Radar Analysis on the Interaction Between Southwesterly Monsoonal Flow and Circulation Associated with Typhoon Morakot (2009)

JournalOctober of2012 the Meteorological Society of Japan, Vol. C.-H.90, No. WEI 5, pp.et al.617−628, 2012 617 DOI: 10.2151/jmsj.2012-503

Radar Analysis on the Interaction between Southwesterly Monsoonal Flow and Circulation Associated with Typhoon Morakot (2009)

Chih-Hsien WEI

Department of Environmental Information and Engineering, National Defense University, Taoyuan, Taiwan

Tai-Hwa HOR

General Education Center, Lunghwa University of Science and Technology Taoyuan, Taiwan

Yao-Chung CHUANG

Department of Environmental Information and Engineering, National Defense University, Taoyuan, Taiwan

Tai-Chi CHEN WANG

Department of Atmopheric Sciences, National Central University Chungli, Taiwan

and

Jian-Liang WANG

Department of Environmental Information and Engineering, National Defense University, Taoyuan, Taiwan

(Manuscript received 18 January 2012, in final form 3 May 2012)

Abstract

This study investigates the mesoscale interaction of typhoon circulation and southwesterly flow during the passage of Typhoon Morakot (2009) over Taiwan using radar observations. Single Doppler radar analysis characterized typhoon features with remarkably distant rainbands, and identified the strong southwesterly monsoonal flow with a maximum speed of more than 45 m s–1 in the southern flank of distant rainbands. Dual-Doppler synthesized winds elucidated a confluent mechanism on the northern side of the rainbands with a maximum convergence over1.5 × 10–3 s–1. Velocity azimuth display (VAD) winds showed the intensification of southwesterly flow at low levels. The southwesterly monsoonal flow initiated about 6 hrs before typhoon landfall, and then became prominently involved with typhoon circulation. Also, the radial component with respect to the typhoon center was enhanced and became comparable with the tangential one (~30 m s–1) about 7.5 hrs after landfall. The variation in intensity of radial components can be regarded as a unique precursor for the extension of influence of the southwesterly monsoonal flow onto the typhoon circulation. Furthermore, the strong convergence showed that the interaction between the southwesterly flow and typhoon circulation might contribute to the development of rainbands, as well as the intensification of the inward radial flow embedded within typhoon.

Corresponding author: Chih-Hsien Wei, Department of 1. Introduction Environmental Information and Engineering, Chung Cheng Institute of Technology, National Defense Univer- The interaction of a developing tropical cyclone sity, 75, Shiyuan Rd., Daxi Township, Taoyuan County 33551, Taiwan, Republic of China (TC) with the Asian monsoon system is a significant E-mail: [email protected] phenomenon that might trigger severe weather events ©2012, Meteorological Society of Japan over the Taiwan area. For instance, the warm, moist 618 Journal of the Meteorological Society of Japan Vol. 90, No. 5 air in the TC circulation can interact with the cold, dry sphere was high convective instability. The pressure winter-monsoonal northeasterlies and create a fron- gradient between the northern South China Sea and the tal-type system during autumn and early winter (Wu Philippines thus strengthened the SW flow. Research et al. 2009). The phenomenon is as important as topo- from numerical simulation concluded that the SW graphical effect to create the asymmetric distribution flow, which was induced after Typhoon Mindulle left of rainbands embedded with in a TC system, which Taiwan, brought highly convective unstable air into the causes extreme difficulty in predicting intense wind southern Taiwan area. A secondary low formed over and heavy rainfall in Taiwan. The TC-terrain interac- the Taiwan Strait during the passage of Taiwan, and tion problem has led to plenty of significant studies, accelerated the SW flow to trigger strong mesoscale including numerical simulations (Wu and Kuo 1999; convective systems (MCSs) later on (Chien et al. 2008; Wu 2001; Wu et al. 2002; Lin et al. 2002; Chiao and Lee et al. 2008). Recently, Chen et al. (2010) learned Lin 2003; Lin et al. 2005; Jian and Wu 2008; Ge et al. that from a climatologic point of view, the local rainfall 2010; Chien and Kuo, 2011; Wu et al. 2009; Liang in Taiwan can be separated into two subcomponents: et al. 2011) and observations (Hor et al. 2005; Yu and tropical cycle rainfall (PTC) and seasonal monsoon Cheng 2008; Wei et al., 2011). In contrast, few system- rainfall (PSM); and two dominant rainfall variability atic studies have been done on the TC interaction with types are found: enhanced PTC but suppressed PSM and the summer monsoon system (Lin et al. 2011; Wu et al. suppressed PTC but enhanced PSM. 2011; Chien and Kuo 2011). The primary goal of the study is to investigate meso- Over the Taiwan area, the southwesterly monsoonal scale features of the interaction between the typhoon flow (hereafter, the SW flow) prevailed in the summer, circulation and the SW flow. By analyzing Doppler while the northeastern monsoonal flow dominated radar data collected during the impact of Typhoon in the winter. As early as the 1980s, the statistical Morakot (2009) on Taiwan Island, an expectation in study on typhoon track and the accumulated rainfall finding a significant indicator for the onset of severe rate collected during 1952–1981 in the Taiwan area rainfall events based upon such an interaction is suggested that moist and unstable flow would be also the focus of this study. Due to the uniqueness introduced from the South China Sea while a typhoon of Typhoon Morakot, Lin et al. (2011) studied the was passing over the seashore of northeastern Taiwan mesoscale processes for the precipitation of Typhoon (Yu 1982). The numerical model simulation by Chiao Morakot by Weather Research and Forecasting (WRF) et al. (2003) indicated that the enhancing south- model simulation. They found that the most prominent west monsoonal flow and typhoon circulation would features were the wave train convective cells inside construct a low-level convergence before and after the the main rainband and the convergence zone at the passage of a typhoon over Taiwan, and new convec- southern boundary of the rainband. However, some of tive cells were triggered in southwestern Taiwan by their findings need to be verified by using more detailed the orographic lifting of the circulation associated observational data in spatial and temporal resolutions. with the convergent zone. A prominent case which is Basically, the rawinsonde and dropsonde observational similar to this concept is Typhoon Mindulle (2004). data are significant to reaching that idea. Unfortu- Typhoon Mindulle formed in the vicinity of Guam nately, the rawinsonde observations were lacking over Island on 23 June 2004, and then moved west-north- the southwestern Taiwan area due to the shortage of westward. Its path was nearly north-northwestward sondes. Even the sounding observations at Dongsha while approaching Taiwan Island. According to the Island (20°43′N 116°42′E), upstream of the SW flow, announcement by the Central Weather Bureau of were provided just once a day (at 0000 UTC) for to the Taiwan (CWB), its intensity had weakened to a slight same reason (Lin et al. 2010). Also, dropsonde obser- typhoon (maximum wind speed of 17.2–32.6 m s–1) vations were not available after 0600 UTC 6 August before making landfall on the eastern coast of Taiwan 2009. Therefore, the unique radar data are invaluable on 1 July 2004. Most of intense rainbands developed to elucidating the mesoscale characteristics between and distributed over the south of the storm system while typhoon circulation and the SW flow. Descriptions of the typhoon was moving further north. Therefore, the the environmental conditions are presented in Section organization of typhoon entity was asymmetrical from 2. Results from the single Doppler radar analysis is both satellite and radar observations. The SW flow demonstrated in Sections 3, and the velocity azimuth in the Typhoon Mindulle case, which was character- display (VAD) approach as well as the dual-Doppler ized by warm and moist air, induced a mesolow in radar analysis are presented in Section 4. Discussion the northern South China Sea where the lower atmo- and summary are shown in Section 5. October 2012 C.-H. WEI et al. 619

2. Environmental conditions Based upon the warning issued by the CWB in Taiwan, Typhoon Morakot formed at a distance of almost 1500 km away from the east of the southern tip of Taiwan on 4 August 2009. It moved westward and passed Taiwan Island between 6 and 9 August 2009, sustaining its impact on the Taiwan area for almost 64 hours (Fig. 1). Table 1 depicts the time sequence of observed intensity, minimum central pressure and position of Typhoon Morakot as reported by CWB. Its passage induced the most serious flooding event over the past five decades in Taiwan, causing devastating disasters of property and casualty. Some research attributed this feature to the interaction between the well pronounced SW flow and the circulation of typhoon. Fig. 1. Track map of Typhoon Morakot (2009) Hong et al. (2010) found that Typhoon Morakot with center pressure (hPa) reported by the Cen- was associated with a large-scale convection region tral Weather Bureau in Taiwan. The intensity of with monsoon circulation of a different time scale in typhoon wind speeds; intense >51.0 m s–1, mod- the tropical western North Pacific. Hsu et al. (2010) erate intensity 32.7–50.9 m s–1 (in green circle), learned that the SW flow in the Typhoon Morakot case slight intensity 17.2–32.6 m s–1 (in blue circle) supplied abundant moisture and formed a large-scale and tropical depression < 17.2 m s–1 (in red cir- convection-active region, which is much larger than the cle). (Adopted from the Central Weather Bureau size of typhoon vortex. Chien and Kuo (2011) pointed of Taiwan.) out two unique features of Morakot based upon the observations: the slow translation speed of the system (~10 km h–1), which caused a long duration of typhoon and isobars was large enough (30–60 degrees) at low influenced rainfall; the strong southwesterly flow that levels (Fig. 2), it deemed that the interaction between transported moisture-laden air to the northern South typhoon circulation and the monsoonal flow may lead China Sea and the southern Taiwan Strait. to reinforcing of the typhoon entity or the intensifica- At 0000 UTC 7 August 2009, a low pressure system tion of SW flow due to the confluent mechanism. The developed over the Northwest Pacific in the vicinity results will be shown in Section 4. Therefore, the more of Taiwan prior to the passage of Typhoon Morakot detailed mesoscale analysis on the interaction between (Fig. 2a). There were three typhoons embedded within the circulation of Typhoon Morakot and the SW flow the low. One was Typhoon Goni (2009), with a center using intensive observations in temporal and spatial pressure of 994 hPa, which was located in the South scale is quite necessary. China Sea near Hainan Island. The other one was 3. Mesoscale analysis on the typhoon circulation Typhoon Morakot which was on the way to Taiwan, and the third one named Etau, with a center pressure The time sequence of integrated reflectivity around of 999 hPa, was located at the east of Morakot over the Taiwan area (Fig. 3) illustrated that, besides the the open ocean. Hong et al. (2010) reported that these typhoon eyewall, the intense rainbands developed in three tropical cyclones were spawned from the large- the vicinity of the southwestern Taiwan. They were scale cyclonic flow (i.e., monsoon gyre). The low pres- regarded as “distant rainbands” since their distances sure zone slowly moved westward with the extension were over 250 km away from the typhoon center. of the subtropical ridge. It introduced intense SW flow The most remarkable rainband, which was denoted as (shown on the surface and 850 hPa charts in Fig. 2) A, existed off the southwestern coast of Taiwan and on its southern edge and an abundant moisture zone became well organized since 1100 UTC 7 August 2009. was extended from the Bay of Bengal to the east of It moved cyclonically to the south of Taiwan (Figs. 3b, Taiwan (Hong et al. 2010). The prevailing SW flow 3c). After lasting more than 3 hours, the rainband was began to interact drastically with the typhoon circu- approximately stationary and became less organized lation as Typhoon Morakot propagated near Taiwan (Figs. 3d, 3e). Before Typhoon Morakot made landfall Island. Since the angle between the southwesterly wind in eastern Taiwan at 1530 UTC 7 August, a number 620 Journal of the Meteorological Society of Japan Vol. 90, No. 5

Table 1. The time sequence of observed intensity, center pressure (hPa) and center location (in longitude and latitude) of Typhoon Morakot (2009) according to the Typhoon Data Base of the Central Weather Bureau of Taiwan. Time Center Pressure Position Intensity (UTC) (hPa) (longitude and latitude)

2009.08.03.0000 Slight Typhoon 994 133.5E, 20.5N 2009.08.04.0000 Slight Typhoon 992 136.1E, 21.6N 2009.08.05.0000 Slight Typhoon 980 133.2E, 22.8N 2009.08.06.0000 Moderate Typhoon 960 127.9E, 23.3N 2009.08.07.0000 Moderate Typhoon 955 123.2E, 23.5N 2009.08.07.0300 Moderate Typhoon 955 122.6E, 23.4N 2009.08.07.0600 Moderate Typhoon 955 122.4E, 23.4N 2009.08.07.0900 Moderate Typhoon 955 122.3E, 23.5N 2009.08.07.1200 Moderate Typhoon 955 122.1E, 23.6N 2009.08.07.1500 Moderate Typhoon 955 121.9E, 23.9N 2009.08.07.1800 Moderate Typhoon 965 121.6E, 24.0N 2009.08.07.2100 Moderate Typhoon 965 121.5E, 24.3N 2009.08.08.0000 Moderate Typhoon 970 121.5E, 24.6N 2009.08.09.0000 Slight Typhoon 975 120.5E, 25.9N 2009.08.10.0000 Slight Typhoon 988 119.6E, 28.2N

of marked convective cells initiated after Rainband A moved eastward (Fig. 3e), they then organized into a new linear rainband, which was denoted as Rainband B at 1600 UTC (Fig. 3f). Simultaneously, Rainband A still existed at the southern tip of Taiwan, although it was not as intense as previous. Later, Rainband B replaced Rainband A after typhoon landfall. The evolution of rainbands demonstrated that the environmental conditions over southwestern Taiwan area are favor- able for triggering new convection and the organization of a remarkable rainband. Since the area was far away from the inner core of typhoon, there may have existed significant dynamic mechanisms for the development of rainbands. The Doppler velocity data collected by Chiku (RCCG) and Kenting (RCKT) radars, which are oper- ational and ground-based radars, were vital for understanding the flow patterns in the vicinity of typhoon in this study. The scanning strategy and specification for both radars are shown in Table 2. Doppler velocity data were edited by using the NCAR SOLO II software in order to eliminate noise and unfold velocities. Figure 4a demonstrated the plan position indicator (PPI) at 0.5 degree elevation observed by Chiku radar before Fig. 2. Weather charts for the East Asian region at (a) surface and (b) 850 hPa at 0000 UTC 7 typhoon landfall. The zero contours extended from the August 2009. The arrow indicates the prevailing radar site where the real wind direction was perpendic- southwesterly flow over the South China Sea. The ular to the radar beams. It depicted the north-northwest- characters of ‘W’ and ‘C’ in panel (b) indicate rel- erly and northwesterly flows prevailed to the southwest atively warm and cold area, respectively. (Adopt- of radar site, implying that the typhoon circulation ed from the Central Weather Bureau of Taiwan.) dominated the flow regime over this area. Another zero October 2012 C.-H. WEI et al. 621

a b 2009-08-07 2009-08-07 1100 UTC 1200 UTC

c d (dBZ) 2009-08-07 2009-08-07 1300 UTC 1400 UTC

A A

e f 2009-08-07 2009-08-07 1500 UTC 1600 UTC (dBZ)

A A

Fig. 3. The integrated column vector (CV) charts of reflectivity around Taiwan area at (a) 1100 UTC, (b) 1200 UTC, (c) 1300 UTC, (4) 1400 UTC, (e) 1500 UTC and (f) 1600 UTC 7 August 2009, respectively. Rainbands A and B are identified. The red dotted line in (a) represents the path of typhoon. The symbol “ ” stands for the positions of typhoon center. The symbol “+” and “×” represent the Kenting and Chiku radar sites, respectively. The black dashed squares in (b) and (f) indicate the area for dual-Doppler wind synthesis shown in Figs. 6, 7, 8. 622 Journal of the Meteorological Society of Japan Vol. 90, No. 5

Table 2. The characteristics of Chiku and Kenting Doppler weather radars. The letter “D” stands for Doppler mode and “ND” non-Doppler mode. Radars Chiku Radar Kenting Radar Items (RCCG) (RCKT)

Wavelegth 10 cm (S Band) Location 23.15°N 120.07°E 21.9°N 121.83°E Elevation (m) 38 29 Manufactory Gematronik Type Meteor-1000S Scanning range (km) 230 (D), 460 (ND) Angular resolution (degree) 1 Range resolution (km) 0.25 (D), 1 (ND) Sample size 131 (D), 32 (ND) Ray width (deg/s) 11.36 Nyquist velocity 20.0 49.51 Pulse width (μs) 0.4 (D), 10.0 (ND)

Doppler velocity was also located to the southwest of The analyzed range for both radars was 60 km. The the radar site at a distance of 120 km, indicating that VAD analysis at Chiku radar (Fig. 5a) depicted that a wind transition zone existed there since the zero the north-northwesterly flow prevailed below 5 km contours depict the real wind direction perpendicular level in altitude with a maximum speed of 32 m s–1 to the radar beams. A similar pattern remained during before 1800 UTC on 7 August 2009, revealing that the the landfall of typhoon (Fig. 4b), except the transition typhoon circulation dominated the flow pattern in the zone shifted close to the radar site. Furthermore, the north of Rainband A. In the same area, the wind field observations from the Kenting radar illustrated that at low levels (< 2 km) gradually shifted from westerly the SW flow dominated the circulation to the south to north-northwesterly after 2100 UTC. The veering of of Rainband A (Fig. 4c) before typhoon landfall. The the vertical wind profile could be extended to 5 km in maximum inbound Doppler velocity enhanced and altitude. The SW flow with a maximum speed of 45 split into two branches while the typhoon was making m s–1 in the vicinity of the Kenting radar site (Fig. 5b) landfall, suggesting that a strong westerly wind inter- initiated after 0700 UTC on 7 August 2009 in the lower acted with the SW flow (Fig. 4d). The two distinct layer (~0.5 km). It apparently prevailed to the south flows would trigger an offshore confluence along the of the rainband A with an abundant moisture supply, distant rainbands, which will be shown in Fig. 6. making the atmosphere more unstable from the south- The vertical profile of sounding observations may westerly winds during the passage of typhoon (Hong clearly reveal the temporal circulation variation over et al. 2010; Ge et al. 2010). the southwestern Taiwan area. Since neither rawin- 4. Interaction processes sondes nor dropwinsondes were deployed at that time, analyzing the Doppler wind retrieval was the only 4.1 Interaction of typhoon circulation and SW flow approach for dynamic interaction between typhoon The above phenomena need further study in order to circulation and SW flow. understand its dynamic scenario. The co-coverage of The velocity azimuth display (VAD) (Lhermitte Chiku and Kenting radars with a baseline of 158 km and Atlas 1962; Browning and Wexler 1968; Srivas- has allowed for dual-Doppler analysis. The radar data tava et al. 1986) is an algorithm which is used to were interpolated onto a 1.5 km × 1.5 km × 0.5 km (x, obtain the vertical profile of horizontal winds for the y, z) grid of a Cartesian coordinate with a proper radius atmosphere in a certain range surrounding a Doppler of influence using Cressman interpolation. To date, radar. Therefore, radial velocity data collected from the NCAR CEDRIC (Custom Editing and Display of Chiku and Kenting radars scanning every 7.5 minutes Reduced Information in Cartesian space) package is were regarded as intensive sounding observations. ready to synthesize the grid Doppler velocity into the October 2012 C.-H. WEI et al. 623

Fig. 4. The Doppler velocity of Chiku radar at (a) 1201 UTC and (b) 1616 UTC, as well as that of Kenting radar at (c) 1201 UTC and (d) 1616 UTC 7 August 2009 at a 0.5 elevation angle, respectively. Radar site is located at the center of rings with interval of 30 km. The bold arrows represent the flow directions.

three dimensional wind fields. More detailed processes ysis can simply explain the flow pattern in the vicinity for synthesized wind are referred to in the study by Lee of Rainbands A and B. The stream blew cyclonically et al. (2008). The domain of dual-Doppler analysis was along the tangential direction of typhoon in the north shown as a black square (in broken line) in Fig. 3b, of Rainband A at 1201 UTC August 2009 (Fig. 6a). which was far from the baseline between these two It was abruptly deflected and turned inward when radars. Therefore, the synthesized wind fields would approaching the northern edge of the rainband. The not exhibit a singularity or discontinuity in the domain. angular differences between streamline and tangen- However, the long baseline would smooth the retrieved tial direction of typhoon circulation could reach 45 wind fields due to coarse sampling (Davies-Jones degrees. A similar flow pattern was also presented in 1979). The synthesized data at 1201 and 1616 UTC on the vicinity of Rainband B at 1616 UTC (Fig. 6b). The 7 August 2009 were selected to analyze the wind field remarkable deflection of the streamline was accompa- before and during the landfall of Typhoon Morakot, nied with a maximum angular difference of 30 degree. respectively. Figure 6 showed that the streamline anal- The drastic deflection of flow, which changed the wind 624 Journal of the Meteorological Society of Japan Vol. 90, No. 5

Fig. 5. The time sequences of the velocity azimuth display (VAD) wind at (a) Chiku radar and (b) Kenting radar. The unit of wind speed is knots. Yellow shaded area stands for the wind direction less than 270 degrees. The red arrow indicates the time of landfall of Typhoon Morakot at 1530 UTC 7 August 2009. Some wind barbs above 11 km in altitude are not reliable due to incomplete Fig. 6. Dual-Doppler syntheses at 2 km level data coverage. shown in Fig. 2 at (a) 1201 UTC for Rainband A and (b) 1616 UTC for Rainband B on 7 August 2009. The synthesized wind field is demonstrated by streamline. The bold black circles are range direction from northwesterly to west-nothwesterly, rings centered at the typhoon center with interval implied that the circulation of the typhoon was experi- of 40 km, and the axes indicate relative distanc- encing an interaction with other flow patterns, such as, es from the Kenting radar site (x = 0 km, y = 0 SW flow. Since the obvious angular difference corre- km). The Chiku radar is located at the position of sponded to the existence of rainbands, the intensity of (x = –81 km, y = 138 km). The typhoon center is inward flow might play a key role for the maintenance located at the position of (x = 128 km, y = 189 and development of rainbands. km) at 1201 UTC and (x = 94 km, y = 228 km) In order to realize the influence of SW flow on at 1616 UTC. The arrow indicates the confluence the typhoon circulation, the synthesized wind was along the distant rainband. deducted by the mean typhoon circulation, which was determined from the VAD wind profile of Chiku October 2012 C.-H. WEI et al. 625

Fig. 8. Same as Fig. 6, except that the bold gray Fig. 7. Same as Fig.6, except subtracting the ty- contours stands for the convergence of 1 × 10–3 phoon circulation based on the VAD wind of s–1 and 1.5 × 10–3 s–1, respectively. The black Chiku radar at (a) 1201 UTC for Rainband A and solid contours represent inward radial component (b) 1616 UTC for Rainband B on 7 August 2009, (m s–1) of typhoon circulation, while the outward respectively. The blue thin lines represent spokes radial component (m s–1) is plotted by black from the typhoon center. dashed contours. radar because the flow patterns around the Chiku and the northern edge of Rainband B, where typhoon circu- Kenting radars were comparable with the VAD winds. lation dominated. The SW flow almost corresponded Figure 7a showed that the SW flow presented in the to the direction of spokes from the typhoon center, south flank of Rainband A with a maximum speed over revealing that it might enhance the radial component 40 m s–1 and decelerated in the north flank of the rain- of typhoon circulation. Accordingly, the kinematics band. Therefore, Rainband A was located at a transition of typhoon circulation could be further analyzed in zone of wind speed. The SW flow also exceeded 40 m terms of radial components only (Fig 8). The pattern s–1 at 1616 UTC (Fig. 7b), although it could extend to of inward radial flow was similar to that of SW flow, 626 Journal of the Meteorological Society of Japan Vol. 90, No. 5 which was intense in the south flank of rainband and became weak in the north of rainband. The maximum speed of radial flow was over 21 m s–1 at 1201 UTC for Rainband A (Fig. 8a), possessing almost a half of the total wind speed. The maximum speed of radial wind at 1616 UTC decreased to roughly 15 m s–1 (Fig. 8b), which is weaker than that at 1201 UTC. The decline of radial wind weakened the accompanying convergence. Therefore, the convective cells embedded within the Rainband B were relatively small at 1616 UTC. The above analyses evidenced that the variation of the radial flow in the north of Rainband A was dominated by the typhoon circulation. However, its variation in the south of the rainband was clearly influenced by the interaction of the typhoon circulation and the SW flow. It confirmed that there was a confluent mechanism along the rainband which contributed to the pronounced interaction between the typhoon circulation and the SW flow, causing remarkable conver- gences along the rainband which exceeded 1.5 × 10–3 s–1 at low levels associated with intense cells both at 1201 UTC (Fig.8a) and 1616 UTC (Fig.8b). Lin et al. (2011) suggested details of the dynamical processes of Typhoon Morakot, i.e., a convergence zone, using a fine-resolution numerical simulation (in horizontal resolution of 3 km). When the SW monsoon made its advance northward with the support of the northerly component of the typhoon circulation, the convergence zone appeared on the southern side of the main rainband. The present study based on the radar analyses (in Fig. 9. The time sequences of the tangential (black horizontal resolution of 1.5 km), however, showed that line) and radial (red line) components with re- most of the prominent convergence zones existed at spect to the typhoon center at (a) Chiku radar northern side of Rainbands A and B at 1201 and 1616 and (b) Kenting radar at 2 km level. The vertical UTC, respectively. Remarkably, the strong conver- axis is the wind speed (m s–1) and horizontal one gence zone was accompanied with an intense gradient represents the time (UTC). The positive values of radial flow. The deceleration of radial velocity in the red line stand for inward radial wind with would generate speed convergence at the northern side respect to the typhoon center. The red arrow in- of rainband. dicates the time of landfall of Typhoon Morakot at 1530 UTC 7 August 2009. 4.2 Intensification of SW flow The dual-Doppler analysis provides a detailed description for the interaction between typhoon circu- into tangential and radial components with respect to lation and SW flow. The results demonstrated the the typhoon center in order to further analyze the pres- disagreement between numerical simulation and radar ence of SW flow and its interaction with the typhoon observation may be attributed to the lack of sounding circulation. The flow deduced from VAD wind at observations over southwestern Taiwan area, which are Chiku radar was comprehensive since the tangential essential for the initialization of a numerical model. component was around 25–30 m s–1, which was at least Therefore, the intensive radar observations deliver twice as large than the radial (< 10 m s–1) (Fig. 9a). much finer and more reliable information for the iden- The magnitude of radial component of VAD wind at tification of significant features. Kenting radar (Fig. 9b) was less than that of tangen- According to the dual-Doppler analysis in the tial in 25 m s–1 about 6 hours before landfall, which previous section, the VAD winds also could be divided was similar to the finding from Chiku radar. It abruptly October 2012 C.-H. WEI et al. 627 increased after 0900 UTC 7 Aug. 2009 and eventually to the early warning of upcoming invasions of typhoon reached to 30 m s–1, comparable with the tangential over the Taiwan area. (~30 m s–1) about 7.5 hours after landfall. The differ- Acknowledgements ence in radial components analyzed by using Doppler radar data indicated that the southerly flow contributed The radar data were kindly provided by Central to the enhancement of the radial component south of Weather Bureau (Taiwan). Discussions with Dr. Rainband A and B, which was also consistent with Wen-Chau Lee (EOL/NCAR) are very much appre- the dual-Doppler analysis. These significant features ciated. The authors would like to thank Rev. Christo- provide unique precursors to identify the beginning of pher Wright, a native English speaker, for correcting the interaction between typhoon circulation and SW grammar and writing the original version. And we also flow, as well as the extent of the influence of SW flow thank two anonymous reviewers for providing valuable on typhoon circulation. suggestions. We are indebted to the National Science Council of the Republic of China for the financial 5. Summary support through Grant NSC 101-2623-E-606-010-D. Most of studies on the mesoscale interaction References between typhoon circulation and SW flow focused on the numerical simulation, but rarely on the obser- Browning, K. A., and R. Wexler, 1968: The determination vational analysis. The interaction, which causes the of kinematic properties of a wind field using Doppler asymmetry of typhoon entity, is of considerable impor- radar. J. Appl. Meteor., 7, 105–113. tance as the topographical effect for predicting signif- Chen, J.-M., T. Li, C.-F. Shih, 2010: Tropical Cyclone– and icant wind speed and heavy rainfall in Taiwan. During Monsoon-Induced Rainfall Variability in Taiwan. J. Climate, 23, 4107–4120. the passage of Typhoon Morakot, the asymmetry of Chiao, S., and Y. L. Lin, 2003: Numerical simulations of an typhoon circulation induced the most serious flooding orographic rainfall event associated with the passage event causing devastating casualties and destruction of of a tropical storm over a mesoscale mountain. Wea. property. The paper uses Doppler radar data to docu- Forecasting, 18, 325–344. ment the presence of the SW flow and its influence on Chien, F. C., Y. C. Liu, and C. S. Lee, 2008: Heavy rainfall typhoon circulation off the southwest coast of Taiwan and southwesterly flow after the leaving of Typhoon during the transverse of Typhoon Morakot. Intensive Mindulle (2004) from Taiwan. J. Meteor. Soc. Japan, VAD winds showed the intensification of SW flow 86, 17–41. doi:10.2151/jmsj.86.17. at low levels. The SW flow started about 6 hr before Chien, F.-C., and H.-C. Kuo (2011), On the extreme rainfall typhoon landfall, and then it became prominently of Typhoon Morakot (2009), J. Geophys. Res., 116, involved with typhoon circulation. Also, the radial D05104, doi:10.1029/2010JD015092. component of VAD winds with respect to the typhoon Davies-Jones, R. P., 1979: Dual-Doppler Radar Coverage center was enhanced and became comparable with the Area as a Function of Measurement Accuracy and Spatial Resolution. J. Appl. Meteor., 18, 1229–1233. tangential (~30 m s–1) about 7.5 hr after landfall. The doi: http://dx.doi.org/10.1175/1520-0450-18.9.1229 inward radial component of typhoon circulation in the Ge, X., T. Li, S. Zhang, and M. Peng, 2010: What causes south of distant rainbands was enhanced under the the extremely heavy rainfall in Taiwan during influence of the SW flow. Therefore, the variation in Typhoon Morakot(2009)? Atmos. Sci. Lett., 11, 46–50, the radial component in the VAD wind can be regarded doi:10.1002/asl.255. as a useful indicator to the extent of the SW flow influ- Hong, C. C., M. Y. Lee, H. H. Hsu, and J. L. Kuo, 2010: ence on the typhoon circulation. Role of submonthly disturbance and 40–50 day ISO The dual-Doppler synthesized analysis discovered on the extreme rainfall event associated with Typhoon an intense rainband embedded within the most intense Morakot (2009) in Southern Taiwan. Geophys Res. inward radial flow. The interaction between the typhoon Let., 37, L08805. doi:10.1029/2010GL042761. circulation and the prevailing SW flow accompanying Hor, T. H., C. H. Wei, M. H. Chang, and C. S. Cheng, 2005: confluence with maximum convergence of 1.5 × 10–3 Doppler radar analysis of Typhoon Otto (1998) - Char- acteristics of eyewall and rainbands with and without s–1 attributed to the development and maintenance the influence of Taiwan orography. J. Meteor. Soc. of the distant rainbands as well as the intensification Japan, 83, 1001–1023. of the inward radial flow embedded within typhoon Hsu, H. H., C. C. Hong, C. H. Weng, M. Y. Lee, T. T. Lo, circulation. Expectedly, the results can illustrate the J. L. Kuo, K. C. Ko, and C. Chou, 2010: Multiscale asymmetric distribution of the typhoon circulation, the background flow of Typhoon Morakot.Atmos. Sci., 38, evolution and features of distant rainbands, and apply 628 Journal of the Meteorological Society of Japan Vol. 90, No. 5

1–19. (in Chinese with English abstract) Srivastava, R.C., T.J. Matejka, and T.J. Lorello, 1986: Jian, G-J., and C-C. Wu, 2008: A numerical study of the track Doppler radar study of the trailing anvil region asso- deflection of super-Typhoon Haitang (2005) prior to ciated with a squall line. J. Atmos. Sci., 43, 356–377. its landfall in Taiwan. Mon. Wea. Rev., 136, 598–615. Wei, C. H., T. H. Hor, and J. L. Wang, 2011: Elucidating char- Lee, C. S., Y. C. Liu, and F. C. Chien, 2008: The secondary acteristics of eyewalls and rainbands associated with a low and heavy rainfall associated with Typhoon severe typhoon influenced by Taiwan’s high orography Mindulle (2004). Mon. Wea. Rev., 136, 1260–1283. using Doppler radar analysis. Meteor. Atmos. Phys., doi:10.1175/2007MWR2069.1. 112, 1–14. doi 10.1007/s00703-011-0133-0 Lee, W. C., M. M. Bell, and K.E. Goodman, 2008: Super- Willoughby, H.E., F. D. Marks, and R. J. Feinberg, 1984: cells and mesocyclones in outer rainbands of Hurri- Stationary and moving convective bands in hurricanes. cane Katrina (2005). Geophys. Res. Lett., 35. L16803, J. Atmos. Sci., 41, 3189–3211. doi:10.1029/2008GL034724. Wu, C.-C., and Y.-H. Kuo, 1999: Typhoons affecting Lhermitte, R. M., and D. Atlas, 1962: Precipitation motion Taiwan—Current understanding and future challenges. by pulse Doppler. Proc 9th Weather Radar Conference, Bull. Amer. Meteor. Soc., 80, 67–80. Kansas City, AMS 218–223. Wu, C.-C., 2001: Numerical simulation of Typhoon Gladys Liang J., L. Wu, X. Ge, and C-C. Wu, 2011: Monsoonal influ- (1994) and its interaction with Taiwan terrain using the ence on Typhoon Morakot (2009). Part II: Numerical GFDL hurricane model. Mon. Wea. Rev., 129, 1533– study. J. Atmos. Sci., 68, 2222–2235. 1549. Lin, C.Y., H. M. Hsu, Y. F. Sheng, C. H. Kuo, and Y. A. Liou, Wu, C.-C., T.-H. Yen, Y.-H. Kuo, and W. Wang, 2002: Rain- 2011: Mesoscale processes for super heavy rainfall fall simulation associated with Typhoon Herb (1996) of Typhoon Morakot (2009) over Southern Taiwan. near Taiwan. Part I: The topographic effect. Wea. Fore- Atmos. Chem. Phys., 11, 345–361. doi:10.5194/ casting, 17, 1001–1015. acp-11-345-2011, Wu, C.-C., K-W. Cheung, and Y-Y. Lo, 2009: Numerical Lin, P. H., C. Y. Tu, G. R. Liu, W. J. Chen, I. I. Lin, T. H. Lin, study of the rainfall event due to the interaction of L. H. Chi, C. B. Chou, S. W. Chen, Y. Yang, C. C. Liu, Typhoon Babs (1998) and the northeasterly monsoon. B. J. Tsuang, J. L. Tsai, and M. H. Li, 2010: The over- Mon. Wea. Rev., 137, 2049–2064. view of satellite, upper-air sounding and hydrological Wu, L., J. Liang, and C-C. Wu, 2011: Monsoonal influence observation pre-and-post landfall of Typhoon Morakot on Typhoon Morakot (2009). Part I: Observational at Taiwan. Atmos. Sci., 38, 63–84. (in Chinese with analysis. J. Atmos. Sci., 68, 2208–2221. English abstract) Yu, C.C., 1982: A study on the relationship between heavy Lin, Y.-L., D. B. Ensley, and S. Chiao, 2002: Orographic rainfall over southwestern Taiwan and typhoons influences on rainfall and track deflection associated passing over seas north of Taiwan. Atmos. Sci., 9, with the passage of a tropical cyclone. Mon. Wea. Rev., 95–112. (in Chinese with English abstract) 130, 2929–2950. Yu, C.C., and L.W. Cheng, 2008: Radar observations of Lin, Y.-L., S.-Y. Chen, C. M. Hill, and C.-Y. Huang, 2005: intense orographic precipitation associated with Control parameters for the influence of a mesoscale Typhoon Xangsane (2000). Mon. Wea. Rev., 136, mountain range on cyclone track continuity and deflec- 497–521. doi:10.1175/2007MWR2029.1 tion. J. Atmos. Sci., 62, 1849–1866.