPa (Figs. 10c,d) for the CTL northeasterlies develop between Saomai and Taiwan due and H25 simulations at 1200 UTC 9 August. Compared to the merging of flows from the anticyclonic circulation to the CTL simulation, the strong divergence from the north of Taiwan and the cyclonic circulation west of enhanced Bopha in the H25 simulation, as shown in Saomai. In the H50 and H25 simulations, the anticyclonic Fig. 5b, moves warm air from the west of Taiwan to the circulation in the northwest of Taiwan gains strength eastern part of China by means of strong southeasterlies. and strong northerlies develop in the west of Saomai. The southeasterlies change to southwesterlies as they Owing to the strong northerlies in the west of Saomai, move farther north, and the warm southwesterlies and especially in the H25 simulation, the cyclonic circulation cold northwesterlies that blow from the north of China and associated convective system of the western portion induce westerlies through the horizontal stretching de- of Saomai weaken significantly, as shown in Fig. 8. formation (Holton 1992). The strong northwesterlies To understand the strong anticyclonic circulation at consisting of the anticyclonic circulation shown in Fig. 10b 200 hPa shown in the H25 simulation of Fig. 9, we cal- can be explained by the thermal wind relationship using culate and show in Fig. 10 the equivalent potential the equivalent potential temperature at 300 hPa. In the temperature and horizontal wind vector at 200 hPa H25 simulation (Fig. 10d), warm regions expand into

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FIG. 9. The horizontal wind vector at 200 hPa from the (a) CTL, (b) H75, (c) H50, and (d) H25 simulations at 1200 UTC 9 Aug. The results are obtained from D1. a wide area including east of Taiwan in which cold air anticyclonic circulation in the H25 simulation shown in still remains in the CTL simulation. Therefore, strong Fig. 9 is due to the expansion of warm air by stronger negative temperature gradient in the zonal direction is outflow associated with enhanced convective activity of formed in the west and northwest of Saomai. Through Bopha. the thermal wind relationship, vertical shear of the me- The change in the horizontal wind with topography in ridional wind becomes negative in the regions of the the upper troposphere was discussed in the previous negative temperature gradient, where strong northerlies paragraphs, and this change eventually modifies vertical form at 200 hPa as shown in Fig. 10b. In addition, these wind shear. The effect of the vertical wind shear on ty- northerlies become stronger by combining with cyclonic phoon evolution has been emphasized in several pre- circulation from Saomai. In summary, the formation of vious studies (McBride and Zehr 1981; Merrill 1988;

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FIG. 10. The equivalent potential temperature and horizontal wind vector at 200 hPa from the (a) CTL and (b) H25 simulations and at 300 hPa from (c) CTL and (d) H25 simulations at 1200 UTC 9 Aug. The results are obtained from D1.

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FIG. 11. Time series of the magnitude of the deep-layer wind shear vector (jV200 2 V850j) averaged over D3 from the CTL (solid line), H75 (dotted line), H50 (dashed–dotted line), and H25 (dashed line) simulations.

DeMaria 1996; Frank and Ritchie 2001; Chambers and the CTL, H75, H50, and H25 simulations, respectively. Li 2011). Among these studies, Frank and Ritchie (2001) Here, the positive lag means that the wind shear begins suggested that larger vertical wind shear causes the de- to decrease earlier than the sea level pressure. The ex- velopment of a highly asymmetric eyewall structure and tremely low correlation seen in the H50 simulation is allows the ventilation of the TC eye, resulting in the probably due to the decrease in wind shear that begins disappearance of the warm core in the upper levels. after 1800 UTC 9 August and continues until 1800 UTC These changes, in turn, increase the central pressure of 10 August, although the sea level pressure increases the TC and therefore decrease its strength. continually throughout that period following decreases Figure 11 shows the time series of the deep-layer wind near 0600 UTC 10 August (Fig. 6b). shear averaged over D3, which is calculated by the Figure 12 shows the equivalent potential tempera- magnitude of the difference in the horizontal wind ture at 200 hPa and the magnitude of the deep layer vector between 850 and 200 hPa. In the CTL and H75 (between 200 and 850 hPa) wind shear at 1800 UTC simulations, the deep-layer wind shear decreases rapidly 9 August. In the CTL simulation, a strong warm region from 1800 UTC 8 August to 1200 UTC 9 August as the with a maximum equivalent potential temperature of typhoon intensifies (Fig. 1b). The wind shear in the H75 372 K is presented near the typhoon center, and the simulation shows a generally similar trend to that in equivalent potential temperature decreases rapidly with the CTL simulation (although with larger magnitude), increasing radial distance from the center. Relatively except that its increase from 1200 UTC 9 August to small and large magnitudes of the deep-layer wind shear 0000 UTC 10 August occurs more rapidly than in the are apparent in the eye and the eyewall, respectively, 2 CTL simulation. A similar decreasing trend is evident although with no regions exceeding 20 m s 1. In the in the H50 and H25 simulations before 0000 UTC H75 simulation, the concentric shape of the equivalent 9 August, but the wind shear increases continuously potential temperature field near the typhoon center is from 0000 UTC 9 August to 1800 UTC 9 August, at slightly collapsed, and the maximum value of the equiv- which the differences in sea level pressure between the alent potential temperature decreases slightly (371 K). In simulations become larger (Fig. 6b). The increases in addition, the vertical wind shear in the northern part of wind shear seen after 1200 UTC 9 August in the H50 and the typhoon center is somewhat strengthened. In the H25 simulations are due to the northerly flow near the H50 and H25 simulations, the concentric form of the typhoon center in the upper troposphere, as evidenced equivalent potential temperature largely disappears, by the environmental steering flow (Fig. 7). The time series and the warm region near the typhoon center shrinks as of wind shear (Fig. 11) and minimum sea level pres- the regions outside the center increase in temperature sure (Fig. 6b) are well correlated, and the maximum cor- (see the thick contours of 368 and 358 K in Fig. 12). The relation coefficients between the two variables are 0.76, magnitude of the vertical wind shear increases signifi- 0.74, 0.18, and 0.72 at lags of 14, 16, 16, and 16hfor cantly, especially in the northern and southern parts of

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FIG. 12. The magnitudes of the deep-layer wind shear vector (jV200 2 V850j; shaded) and the equivalent potential temperature at 200 hPa (contour) from the (a) CTL, (b) H75, (c) H50, and (d) H25 simulations at 1800 UTC 9 Aug. The contour intervals of the equivalent potential temperature are 1 K, and the inner and outer thick, solid lines represent 368 and 358 K, respectively. The results are obtained from D3. the typhoon center. In summary, the simulations show troposphere, resulting in increased typhoon intensity. that increasing the height of the CMR induces weaker The influence of topography on the development of the vertical wind shear and enhances eyewall structure typhoon presented in the current study can be ex- symmetry. These changes cause the warm air to be plained by the mechanism proposed by Frank and maintained near the typhoon center in the upper Ritchie (2001).

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9 August and 0000 UTC 10 August, Bopha and Saomai are weakened rapidly in the H50 and H25 simulations, whereas the intensities of the two vortices in the CTL and H75 simulations do not change significantly. Figure 14 shows the centroid-relative tracks from 1200 UTC 8 August to 0000 UTC 10 August at intervals of 6 h. Centroid-relative tracks denote the locations of two vortices relative to the geographical center of the two vortices. Note that the center location (08 in longi- tude and latitude), which denotes a geographical center between the two vortices, differs between the simulations and at the same time in each simulation. In the CTL and H75 simulations, mutual cyclonic circulations of the two vortices are evident without a significant change in the distance between them. Saomai moves more northward and Bopha moves more southward with respect to their geographical centers. In the H50 and H25 simulations, however, the zonal movements of the two vortices be- come dominant, especially after 1800 UTC 9 August, FIG. 13. The minimum sea level pressure of Saomai (PS_min) and Bopha (PB_min) from 1200 UTC 8 Aug to 0000 UTC 10 Aug when the distance between the two vortices decreases at intervals of 12 h in the CTL (circle), H75 (square), H50 (tri- continuously. The distances between the two vortices angle), and H25 (circle with dot) simulations. from 1200 UTC 8 August to 0000 UTC 10 August change from 1090 (1090) to 521 km (698 km) in the H25 Moisture convergence upstream of the mountains can (H50) simulation, whereas those in the H75 (CTL) maintain the intensity of the typhoon (Bender et al. simulation change from 862 (858) to 824 km (766 km). 1985; Chambers and Li 2011), and the changes in the The distances between the two vortices at 1200 UTC moisture flux in the lower troposphere with respect to 8 August in the CTL and H75 simulations are quite terrain height are examined from the present simula- different from those in the H50 and H25 simulations, as tions (not shown). The results show that the moisture the tracks of Bopha in the H50 and H25 simulations are flux at 850 hPa is not significantly different from each significantly different from those in the CTL and H75 simulation, except that a slight contraction of conver- simulations during the first 6 h of the integration (0600– gence is seen near Taiwan in the simulations with higher 1200 UTC 8 August). topography. Most previous studies of the interaction between two vortices are based on ideal simulations, given certain c. Interaction between Saomai and Bopha assumptions, such as the same magnitudes of vorticity Because Typhoon Saomai and Tropical Storm Bopha with different vortex sizes (e.g., Dritschel and Waugh exist simultaneously in the western North Pacific region, 1992) or the same magnitudes of vorticity and size (e.g., as shown in Fig. 4, the interaction between the two vor- Shin et al. 2006). Therefore, previously employed tices should be considered. Figure 13 shows the minimum methods of categorizing interaction regimes cannot be sea level pressure of the two vortices at the four times easily applied to the present real-storm case. Never- shown in Fig. 4. First, the figure shows that the intensity theless, we estimated the nondimensional distance (ND) of Saomai is significantly stronger than that of Bopha between Saomai and Bopha and determined the ratio of from 1200 UTC 8 August to 0000 UTC 10 August. the two vortex radii (VR), as suggested by Dritschel and However, the relative strength of the two vortices differs Waugh (1992). The results are shown in Table 1 along with time, depending on mountain height. At 1200 UTC with the interaction regimes selected. A total of 13 among 8 August, the intensity of Bopha increases significantly 16 cases (4 times 3 4 simulations shown in Fig. 13) be- with decreasing mountain height, whereas there is long in the ‘‘elastic interaction’’ category described by no significant difference in the intensity of Saomai. At Dritschel and Waugh, a mutual cyclonic rotation of two 0000 UTC 9 August, the intensity of Bopha increases in vortices without significant changes in the strength of all simulations, whereas that of Saomai decreases. Be- each vortex. Only three cases in the H25 (at 1200 UTC tween 0000 and 1200 UTC 9 August, Saomai intensifies 9 August and 0000 UTC 10 August) and H50 (at for all simulations, and the degree of intensification is 0000 UTC 10 August) simulations fall within the ‘‘com- proportional to the mountain height. Between 1200 UTC plete merger’’ and ‘‘partial straining out’’ categories,

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FIG. 14. The centroid-relative tracks from the (a) CTL, (b) H75, (c) H50, and (d) H25 simulations. Each symbol is marked at every 6 h from 1200 UTC 8 Aug, and the triangle in each panel denotes the geographical center between the two vortices. The results are obtained from D1. respectively. This result is mainly due to the decreased Tropical Storm Bopha, which was probably more influ- nondimensional distance resulting from the increasing enced by topography upon landfall in Taiwan before size of Bopha with decreasing mountain height. Although Saomai passed north of Taiwan. Thus, both the direct the degrees of interaction between the two vortices for and indirect effects of Taiwan’s topography on Saomai these two interaction regimes are somewhat stronger through the changes in Bopha are included in the pres- (especially for the complete merger regime) than the real ent simulations. Here, the direct effect refers to the in- interactions between Saomai and Bopha shown in Figs. 4 fluence of topography on Typhoon Saomai when Bopha and 14, Dritschel and Waugh’s (1992) interaction regimes does not exist, whereas the indirect effect denotes the are likely to be applicable to the present real-storm case influence of topography on Saomai through the changes simulations, at least in a qualitative sense. in the strength of Bopha caused by the topography. To Previous modeling results showed that the effects of examine the direct and indirect effects of topography topography on Typhoon Saomai’s track and intensity separately, two additional experiments are conducted by were strongly related to the effects of topography on removing Bopha with 100% (NBH100 simulation) and

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TABLE 1. The nondimensional distance (ND), ratio of vortex takes a more southern track. The intensity of the ty- radii (VR), and interaction regime according to Dritschel and phoon is generally stronger when Bopha is removed (not Waugh (1992) calculated using the simulation results from shown), probably because the northerly in the west of 1200 UTC 8 Aug to 0000 UTC 10 Aug at intervals of 12 h. ‘‘EI,’’ ‘‘PS,’’ and ‘‘CM’’ denote ‘‘elastic interaction,’’ ‘‘complete merger,’’ Saomai associated with the cyclonic circulation of the and ‘‘partial straining out,’’ respectively. typhoon is maintained without Bopha. Consequently, the typhoon is stronger without Bopha, as shown in the Hour and date CTL H75 H50 H25 surface pressure field (Fig. 15c). The stronger cyclonic 1200 UTC 8 Aug ND 2.82 2.81 3.66 3.33 circulation induces weaker southeasterlies in the low- VR 0.44 0.49 0.78 0.98 level environmental steering flows, as shown in Fig. 16; Regime EI EI EI EI 0000 UTC 9 Aug ND 5.05 4.62 2.55 2.06 consequently, Saomai moves at a slower speed without VR 0.48 0.97 0.79 0.75 Bopha. The difference in the track between the NBH100 Regime EI EI EI EI and NBH50 simulations is not significant and does 1200 UTC 9 Aug ND 4.34 3.23 1.82 0.42 not increase with time (Fig. 15a). The intensities of the VR 0.37 0.91 0.64 0.47 typhoon in NBH100 and NBH50 are also similar, except Regime EI EI EI CM 0000 UTC 10 Aug ND 4.32 3.43 1.01 0.03 that its decay after 0600 UTC 10 August is slightly VR 0.35 0.86 0.53 0.32 slower in NBH100 than in NBH50. This result implies Regime EI EI PS CM that the direct effects of topography on the track and intensity of Saomai are not significant when Bopha is removed and that the differences in the track and in- 50% (NBH50 simulation) topography heights. To remove tensity of Saomai among the four simulations with re- Bopha from the initial condition, the vortex removal spect to the mountain heights shown in Fig. 6 are largely scheme used in WRF version 3.1 (Davis and Low-Nam due to the influence of topography on Bopha and the 2001; Skamarock et al. 2008) is employed. First, non- corresponding flow changes in the southwest of Saomai. divergent wind is removed by solving the Laplace equation Figure 17 shows the horizontal wind vector at 200 hPa based on the relationship between nondivergent wind at 1200 UTC 9 August from the H50 and NBH50 sim- and relative vorticity. Next, divergent wind and geo- ulations. In the NBH50 simulation (Fig. 17b), the anti- potential height anomalies are removed using the Laplace cyclonic circulation in the northwest of Taiwan is not relationship between velocity potential and divergence presented. This result demonstrates that the strong an- and between geopotential and geostrophic vorticity, re- ticyclonic circulations in the upper troposphere, as spectively. The temperature anomaly is also removed by shown in the H50 and H25 simulations (Figs. 9c,d), are using the hydrostatic relation. Finally, the water vapor induced by the circulations associated primarily with mixing ratio is modified using a linear interpolation be- Bopha. The horizontal wind field at 200 hPa from the tween the original mixing ratio and averaged mixing ratio NBH100 simulation is similar to that from NBH50 (not surrounding the storm. shown), implying no significant contribution of topog- Figure 15a shows tracks of Typhoon Saomai from the raphy without Bopha. In summary, the results shown in CTL, H50, NBH100, and NBH50 simulations. Gener- Figs. 15–17 demonstrate that Taiwan’s topography ally, the typhoon moves more southward at a slower influenced the track and intensity of Typhoon Saomai, speed when Bopha is removed. These results can be which passed to the north of Taiwan, primarily through explained by the changes in the environmental steering changes in Tropical Storm Bopha, which passed over flow (Fig. 16). In the NBH100 simulation (Fig. 16b), the Taiwan’s topography prior to Saomai, and due to the southeasterly below 500 hPa becomes weaker than in associated flow change between the two storms. the CTL simulation (Fig. 16a) after 0600 UTC 9 August, which is also true for the H50 (Fig. 16c) and NBH50 5. Summary and conclusions (Fig. 16d) simulations. The weakening of the environ- mental steering flows in the NBH100 and NBH50 sim- The effects of topography on the evolution of Typhoon ulations is associated with the changes in the flows Saomai (2006) are investigated by conducting a series of between Bopha and Saomai. Figures 15b,c show the sea numerical simulations using the WRF with 100%, 75%, level pressure at 1200 UTC 9 August from the CTL and 50%, and 25% of the terrain heights of Taiwan. In the NBH100 simulations, respectively. In the NBH100 CTL simulation with the actual topography (100%), the simulation, a closed low of 998 hPa representing Bopha, observed track and intensity of Typhoon Saomai are re- which appears in the southwest of Taiwan in the CTL produced reasonably well. simulation, disappears. As a result, the southerlies in the Differences in the track and intensity of Typhoon southwest of Saomai disappear; consequently, Saomai Saomai between the experiments with different mountain

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FIG. 15. (a) Simulated typhoon tracks in D3 from 1200 UTC 8 Aug to 0600 UTC 10 Aug in the CTL, H50, NBH100, and NBH50 experiments. Every symbol is marked at intervals of 6 h. The sea level pressure at 1200 UTC 9 Aug in D1 from the (b) CTL and (c) NBH100 simulations. The contour intervals in (b) and (c) are 4 hPa. heights are strongly related to those of Tropical Storm much farther north of Taiwan (relatively northward) in Bopha, which passed Taiwan earlier than the typhoon. the simulations with reduced topography (H50 and In the experiments employing reduced terrain height, H25). This is because higher CMR weakens the easterly Bopha strengthens near Taiwan. This is because higher flow in the lower troposphere and suppresses the gen- topography prohibits the interaction between the to- eration of the northerly wind component in the upper pography and Bopha by shifting Bopha southward. In troposphere by restraining the expansion of warm air the simulations with relatively higher mountain heights from Bopha. The resultant weaker vertical wind shear (CTL and H75), the typhoon passes over the northern keeps warm air near the center of the typhoon in the edge of Taiwan (relatively southward), whereas it moves upper troposphere, resulting in the intensification of the

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FIG. 16. As in Fig. 7, but for the comparison between the simulations with and without Bopha: (a) CTL, (b) NHB100, (c) H50, and (d) NBH50 simulations. typhoon. It is likely that the topography of Taiwan pro- which can be determined by their intensity and the dis- moted the development of Typhoon Saomai by pre- tance between them. As mountain height increases, venting the growth of Tropical Storm Bopha. Bopha weakens and Saomai strengthens in all simula- The influence of topography on the typhoon track can tions, although the degree of change in the sea level be explained by the interaction between the two vortices, pressure for the two vortices differs with time. Bopha is

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FIG. 17. The horizontal wind vector at 200 hPa at 1200 UTC 9 Aug from the (a) H50 and (b) NBH50 simulations. The results are obtained from D1. located at nearly the same latitude and approximately To examine the direct effects of topography on the 800–1000 km west of Saomai at the beginning of the track and intensity of Saomai, two additional simulations simulation (1200 UTC 8 August); subsequently, the two are conducted that remove Bopha from the initial con- storms rotate counterclockwise with time. In the higher- dition with 100% and 50% of Taiwan’s terrain heights. It topography simulations, the two vortices rotate in is found that the typhoon moves more southward and at a larger circle with respect to their geographical center, a slower speed when Bopha is removed because the and Saomai and Bopha move more northward and southeasterly environmental steering flow weakens in southward, respectively. The interaction between Saomai response to the absence of cyclonic circulation from and Bopha is investigated based on the nondimensional Bopha. Without Bopha, the influence of topography on distance between the two vortices and the ratio of the the track and intensity of Saomai is not significant be- two vortex sizes, as proposed by Dritschel and Waugh cause Saomai passed north of Taiwan without directly (1992). In most of the different mountain height simu- contacting the mountains. This result demonstrates that lations and over time within each simulation, the two topography influences the track and intensity of Saomai vortices fall within the ‘‘elastic interaction’’ category of in the present case through the changes in the devel- Dritschel and Waugh (1992), which is defined as the opment of Bopha and associated circulations. mutual cyclonic circulation of two vortices without In this study, the effects of topography on Typhoon significant changes in the strength of either vortex. Saomai were examined by conducting four numerical Only one case in the H50 simulation and two cases in the simulations with different heights of the CMR in Tai- H25 simulations are categorized as ‘‘partial straining wan. Although several aspects of topographic effects out’’ and ‘‘complete merger,’’ respectively. The inter- were investigated in this study, a secondary low on the action regimes proposed by Dritschel and Waugh (1992) lee side of the CMR was not considered. Although are based on ideal simulations of two vortices with a variety of previous studies found that a secondary low uniform vorticity and different vortex sizes. Therefore, can play an important role in deflecting a typhoon, the their criteria cannot be easily applied to real-storm ca- physical mechanism of the secondary low and its in- ses. Nevertheless, in most cases of different mountain teraction with a primary low are not yet clearly un- heights and storm evolution times, the interaction be- derstood. Therefore, the formation of a secondary low in tween Saomai and Bopha can be explained by Dritschel the lee of the CMR and its influence on Tropical Storm and Waugh’s (1992) regimes, at least in a qualitative Bopha and Typhoon Saomai are subjects that are worthy sense. of future study.

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