The Secondary Low and Heavy Rainfall Associated with Typhoon Mindulle (2004)

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CHENG-SHANG LEE AND YI-CHIN LIU Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

FANG-CHING CHIEN Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan

(Manuscript received 27 October 2006, in final form 30 July 2007)

ABSTRACT

This paper presents an observational and numerical study of Typhoon Mindulle (2004) as it affected Taiwan. Mindulle made landfall on the east coast of Taiwan at 1500 UTC 1 July 2004, and after 13 h, it exited Taiwan from the north coast. Severe rainfall (with a maximum amount of 787 mm) occurred over central-southwestern Taiwan on 2 July 2004. During the landfall of Mindulle’s main circulation, a secondary low formed over the Taiwan Strait. However, the secondary low, after it developed significantly (vorticity exceeded 5 ϫ 10Ϫ4 sϪ1 over a 30-km radius), did not replace the original center as was observed in many other storms. Instead, it moved inland and dissipated after the original center redeveloped near the north coast of Taiwan. In this study, the evolution of the secondary low, the redevelopment of the primary center, and the processes leading to the severe rainfall were examined. Results showed that the processes leading to the formation and the development of the secondary low were similar to those described in previous studies. These processes include the leeside subsidence warming, the horizontal transport of vorticity around the northern tip of the Central Mountain Range (CMR), and the overmountain upper-level vorticity remnant. However, because of the northward track, Mindulle preserved some strong vorticity on the eastern slope of the CMR. This strong vorticity remnant was steered northward over the ocean offshore from the north coast where the redevelopment of the primary center occurred. This “quasi-continuous track” of Mindulle has not been documented in previous studies. The vortex interaction between the redeveloped primary center and the secondary low resulted in the northeastward movement of the secondary low, which then dissipated after making landfall. Analyses also showed that even though heavy rainfall would occur over the mountain area when only the southwesterly flow prevailed, as on 3 July 2004, Typhoon Mindulle and the secondary low provided extra convergence that resulted in the west–east-oriented convective bands. These convective bands and the orographic lifting of the circulation associated with the secondary low resulted in the heavy rainfall over the central-western plains area.

1. Introduction orographic effects on propagating tropical cyclones. As a tropical cyclone (TC) passes over a mountain Wang (1980), studying 53 typhoons from 1946 to 1975, range, its track, structure, and precipitation are often documented that when a typhoon passes over Taiwan, modified by the topography (Wu and Kuo 1999). The its track might be continuous or discontinuous. When Central Mountain Range (CMR) of Taiwan, with an the center of a typhoon with a discontinuous track is average elevation of more than 3000 m (the highest about to make landfall, a secondary center (or second- peak is almost 4000 m) and dimensions of 300 km ϫ 100 ary low) generally forms on the lee side of the CMR km, provides a unique environment for studying the and then replaces the original center. The formation of the secondary center often influences the distribution and intensity of local rainfall and makes the rainfall forecast even more difficult. Corresponding author address: Dr. Cheng-Shang Lee, Depart- ment of Atmospheric Sciences, National Taiwan University, No. A number of studies with ideal (Chang 1982; Bender 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan. et al. 1985, 1987; Yeh and Elsberry 1993a,b; Huang and E-mail: [email protected] Lin 1997; Lin et al. 1999, 2002) or real-case modeling

DOI: 10.1175/2007MWR2069.1

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(Wu 2001; Wu et al. 2002; Jian et al. 2006; Lin 1993; Lin remnants of the initial center crossed over the CMR et al. 2006) examined the orographic influence on the and were entrained into the secondary center, resulting track of a TC passing over Taiwan. Chang (1982) used in a fully developed secondary low, which then replaced a primitive equation model with a 60-km grid resolution the original center. and an idealized symmetric terrain to simulate the pro- Although previous studies have illustrated various cess of a typhoon passing over the terrain. The results important physical processes responsible for the devel- indicated that a secondary vortex can easily be induced opment of the leeside secondary low, which later re- on the lee side when a weak vortex approaches the placed the original center, Mindulle appears to be quite terrain. The secondary vortex develops as a result of unique in several aspects. First, Mindulle, the associ- the horizontal advection (HA) of positive vorticity in ated secondary low, and the following southwesterly conjunction with leeside vortex stretching. In addition, monsoonal flow brought continuous heavy rainfall and Yeh and Elsberry (1993a,b) showed that the evolution caused serious damage over central-southwestern Tai- of a secondary low, which results in cyclone reorgani- wan. (The observed rainfall on 1–3 July 2004 exceeded zation downstream of the mountain, might occur as a 1590 mm at a rain gauge station, marked by a star sym- result of a downward extension from the upper-level bol in Fig. 1b.) Second, the track of Mindulle (north- remnant of the typhoon or an upward growth of the ward) was quite different from those of previous studies low-level secondary low. The former had more chances (westward track) on the formation of the secondary than the latter to maintain the development of the sec- lows. Third, the secondary low associated with Mind- ondary low. Alternatively, Lin et al. (1999) adopted a ulle, while it developed significantly, did not replace the primitive equation model to investigate the orographic original center as observed in other cases. The purpose of this paper is to examine the forma- influence on a drifting cyclone over an idealized topog- tion and the development mechanisms of the secondary raphy similar to the CMR. Their explanation for the low as well as to examine the influences of the second- abrupt increase of surface vorticity and the contraction ary low and the typhoon circulation on the heavy rain- of the cyclone scale on the lee side is because of the fall. Moreover, we compare the track of Mindulle with generation of new potential vorticity due to wave that of previous studies on landfalling typhoons and breaking associated with the severe downslope wind discuss why the secondary low did not replace the origi- and hydraulic jump. nal center. In section 2, we analyze the circulation pat- Wu (2001), simulating the evolution of Typhoon tern and rainfall distribution while Mindulle was affect- Gladys (1994), indicated that the quasi-stationary sec- ing Taiwan. The setup of the MM5 model and verifica- ondary low to the west of the CMR was mainly induced tion of the model simulation are presented in section 3. by the environmental easterly flow over the CMR. The The evolution of the primary and the secondary centers downslope adiabatic warming associated with the cir- are presented in section 4. The budget analyses during culation of Gladys acted to further enhance the low. the formation and development of the leeside second- Jian et al. (2006) used the fifth-generation Pennsylvania ary low are discussed in section 5. The role that the State University–National Center for Atmospheric Re- secondary low and the typhoon circulation played on search Mesoscale Model (MM5) to investigate the the occurrence of heavy rainfall is discussed in section physical process responsible for the discontinuous track 6. Section 7 is the discussion and conclusions. of Typhoon Dot (1990) over Taiwan. They documented that the adiabatic effect and the vortex stretching on the lee side contributed to the formation of the low- 2. Case description level vortex while the midlevel cyclone was still located Typhoon Mindulle, the 10th tropical cyclone issued on the upstream of the CMR. This low-level vortex by the Joint Typhoon Warning Center (JTWC) in 2004, then coupled vertically with the upper-level vortex that first formed near Guam at 0600 UTC 23 June 2004 and passed over the CMR and redeveloped into a mature then moved west-northwestward. Mindulle turned typhoon. Lin et al. (2006) indicated that it was difficult northward on 30 June 2004 (see Fig. 1) and landed on for a weak typhoon like Toraji (2001) to cross over the the east coast of Taiwan (about 20 km south of Hual- CMR. Hence, more air was forced to go around the ien) at approximately 1500 UTC 1 July 2004. At 0600 northern tip of the CMR, resulting in the formation of UTC 2 July 2004, the typhoon center was located over a leeside secondary low. Potential vorticity bands on the ocean to the northwest of Taipei, according to the the northern side of the CMR acted to organize the official fixes of the Central Weather Bureau (CWB) of secondary low, and then the low-level circulation ex- Taiwan. After that, Mindulle moved north-northeast- tended upward to the upper levels. Later, upper-level ward toward the East China Sea.

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FIG. 1. (a) The track of Typhoon Mindulle (2004) issued by the CWB, from 1200 UTC 28 Jun (labeled as 2812) to 0000 UTC 3 Jul (0300) with 6-hourly centers shown in black dots. The simulated track (0–48 h) at 6-h intervals is denoted by black squares (the dashed line shows the location where the surface center is not well defined). Plus-sign line represents the track of the secondary low identified by the visible satellite images from 0000 to 0400 UTC 2 Jul at 2-h intervals. Triangles are the same but for the simulated one from 24 to 32 h. Locations of some cities are also shown in circles. (b) As in (a), but for a close look. The typhoon track is from 1200 UTC 1 Jul (0112) to 0900 UTC 2 Jul (0209) at 3-h intervals, and the simulated track is from 12 to 33 h at 3-h intervals. The observed track of the secondary low is from 2300 UTC 1 Jul (0123) to 0500 UTC 2 Jul (0205) at 1-h intervals, and the simulated one is from 23 to 32 h at 1-h intervals. The dashed line is the track of the model typhoon as determined by the low-level vorticity maximum. The asterisk symbol in (b) indicates the location of a rain gauge mentioned in the text.

The 500-hPa weather maps issued by the CWB (Fig. (marked C) was located immediately offshore from the 2a) show that Mindulle was located over the ocean near west coast (Fig. 3b).1 The center of the secondary low Taiwan at 1200 UTC 1 July 2004. Another typhoon (which was caused initially by the subsidence warming named Tingting was located at about 2000 km to the as will also be discussed later) also could be identified east-northeast of Mindulle. By 1200 UTC 2 July 2004 as a clear region on the visible satellite images at 0000– (Fig. 2b), the Pacific high had intensified and extended 0600 UTC (0800–1400 local time) 2 July 2004. As westward to the South China Sea (SCS). This resulted shown in Fig. 4a, the secondary low was located over in an increase of the pressure gradient between the the Taiwan Strait on the lee side of the CMR at 0023 Pacific high and Mindulle, and consequently the en- UTC 2 July 2004, while the primary typhoon center was hancement of southwesterly flow there. Such a synoptic difficult to identify. After that, the center of the sec- pattern lasted for 24 h until 1200 UTC 3 July 2004, ondary low moved northeastward and made landfall at when the southwesterly flow started to weaken. 0400 UTC 2 July 2004 (Fig. 4b), with its clear center The mesoscale analyses of surface observations show diminishing. Two hours later, the clear center of the that when Mindulle (marked T) was making landfall on secondary low had vanished from the visible satellite the east coast of Taiwan at 1500 UTC 1 July 2004, images (not shown). pressure was lower on the lee side of the CMR (Fig. Although Fig. 4 does not provide strong evidence to 3a). Note that several other datasets were also used in support the existence of the secondary low, the ani- these surface analyses, including a few tens of auto- mated loop of 1-h visible satellite images (not shown) mated stations in Taiwan and the remote sensing data (to be discussed later in this paper). Nine hours later, the primary center of Mindulle had moved to the north 1 The secondary low is a low pressure system with cyclonic coastal region (according to the CWB official fixes), circulation, which forms on the lee side after the primary typhoon and a low pressure system called the secondary low circulation was affected by the terrain.

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the satellite images. In addition, the Doppler radial wind directions (marked by arrows in Figs. 5c,d) also supported the existence of the secondary low. Figure 6a shows the 24-h accumulated rainfall observed from 367 rain gauge stations in Taiwan from 0000 UTC 1 July to 0000 UTC 2 July 2004. During this period, rainfall mainly occurred over eastern Taiwan, with a maximum amount exceeding 380 mm (ϳ15 in.). One day later (Fig. 6b), the area of heavy rainfall shifted to the west of the CMR with a maximum increased amount exceeding 780 mm (Ͼ30 in.). On the third day (Fig. 6c), the heavy rainfall occurred mostly over the mountain area with two rainfall maxima, 350 and 667 mm, and the heavy rainfall over the western plains almost disappeared. There is no mystery about the first day’s and the third day’s rainfall because the typhoon’s circulation (rainbands) and the southwesterly flow impinging upon the CMR produced the observed heavy rainfall. The heavy rainfall on the second day with three rainfall maxima (657, 787, and 715 mm), when Mindulle had weakened over land and then had moved northward away from Taiwan, was somewhat surprising to many local (CWB) forecasters and caused severe damage in many places (called “the 7–2 flood” locally). It is thus logical to state that the 7–2 flood was caused primarily by the strengthening of the southwesterly flow (see Fig. 2) because Mindulle had weakened on 2 July 2004. However, the location or distribution of the heavy rainfall was also related to the effects of the typhoon’s circulation and the secondary low, because the orographic lifting of the southwesterly flow would produce rainfall distribution more like that shown in Fig. 6c. Note that the maximum rainfall amounts on 3 July were smaller than those on 2 July 2004, especially the northernmost one (350 versus 657 mm). The composite radar reflectivity taken by the CWB FIG. 2. The 500-hPa geopotential height (contour interval 60 Ϫ radar network showed a convective line (marked by a gpm) and winds (full barb5ms 1) issued by the CWB at (a) 1200 UTC 1 Jul and (b) 1200 UTC 2 Jul. 1) offshore near Chiayi at 0200 UTC 2 July 2004 (Fig. 7a). Over the region where the secondary low was located, radar reflectivity was very weak (marked C). indicated that clouds rotated cyclonically around the This indicated that the secondary low was rather dry at clear region. The hourly positions of the secondary low, this time. Four hours later (Fig. 7b), another convective determined as the clear region center, have also been line formed over the southern Taiwan Strait (marked shown in Fig. 1b. The existence of the secondary low by a 2) and extended eastward toward southern Tai- was further verified by the 6-min data taken by the wan. By 1200 UTC 2 July 2004 (Fig. 7c), the two con- Doppler radar located at the National Central Univer- vective lines had further intensified and organized into sity (Fig. 5 shows two time periods). The animated loop two east–west-oriented bands near Chiayi and Kaoh- of the reflectivity images showed that banded echoes siung, respectively. Five hours later, at 1700 UTC 2 July (one is marked A in Figs. 5a,b) rotated cyclonically 2004, the convective lines had dissipated (Fig. 7d). around the clear region from 24.5° to 25°N and from These rainbands contributed significantly to the two 120° to 120.75°E, which moved northeastward at a southern rainfall maxima (daily rainfall amount Ͼ700 Ϫ speed of 4.5 m s 1, consistent with that determined by mm) shown in Fig. 6b.

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FIG. 3. Mesoscale surface analyses of Mindulle at (a) 1500 UTC 1 Jul and (b) 0000 UTC 2 Jul. Isobars are from 986 to 998 hPa. The T marks the center of Mindulle, L marks the lower pressure area, and C marks the position of the secondary low based on remote sensing data.

3. Model design and verification unevenly spaced sigma levels, with the finest resolution in the boundary layer. The physics processes applied in a. Model settings the simulation included the Grell cumulus parameter- The MM5 configuration of this study included three ization (Grell et al. 1994), the Medium-Range Forecast nested domains of 45-, 15-, and 5-km horizontal reso- model (MRF) boundary layer parameterization scheme lutions (Fig. 8). A two-way interaction option was used (Hong and Pan 1996), the simple cooling radiation between domains during the simulation. The grids ex- scheme, and the Reisner mixed-phase microphysics tended vertically to 100 hPa and were resolved by 31 scheme (Reisner et al. 1998).

FIG. 4. Visible satellite images around Taiwan at (a) 0023 UTC 2 Jul and (b) 0411 UTC 2 Jul. The dashed circle marks the clear region (indicating the center of the secondary low) discussed in the text.

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FIG. 5. (a),(b) The composite radar reflectivity (dBZ) taken by the Doppler radar located at the National Central University at 0241 and 0316 UTC 2 Jul, respectively. (c),(d) As in (a) and (b), but for the Doppler radial winds (m sϪ1). The letter A marks the location of one rainband, which rotated around the secondary low, and the arrows show the direction of Doppler radial winds.

The initial, lateral boundaries and sea surface tem- replacing procedures proposed by Jian et al. (2006) to perature data were obtained from the European Centre produce a vortex that was more representative of the for Medium-Range Weather Forecasts (ECMWF)/ observed one at the initial time. Observations were also Tropical Ocean and Global Atmosphere (TOGA) used continuously throughout the model simulation in global analyses (1.125° ϫ 1.125° resolution). Surface domain 1, using a four-dimensional data assimilation and sounding observations were utilized to improve the (FDDA) nudging technique (Stauffer and Seaman initial and boundary conditions through an objective 1990; Stauffer et al. 1991). In addition, the wind field analysis procedure, the Little-R. Since the global model data from QuikSCAT were used in the FDDA proce- analyses were too coarse to properly resolve a tropical dure to improve the position of the secondary low. The cyclone, we used the tropical cyclone bogussing and purpose of this FDDA procedure was to obtain a better

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FIG. 6. The 24-h accumulated rainfall (mm) in Tai- wan from (a) 0000 UTC 1 Jul to 0000 UTC 2 Jul, (b) 0000 UTC 2 Jul to 0000 UTC 3 Jul, and (c) 0000 UTC 3 Jul to 0000 UTC 4 Jul. The contour interval is 40 mm. The maximum rainfall amounts are also shown.

simulation of domain 1, which could in turn provide in this experiment were the same as those in the control better lateral boundary conditions for domains 2 and 3. experiment, except that the typhoon circulation was re- Inside these two nested domains from which the simu- placed by a weak environmental flow at the initial time. lation data were used to perform the analyses of this The vortex-removed process was done in the REGRID study, the FDDA option was not applied. The model module of MM5, in which the original typhoon circu- was integrated for 48 h from 0000 UTC 1 July to 0000 lation was replaced by a very weak initial bogus vortex. UTC 3 July 2004. The center location of the bogus vortex is based on the The above simulation was the control experiment official fixes of CWB. The maximum wind speed and and was used for the diagnostic analyses of this study. the radius of maximum wind of the bogus vortex are 1 In addition, a sensitivity experiment (the no-typhoon msϪ1 and 100 km, respectively, which can be treated as run, hereafter NT) in which the typhoon was removed the environmental flow after6hofintegration. After at the initial time was performed. All the model settings 12 h of integration, the environment would become bal-

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FIG. 7. Composite radar reflectivity (dBZ) taken by the CWB radar network at (a) 2000 UTC 1 Jul, (b) 0200 UTC 2 Jul, (c) 1200 UTC 2 Jul, and (d) 1700 UTC 2 Jul. The letter C marks the location of the secondary low. Numbers 1 and 2 denote the convective lines. anced, so there would be no typhoon or secondary low. better simulation for the track after landfall. Because This NT was designed to examine the role of the sec- the intensity estimates might have a large uncertainty ondary low and the typhoon circulation on the mesoscale when the center of the typhoon is over the mountain convection systems (MCSs) and on the heavy rainfall. area, comparison of intensity was done only when the center was over the ocean. At 12 h (before the landfall), b. Verification of the simulation the model typhoon was weaker than that observed (990 The simulated track of Mindulle is shown in Fig. 1, versus 976 hPa). However, after 36 h, the intensity of along with the track issued by the CWB. Comparisons the model system was close to that observed, that is, showed that the typhoon track after 12 h of simulation 994, 994, and 995 hPa at 36, 42, and 48 h (model) versus was reproduced reasonably well, including the location 994, 994, and 997 hPa (observed), respectively. The and time at landfall. The simulated track deviated from simulated 24-h accumulated rainfall from 24 to 48 h the observed one by only 25 km for most of the times (0000 UTC 2 July to 0000 UTC 3 July 2004) showed after 12 h of integration. However, it has to be noted that heavy rainfall occurred mainly over central and that the initial position of Mindulle in the model was southern Taiwan west of the CMR, with three peak placed to the southeast of the observed one to get a values of 500, 739, and 701 mm, from north to south

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FIG. 8. Domain configuration of the MM5 simulation: D1, D2, and D3 denote the domains with grid spacing of 45, 15, and 5 km, respectively.

(Fig. 9). This simulated rainfall distribution, including even the peak values, was fairly consistent with that observed (peak values at 658, 787, and 716 mm) at the rain gauge stations. One important reason why the model reproduced the FIG. 9. The simulated 24-h accumulated rainfall (mm) from 24 rainfall well is that the model simulated the secondary to 48 h (from 0000 UTC 2 Jul to 0000 UTC 3 Jul) in domain 3. low reasonably well as shown in Fig. 1, except that the Contour lines start at 20 mm with an interval of 100 mm. Shading starts at 10 mm with an interval of 40 mm. The maximum rainfall simulated secondary low lasted 3 h longer than the ob- amounts are also shown. served one. As shown in Fig. 10a, after 28 h a secondary low was located over the central Taiwan Strait near the (Fig. 10b). The convection over the southern Taiwan west coast of Taiwan (such a pattern is similar to that Strait developed into two east–west-oriented convec- shown in Fig. 3b, except for the timing). By this time, tive bands (marked by a 1 and a 2) that extended east- the main circulation of Mindulle had been greatly de- ward toward the terrain and caused the two southern stroyed by the terrain. However, judging from the wind rainfall peaks as shown in Fig. 9. In comparison with the field shown in Fig. 10a, the primary typhoon center was radar observations shown in Fig. 7, the model also re- likely located over northern Taiwan. produced the evolution of the MCSs and the convective Figure 10a also shows the simulated reflectivity, bands reasonably well, with the exception of a slight which indicates that at 28 h, mesoscale convective sys- difference in the timing and location. In Fig. 10a, the tems were developing to the south of the secondary low simulation time is 2 h later than the observation, and over the southern Taiwan Strait and the northern SCS. the simulated rainband (marked by a 1) is 20 km to the The convection offshore from Kaohsiung organized south of the observed one. In Fig. 10b, the locations of into a narrow convective line over the ocean at this time the simulated rainbands (marked bya1anda2)are (marked by a 1). On the western slope of the CMR, very close to the observed ones (Fig. 7c), but the simu- there was enhanced convection as a result of terrain lation time is 2 h earlier than the observation. lifting. Three hours later (figure not shown), the secondary low moved northeastward to make landfall on the west coast of Taiwan, while the main circulation of 4. The evolution of the primary and the secondary Mindulle reappeared and was leaving the north coast. centers At 34 h, Mindulle had moved to the ocean north of To examine the evolution of the primary center and Taiwan and the secondary low was dissipating over land the secondary low, we analyzed geopotential height and

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700 hPa and perhaps 500 hPa as well at this time. Eight hours later at 28 h (Fig. 11c), the original typhoon center at 500 hPa had passed over the CMR and merged with the secondary low that developed upward from the lower levels. This resulted in the secondary low reach- ing a stage of maturity that exhibited a vertically stacked structure from the low to midlevel. As shown in Figs. 11b,c, at 500 hPa the primary center passed over the CMR and merged with the secondary low. To highlight this process, the 500-hPa potential vorticity as well as the wind and geopotential fields at 2-h intervals are shown in Fig. 12. At 17–19 h (Figs. 12a,b), when the primary center (marked T) just made landfall, it still had a well-organized cyclonic circulation with close contours. But 2–4 h later (Figs. 12c,d), the potential vorticity of the primary center had weakened significantly and the cyclonic circulation was not well defined. During the same time period, the secondary low (marked C) developed upward to 500 hPa (see also Fig. 11b). Note that the advection of potential vorticity associated with the primary center at 500 hPa might have helped the development of the secondary low and the formation of the 500-hPa cyclonic circulation center. In the next4h(21–25 h), the primary center moved southwestward and merged with the secondary center (Figs. 12e,f). Apparently there were interactions between the primary center and the upward-developing secondary low (similar to that discussed by Jian et al. 2006). At 32 h, the secondary low was making landfall and dissipating over the coastal region (Fig. 11d). On the other hand, the circulation of Mindulle reappeared offshore from the north coast at 925 hPa and was developing upward. After the secondary low made landfall, FIG. 10. Model-simulated (domain 3) 925-hPa geopotential its circulation at 925 hPa was destroyed by the terrain Ϫ1 height (contours interval 10 gpm) and winds (full barb 5 m s ; and dissipated (Fig. 11e). At 500 and 700 hPa, the cir- winds below ground are omitted), and maximum reflectivity culation of the secondary low had also weakened. Dur- (shaded; dBZ) at (a) 28 and (b) 34 h. Arrows 1 and 2 denote the convective lines. ing this period (32–42 h), although the primary center was limited at low levels with no support from the upper level, it still could develop gradually upward be- winds at 925, 700, and 500 hPa as shown in Fig. 11. At cause the ocean provided more moisture for the rede- 12 h, when Mindulle was located offshore from the east veloping process to occur. At 42 h, the low pressure coast, the center of the main circulation was vertically centers at the three levels coupled vertically, but with a stacked from 925 hPa up to above 500 hPa. On the lee tilt toward the southeast (Fig. 11f). side of the CMR, a low-level secondary low with only To help understand the effect of leeside subsidence weak cyclonic circulation at this time was developing warming on the formation of a secondary low, potential and extending upward. By 20 h (Fig. 11b), Mindulle had temperature and winds at 2 km above ground level made landfall and lost its pressure and wind structures (AGL) were examined. (Note that the average eleva- below 700 hPa because of the terrain effect. At 500 hPa, tion of the CMR in the model is about 2000 m.) At 6 h, even though the cyclonic circulation associated with the when Mindulle was located offshore from the southeast low pressure center of Mindulle weakened significantly, coast of Taiwan, there was strong easterly flow over the it was moving across the CMR. The secondary low, with CMR in northern and central Taiwan at 2 km AGL an eastward tilt, intensified and developed upward to (Fig. 13a). This resulted in a sinking motion and adia-

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Ϫ FIG. 11. The 925-, 700-, and 500-hPa geopotential height (contours interval 7 gpm) and winds (full barb 5 m s 1) at (a) 12, (b) 20, (c) 28, (d) 32, (e) 36, and (f) 42 h. The dotted–dashed and dotted lines indicate the vertical extent of the primary and secondary centers, respectively. The black dot (open circle) is the center of circulation with (without) a closed isobar. batic warming on the lee side of the CMR, which moved southward to the southern Taiwan Strait as a formed a region of relatively high potential tempera- result of the horizontal advection by the typhoon’s ture over the Taiwan Strait. With the typhoon center outer circulation (Fig. 13b). Over the area offshore moving closer to the coast at 12 h, the warm air region from the northwest coast, there was another area of

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FIG. 12. The simulated 500-hPa geopotential height (contour interval 4 gpm), potential vorticity (shaded; starting at 3 with an interval of 1.5 PVU), and wind field (full barb 5 m sϪ1) at (a) 17, (b) 19, (c) 21, (d) 23, (e) 25, and (f) 27 h. The letter T indicates the primary center, and C denotes the secondary low. warm air due to the continuous subsiding of the easterly with lower surface pressure. This favored the spinup of flow. This is where the secondary low started to de- cyclonic circulation and the formation of the secondary velop. After the secondary low fully developed at 28 h, low at a low level. After making landfall (Fig. 14c), the low-level flow over the northern CMR became Mindulle lost its low-level circulation pattern, but the southerly. Again, the overmountain flow produced remnant of the center vorticity was stretched along the warming in and offshore from northern Taiwan (not eastern slope of the CMR. The elongated vorticity shown), leading to an area with lower surface pressure, patch was further maintained or even enhanced by the which might be an important precursor to the redevel- strong wind shear between strong southerly winds off- opment of the primary center after 30 h. shore and weak winds inland. The shear vorticity To illustrate the spinup of the secondary low, the sea around the northern tip of the CMR was advected level pressure, 10-m winds, and 925-hPa vorticity are again by the cyclonic outer circulation to enter the re- shown in Fig. 14. From 8 to 12 h, when the typhoon gion of the secondary low (similar to that discussed by center was located near the east coast of Taiwan, a Lin et al. 2006). This further intensified the secondary broad area of weak low pressure appeared over the low, with an asymmetric center and cyclonic vortex Taiwan Strait as shown in Figs. 14a,b. Near the north- forming during 16–20 h. From 20–26 h, the secondary ern boundary of box A (shown in Fig. 14), an elongated low stayed still offshore from central Taiwan and main- band of positive vorticity formed as a result of low-level tained its circulation. After 26 h, the secondary low wind shear created by the typhoon’s cyclonic airflow started to move landward, leaving box A, and became that circled around the northern tip of the CMR. The better coupled with the upper-level vortex at 28 h (Fig. ribbon-shape (shear) vorticity was advected southwest- 11c). Note that the area-averaged 925-hPa vorticity was ward by the cyclonic environmental flow to the region ϳ5 ϫ 10Ϫ4 sϪ1 over an area with a 30-km radius. After

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Ϫ FIG. 13. Distribution of horizontal wind (full barb 5 m s 1) and potential temperature (contours and shading at an interval of 1 K, shading starts at 309 K) at 2 km AGL at (a) 6 and (b) 12 h.

30 h, the primary typhoon center reappeared over the surface pressure, which might be an important precur- ocean near the northern coast (Figs. 14e,f). sor to the redevelopment of the primary center. On the As shown in Fig. 11, the typhoon center at the upper other hand, after making landfall (18 h), Mindulle lost level decoupled from that at the low level. To examine its low-level signature because of the terrain effect, as the decoupling process, we used data from domain 2 to shown in Fig. 15a. Note that contours of terrain used in calculate the steering flows (averaging a 300- to 700-km the model are shown in Fig. 15 to reveal the steep ter- radius) of the low (850–700 hPa), mid- (700–500 hPa), rain slope along the east coast of Taiwan. In the fol- and high (500–300 hPa) levels at 1600 UTC 1 July 2004. lowing hours, the low-level vorticity remnant on the The direction and speed of the computed steering flow eastern slope of the CMR was stretched along the east were 17° (clockwise from due north) and 4.5 m sϪ1,10° coast and was then steered northward on the eastern and 5.3 m sϪ1, and 351° and 4.4 m sϪ1 for the low, mid-, slope of the CMR because of the strong low-level and high levels, respectively. The high-level steering southerly flow (Figs. 15b,c and 16a). At 31 h, when the flow was in good agreement with the movement of the strong vorticity patch was steered offshore, a cyclonic upper-level center (Fig. 12c,e). The midlevel steering circulation started to develop near the north coast (Fig. flow lay between those of the high and the low levels. It 16b). The primary typhoon center reappeared over the is also interesting to note that although the computed ocean at 33 h, when the cyclonic circulation coupled steering flow at the low level was less realistic because with the strong vorticity patch (Fig. 16c). The decrease of the blocking effect of the CMR, it was very close to in surface friction and the increase of the moisture sup- the orientation of the CMR (ϳ20°). Such results indi- ply probably also helped the reorganization of the pri- cated that the environmental steering flow and the mary typhoon center. CMR blocking effect played an important role in the vertical decoupling of the upper-level and the lower- 5. Budget analyses during the formation of level typhoon centers. secondary low One of Mindulle’s unique features is that the secondary low, even though it developed fully, did not replace a. Thermal budget the original center; instead the primary center of Min- As shown in Figs. 13a,b, there was a weak low pres- dulle reappeared at the nearshore region from the sure system developing over the Taiwan Strait before north coast. Figures 15 and 16 show the redevelopment Mindulle made landfall. Previous studies (Yeh and Els- process of the primary typhoon center over the north berry 1993a,b; Lin et al. 1999; Wu 2001; Lin et al. 2006) coastal region. As discussed before, after the secondary have indicated that when a typhoon approaches Tai- low fully developed at 28 h, the flow over the northern wan, downslope easterly winds associated with the ty- CMR became southerly. The overmountain flow pro- phoon circulation can result in strong adiabatic warm- duced a low-level warming in and offshore from north- ing on the lee side of the CMR, which is an important ern Taiwan (not shown), leading to an area with lower process in the formation of a secondary low. Therefore,

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Ϫ FIG. 14. The simulated sea level pressure (contours interval 1 hPa), 10-m winds (full barb 5 m s 1), and 925-hPa vorticity (shading with an interval of 20 ϫ 10Ϫ5 sϪ1) at (a) 8, (b) 12, (c) 16, (d) 20, (e) 30, and (f) 34 h. Box A denotes the location of the average area. The thick line in (e) and (f) connects the centers of the typhoon and the secondary low.

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FIG. 15. The simulated potential vorticity (shading; starting at 0.5 with an interval of 1 PVU) and wind field (full barb 5 m sϪ1) for ␴ ϭ 0.9 at (a) 18, (b) 22, and (c) 26 h. Contours indicate the height of terrain (interval of 500 m; starting at 500 m). to diagnose the thermal process during the formation of tively. Here, the residual accounts for the diabatic ef- the secondary low, the thermodynamics equation is fect, subgrid-scale effects, and numerical errors. Each written in terms of potential temperature ␪: term in Eq. (1) was computed using data from domain Ѩ␪ Ѩ␪ 3 and then averaged horizontally in box A (shown in ١␪ Ϫ ϩ ͑ ͒ Fig. 14). Note that box A was made as large as possible ϭϪ Ѩ V · w Ѩ RT, 1 t z to contain the secondary low as long as possible, but where the term on the left-hand side is the local change without too much land contamination during the aver- (#1),2 the terms on the right-hand side are the horizon- age process. In addition, the condensational heating tal advection (#2), the vertical advection (or the adia- rate (#5) was calculated from the model output. batic effect; #3), and the residual term (#4), respec- Results are shown in Fig. 17 for the 8 and 18 h of simulation, corresponding to the early formation and the later development of the secondary low, respec- 2 The symbol # followed by a number hereinafter represents the tively. At 8 h (Fig. 17a), the temperature increased be- number of the corresponding curve shown in Fig. 17. low 4.2 km AGL except at ϳ2 km and below 1 km

FIG. 16. The simulated potential vorticity for ␴ ϭ 0.9 (shading; starting at 0.5 with an interval of 1 PVU), 925-hPa geopotential height (contours at an interval of 3 gpm), and 925-hPa wind field (full barb 5 m sϪ1) at (a) 29, (b) 31, and (c) 33 h.

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Ten hours later (18 h), when the low-level circulation of the secondary low was developing (see Figs. 14c,d), the local change term was slightly negative below ϳ3 km (Fig. 17b) and the adiabatic term was negative at approximately 0.3–4.2 km AGL. Apparently, the overmountain sinking easterly flow had weakened as Min- dulle made landfall, and then the vertical motion became upward as a result of low-level convergence associated with the development of the secondary low. This resulted in a deep layer of adiabatic cooling below ϳ600 hPa. The cooling, however, was largely compen- sated for by the positive residual term below 3 km AGL, with the main contribution from the condensational heating above 0.5 km AGL. The horizontal advection of warm air located offshore to the northwest coast (see Fig. 19a) also helped to compensate for the adiabatic cooling above 1.5 km. To illustrate the horizontal distributions of various budget terms, the vertical averages of all terms at 950– 600 hPa were analyzed. Results for vertical advection and local change terms inside box A are shown in Fig. 18. At 8 h, the vertical advection was positive over most areas where surface pressure was also lower (Fig. 18a), and it contributed significant heating to the low-level warming shown in Fig. 18b. At 18 h, the vertical advection around the low region became mostly negative (Fig. 18c), which contributed to the low-level cooling to the south of the low center (Fig. 18d). Note that at 18 h, the subsidence warming still occurred around the northeastern corner of box A, or offshore from the northwest coast of Taiwan. In summary, the weak low on the lee side of the CMR early in the simulation (e.g., at 8 h) was produced by the adiabatic warming of the FIG. 17. The vertical profiles of the thermal budget terms at (a) downslope easterly flow, as many other studies have 8 and (b) 18 h. The local change (#1), horizontal advection (#2), shown (Lin et al. 1999, 2002, 2006; Wu 2001; Wu et al. vertical advection (#3), residual term (#4), and the condensational 2002; Jian et al. 2006). However, the adiabatic warming heating (#5) are averaged inside box A of domain 3 (shown in Fig. 14). The x axis is the value (10Ϫ5 KsϪ1). did not really favor the further development of the secondary low because air was descending. During the development phase of the secondary low, the upward mo- AGL, with primary contribution from the adiabatic tion increased and the downslope easterly wind weak- warming (#3). However, it is noted that the subsidence ened. It is therefore clear that the horizontal advection warming did not reach below 0.5 km AGL. The hori- of shear vorticity primarily contributed to the later de- zontal cold advection (#2), which was a result of strong velopment of the secondary low, as aforementioned. northerly winds associated with the typhoon’s outer cir- To further examine the origin of air parcels that con- culation, tended to compensate for the subsidence tributed to the development of the secondary low, tra- warming and caused a slight cooling below 1 km AGL jectories ending at every grid point inside box A at 1.5 and at ϳ2 km. The residual term (#4) was mostly nega- km AGL were integrated backward from 18 to 8 h tive below 4.2 km. Judging from the trivial (near zero) using the simulation data from domain 3. There were condensational heating (#5) and the local time being basically two groups of trajectories as shown in Fig. 19, the afternoon hour, the cooling residual may have come which helps to illustrate the formation mechanisms of primarily from the subgrid-scale effects and numerical the secondary low. Trajectory 1, originating from the errors (as well as the possible reevaporation associated upstream easterly flow of the typhoon’s circulation at with the subsidence). about 1.3 km AGL, climbed over the peak of the CMR

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FIG. 18. The 950–600-hPa averaged vertical advection term (#3) of the thermal budget inside box A (shown in Fig. 14) at (a) 8 and (c) 18 h and local change term (#1) at (b) 8 and (d) 18 h. The contour interval is 8 ϫ 10Ϫ5 K sϪ1 for (a) and (c) and 4 ϫ 10Ϫ5 KsϪ1 for (b) and (d). Solid (dotted) curves are positive (negative) and the zero line is dashed. The low centers [determined by the closed isobars in Fig. 14a for (a) and (b), and Fig. 14c for (c) and (d)] are marked by X.

at 9–10 h and descended on the lee side from 11 to 16 b. Vorticity budget h. The sinking motion resulted in the aforementioned adiabatic warming that contributed to the early stage To further reveal the evolution of the secondary low, formation of the secondary low at the low levels. After a time–height cross section of relative vorticity aver- 17 h, when the secondary low intensified, the air parcel aged horizontally inside box A is presented in Fig. 20. started to rise. Trajectory 2 originated from the low Results show that before 10 h, the domain-averaged level in the typhoon’s circulation and circled around the relative vorticity was positive below 700 hPa, with a Ϫ Ϫ northern tip of the CMR. After 15 h, the air parcel maximum value of ϳ20 ϫ 10 5 s 1 at ϳ950 hPa. (Note approached the secondary low center and started to that the value before 3 h should be ignored because of rise. This trajectory represented the path of air parcels the spinup of the model.) This low-level vorticity was that transported vorticity to the low pressure region, associated with the subsidence-induced low (Fig. 13a). which assisted the development of the secondary low. The 950-hPa vorticity reached a minimum value at These results are similar to those of Jian at al. 2006 and around 10 h, when the initial low pressure system Lin at al. 2006. moved out of box A. After that, the vorticity increased

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FIG. 20. Time–height section of the domain-averaged vorticity (10Ϫ5 sϪ1) inside box A (shown in Fig. 14). The x axis is the simulation time (h).

all played an important role in the development of the secondary low. It is therefore worthwhile to examine the vorticity budget of the secondary low, using the following vorticity equation: Ѩ␨ Ѩ␨ v · f ϩ ␨͒ Ϫ ␻ Ϫ ͑ f ϩ ␨͒١ ١͑ · ϭϪv Ѩt Ѩp Ѩ␻ Ѩ␷ Ѩ␻ Ѩu Ϫ ͩ Ϫ ͪ ϩ RT, ͑2͒ Ѩx Ѩp Ѩy Ѩp where the term on the left-hand side is the local change of relative vorticity (LC), the terms on the right-hand FIG. 19. (a) The potential temperature (shading at an interval of side are, sequentially, the horizontal advection of abso- 1 K) and pressure at 1.5 km AGL (contours at an interval of 0.5 lute vorticity (HA), the vertical advection of relative hPa) at 18 h. Numbers 1 and 2 show two 10-h backward trajecto- vorticity (VA), the divergence term (DT), the tilting ries ending at 18 h and 1.5 km AGL. (b) The projected heights of the two trajectories; the thick line is the terrain. term (TT), and the residual term (RT), which accounts for the friction, subgrid-scale effects, and numerical errors. Each term of Eq. (2) was averaged vertically for not only at low levels but also at the mid- and high two levels: the low level (950–800 hPa) and the midlevel levels. The former was primarily formed by the low- (800–500 hPa) inside box A, and then a time average level horizontal advection (see Figs. 14b,c), and the lat- was taken for three periods: 12–20 h, 20–26 h, and 26–32 ter by the approaching of the overmountain typhoon’s h, corresponding to the developing, mature, and disap- upper-level center (see Fig. 11b). The vorticity at 950– pearing stages of the low-level vortex inside box A, 3 900 hPa also was maintained at a value greater than respectively. Note that the low-level vorticity reached 2.5 ϫ 10Ϫ4 sϪ1 at 20–26 h. After 26 h, the low-level its maximum values at 21–22 h and started to decrease vorticity started to decrease because the secondary low significantly after 26 h. started leaving box A. However the vorticity above 500 At the low level (Table 1a), LC was positive at 12–20 hPa was still increasing until 28 h because the typhoon’s h, decreased to almost zero at 20–26 h, and became upper-level center moved into box A and coupled with the low-level secondary low (Figs. 11d and 12f). 3 The low level is defined between 950 and 800 hPa to exclude As aforementioned, the horizontal vorticity advec- from the average most of the surface frictional effect but to keep tion and the vorticity of the upper-level typhoon center the low-level characteristics.

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TABLE 1. The averaged vorticity budget inside box A (shown in negative at the disappearing stage when the secondary Fig. 14) for (a) the low level (950–800 hPa) and (b) the midlevel low was moving out of box A, leading to the large (800–500 hPa) at 3 periods: 12–20, 20–26, and 26–32 h. The unit is Ϫ Ϫ negative LC at this stage. On the other hand, DT in- 10 9 s 2. creased significantly at the mature and the disappearing (a) The low level stages because of the increased low-level convergence Times LC HA VA DT TT RT and vorticity. The VA was slightly negative at the de- 12–20 4.3 3.5 Ϫ0.03 1.0 1.6 Ϫ1.8 veloping stage but became largely positive at the two 20–26 0.16 Ϫ5.4 9.1 9.9 Ϫ7.4 Ϫ6.1 later stages because the positive vorticity was trans- 26–32 Ϫ6.0 Ϫ17.0 5.6 10.5 Ϫ3.3 Ϫ1.9 ported upward when the boundary layer circulation in- (b) The midlevel tensified. To illustrate how HA and DT acted together to spin Times LC HA VA DT TT RT up the vortex at the developing stage, Fig. 21 shows Ϫ Ϫ 12–20 2.9 3.6 0.56 1.6 0.53 0.16 950-hPa LC, HA, and DT inside box A at 12–16 and 20–26 2.1 0.74 1.9 Ϫ0.83 0.83 Ϫ0.57 26–32 Ϫ1.1 Ϫ3.6 3.4 0.90 Ϫ1.4 Ϫ0.36 16–20 h. At the earlier developing phase (12–16 h), HA and DT had positive values to the northeast and the south-southwest of the low center (at 16 h), respec- negative at 26–32 h. Such results are consistent with the tively. The area with positive HA extended southwest- stage classification based on the evolution of the low- ward at 16–20 h. This HA effect is consistent with the level vortex. The positive LC at the developing stage southwestward transport of shear vorticity as shown in was formed mostly by HA and partially by DT. Here, Figs. 14b,c. Note that the other terms (VA, TT, and HA became negative at the mature stage when the vor- RT) are relatively noisier or small (except near the ticity reached the maximum value; it became even more eastern boundary), thus they are not shown in Fig. 21.

FIG. 21. The 950-hPa vorticity budgets inside box A (shown in Fig. 14) at (top) 12–16 h and (bottom) 16–20 h for (a),(d) LC, (b),(e) HA, and (c),(f) DT. Solid (dotted) curves are positive (negative) and the zero line is dashed. The low centers in (a)–(c) at 16 h and in (d)–(f) at 20 h (estimated by the closed isobars in Figs. 14c,d, respectively) are marked with X. The contour interval is 10 ϫ 10Ϫ9 sϪ2 for LC and 20 ϫ 10Ϫ9 sϪ2 for HA and DT.

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In addition, HA, DT, and TT tend to have larger values tween Mindulle and typhoons studied by others is the near the eastern boundary. These large values are due direction of movement. In our case, the fully developed to the terrain effect and tend to cancel out. secondary low was located near the west coast of cen- At the midlevel (Table 1b), LC was positive during tral Taiwan at 30 h, as shown in Fig. 14e. However, the the first two periods (12–20 and 20–26 h) but decreased original low-level center was located near the north to a small negative value in the third period because the coast of Taiwan and was redeveloping. Four hours maximum vorticity occurred at a later time. During the later, the secondary low moved northeastward over first period, the positive LC was mainly from the posi- land and dissipated (Fig. 14f), and the primary typhoon tive HA, which was also positive in the second period. circulation moved north-northwestward over the This was related to the approaching of the upper-level ocean. As shown in Figs. 14e,f, the line connecting the center over the CMR (Figs. 11a–c). The VA was centers of the two lows rotated cyclonically over time, slightly positive during the first period but increased its indicating that there was vortex interaction between magnitude through the third period when the low in- these two systems. This vortex interaction (which oc- tensified and transported more positive vorticity up- curred under such a track pattern) appears to be one ward. The DT, however, was negative during the first important reason why the fully developed secondary period because the vortex was undergoing development low did not replace the original center. and because air diverged at this layer as the convection b. Impact of the secondary low and the typhoon developed upward. The DT became positive during the circulation on rainfall third period when the midlevel vortex was the strongest. After Mindulle left northern Taiwan, there were con- In summary, the horizontal vorticity transported by vective lines developing over the southern Taiwan the flow circling around the northern tip of the CMR Strait (Fig. 7), which were also simulated by MM5 (Fig. and the overmountain upper-level typhoon center had 10). Apparently the southwesterly flow had played an an important impact on the development of the second- important role in the development of these MCSs. ary low. During the mature stage, VA was also playing However, it is still important to examine the impact of a (negative) role at the low level when both the vortic- the secondary low and the typhoon circulation on the ity and divergence were large (large DT), leading to a formation of the convective lines shown in Fig. 10 and trivial local change of vorticity. It is also interesting to the accompanying heavy rainfall of this Mindulle case. note that TT is positive (negative) if HA was positive Therefore, the low-level (␴ ϭ 0.9; ϳ925 hPa) winds, (negative) at the low level, when the positive vorticity divergence, and trajectories were examined. At 28 h was entering (leaving) the domain. Finally, it is impor- (Fig. 22a), there was a convergence zone (marked I) tant to point out that although there might be some over the southern Taiwan Strait offshore from the numerical errors during computation, the current re- southwest coast of Taiwan, which corresponds to the sults are reasonable for two reasons. First, the changes convective line 1 shown in Fig. 10a. The backward tra- of LC, HA, VA, and DT were consistent with the jectory analyses (as shown in Fig. 22) indicate that this changes in the circulation pattern and were physically convergence zone was caused by the confluence of the meaningful. Second, the magnitude of RT was small at outer circulation of the typhoon and the mesoscale cir- the midlevel, where the frictional effect was small ex- culation of the secondary low. To the south of conver- cept at the second stage when the midlevel vorticity was gence zone I, there was another broader convergence the strongest. zone that was caused by the southwesterly flow. At 34 h, the circulation of the secondary low weak- 6. The roles of the secondary low ened as it moved northeastward and made landfall, while the primary center had moved over the ocean a. The interaction between the primary center and with better organized cyclonic circulation (Fig. 22b). the secondary center The flow over the northern SCS also increased its As shown in Figs. 11 and 14, the secondary low after southerly component, especially on the western side. becoming fully developed moved northeastward to- As a result, the convergence zone I moved northward ward land and then dissipated after making landfall. slightly, and another convergence zone (marked II) This feature is different from the previous studies formed to the south of zone I, which corresponds to the (Wang 1980; Yeh and Elsberry 1993a,b; Lin et al. 1999; convective line 2 shown in Fig. 10b. The backward tra- Wu 2001; among others) that documented the second- jectory analyses (from 34 h) indicate that the conflu- ary low developing and replacing the original center for ence of the outer circulation of typhoon and the southwestward-moving typhoons. The major difference be- westerly flow helped organize this convergence zone.

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Ϫ1 FIG. 23. As in Fig. 22, but for the NT experiment. Dashed lines FIG. 22. The simulated wind (full barb 5 m s ) and divergence Ϫ1 Ϫ Ϫ represent the 12 m s isotach. The dotted–dashed lines denote (shading 10 5 s 1)at␴ ϭ 0.9 (ϳ925 hPa) at (a) 28 and (b) 34 h. the wind shear. Boxes I and II in (b) indicate the convergence zones, and numbers 1–8 denote the backward trajectories ending at 100 m AGL from (a) 28–18 h and (b) 34–18 h. Arrows point to the positions of air parcels every 2 h, and their sizes indicate the heights of air parcels, lation (Fig. 23), there was no circulation of typhoon or with legend shown in the lower-right-hand corner. secondary low in the domain. At 28 h (Fig. 23a), the southwesterly flow over the northern SCS was still The convergence zone caused by the southwesterly present, with strong convergence located near the lead- flow still existed at the same place (south of zone II) but ing edge. Simulated reflectivity (not included here) weakened somewhat. It was therefore clear that the showed that many intense MCSs formed over this con- existence of the secondary low and the circulation of vergence area. The magnitude of convergence is com- Mindulle provided extra convergence to further en- parable with (but slightly weaker than) that of the con- hance the convection, which helped produce the two trol experiment (see Fig. 22a). However, the distribu- southern rainfall maxima (Ͼ700 mm) and the heavy tion of convergence was different from that of the rainfall over the western plain area shown in Fig. 9. control experiment, especially over the southern Tai- To further examine the role of the secondary low and wan Strait. Apparently, without the typhoon and the the typhoon circulation on the MCSs and heavy rain- secondary vortex in the NT, there was still convection fall, the NT in which the typhoon was removed at the downstream (northern SCS) of the southwesterly flow, initial time was performed. After 28 and 34 h of simu- but the convergence did not extend farther to the Tai-

Unauthenticated | Downloaded 09/23/21 05:38 PM UTC APRIL 2008 LEEETAL. 1281 wan Strait. Consequently, there was no convective line occurred as a result of downslope adiabatic warming over the southern Taiwan Strait as seen in the control associated with the overmountain easterly flow of the experiment. It is noted that in areas near the typhoon typhoon circulation. At the same time, the shear vor- center, the wind speeds of NT would be weaker than ticity associated with the flow that circled around the those in the control experiment because of the absence northern tip of the CMR was advected by the environ- of typhoon circulation. Therefore, the low-level mois- mental cyclonic flow into the low pressure region. ture content of NT would be lower because of less sur- These two processes resulted in the formation of a sec- face evaporation. This might be another reason why ondary low with cyclonic circulation over the central there was less convection over the Taiwan Strait. Taiwan Strait. After Mindulle made landfall (the sec- At 34 h, the southerly component of flow over the ond stage), the low-level circulation of the primary cen- northern SCS increased, resulting in an east–west- ter was destroyed by the terrain, with the vorticity rem- oriented shear line located about 22.5°N, where a nar- nant on the eastern slope of the CMR being stretched row convergence zone existed. This convergence zone along the east coast. The vorticity remnant was main- was weaker than that in the control run (zone II in Fig. tained or even enhanced by the strong wind shear along 22b), without the existence of a typhoon circulation or the east coast. At the same time, the northern part of a secondary low. Furthermore, the lower surface pres- the vorticity patch was advected, again by the cyclonic sure associated with the typhoon and the secondary low flow, to circle around the northern tip of the CMR and could enhance the southwesterly flow over the areas to then to the region of the secondary low. On the other the southwest-southeast of Taiwan because of the en- hand, the positive vorticity associated with the upper- hanced pressure gradient between Mindulle and the Pa- level typhoon center moved over the CMR and coupled cific high. Therefore, the existence of Mindulle and the with the secondary low. Consequently, the secondary associated secondary low has played a significant role in low intensified significantly and reached its maximum the occurrence of the 7–2 flood in Taiwan. intensity stage. The above mechanisms were also verified by the trajectory analyses as well as the thermal and vorticity 7. Discussion and conclusions budget analyses. Note that the thermal process (subsi- This paper presents an observational and numerical dence warming) contributed only to the formation of a study of Typhoon Mindulle (2004) as it affected Tai- weak low pressure system at lower levels over a broad wan. Mindulle made landfall on the east coast of Tai- region. The low-level horizontal vorticity advection and wan at 1500 UTC 1 July 2004, and left the north coast then the local convergence further concentrated the of Taiwan 13 h later. Right before and during landfall of vorticity, leading to the development of the secondary the primary typhoon center, a secondary low formed low (also called secondary vortex afterward). These re- over the Taiwan Strait. The secondary low developed sults are consistent with the conclusions of Chang with cyclonic circulation extending upward to about 300 (1982), which showed that the vortex stretching and hPa, but dissipated over land after Mindulle redevel- advection are dominant terms during the formation of a oped over the ocean north of Taiwan. The focus of this secondary low. study is placed on the formation and development of The formation and the evolution mechanisms of the the leeside secondary low, the physical process of re- secondary low in our study agree to some extent with development of the primary center, the interactions be- the studies of Jian et al. (2006) and Lin et al. (2006), tween the primary and the secondary centers, and their which examined Typhoons Dot (1990) and Toraji impact on the heavy rainfall. (2001), respectively. However, there are still some dif- The MM5 with three nested domains and the finest ferences. After Typhoons Dot and Toraji made land- grid of 5 km was used for the model simulation. By fall, the secondary low developed on the lee side of the using the FDDA nudging scheme to assimilate the CMR and replaced the original center. This is the so- QuikSCAT data, the model nicely reproduced the track called discontinuous track. In our case, however, the of Mindulle, the evolution of the secondary low, and secondary low had a maximum vorticity greater than the 24-h accumulated rainfall. Analyses of simulated 5 ϫ 10Ϫ4 sϪ1 over an area with a 30-km radius and did results showed that the development of the secondary not replace the original center. Instead, it moved land- low could be divided into two stages separated by the ward and dissipated over land. Moreover, the low-level landfall time of the typhoon. During the first stage primary typhoon center reappeared over the north when the primary typhoon center was located near the coast after its disappearing over land. According to Lin east coast of Taiwan, a broad area with slightly lower et al. (1999), the formation of a secondary center is surface pressure on the lee (west) side of the CMR closely linked with the track, the terrain orientation,

Unauthenticated | Downloaded 09/23/21 05:38 PM UTC 1282 MONTHLY WEATHER REVIEW VOLUME 136 and the landfall location of the primary vortex. In our ented convections and heavy rainfall to the south of the case study, Mindulle has a northward track with a slight secondary low. Note that heavy rainfall also occurred shift toward the west near Taiwan, which is different over the plains area on 2 July 2004, not just over the from the westward-moving Typhoons Dot and Toraji. mountain areas as on 3 July 2004, when only southwest- Several processes are thus proposed here to explain the erly flow prevailed. Unfortunately, it is almost impos- difference. First, because of the northward track, Min- sible to isolate the role of the typhoon circulation and dulle preserved some strong vorticity on the eastern the secondary low on the heavy rainfall because of the slope of CMR, which was further steered northward nonlinear interactions among the typhoon, the second- over the ocean north of Taiwan where the redevelop- ary low, the environmental flow, and the terrain. In ment occurred. Moreover, although the secondary low addition, the secondary low was likely not contributing was once stronger than the original typhoon, the pri- as much as the primary center contributed to providing mary circulation after redeveloping was able to interact extra convergence because of its relatively short life with the secondary low, resulting in the northeastward span. The secondary low should play a role as a modi- movement of the secondary low and its dissipation after fication of environmental flow, which could result in it made landfall. Therefore Mindulle can be treated as the west–east-oriented convergence lines located in the a “quasi-continuous track,” which is different from our Taiwan Strait. previous understanding that the track of a typhoon is Last, it is noted that Mindulle appeared to be quite usually discontinuous if it is accompanied by a strong unique as to its quasi-continuous track and its impact of secondary low after making landfall. the secondary low on the local rainfall distribution Since the model typhoon is weaker than that ob- when compared with the typhoons studied by Jian et al. served during the first 12 h of simulation, it is logical to (2006) and Lin et al. (2006). More studies on the sec- suspect that the interaction between the terrain and ondary low are needed to address different roles that it Mindulle was not well simulated. Therefore, we have might play in modifying the environmental flow, result- also done other experiments with different initial ty- ing in the change of rainfall distribution associated with phoon intensities. Although results show that the a landfalling typhoon in Taiwan. model typhoon might have different initial intensities (one is comparable to that observed), it still has the Acknowledgments. The authors thank Miss Erin same track pattern (quasi continuous). Therefore, the Huang for her help in manuscript preparation and occurrence of a quasi-continuous track might depend thank Dr. Jen-Hsin Teng of CWB and Dr. P.-L. Lin of more on the typhoon’s moving direction than the in- the National Central University for their help in radar tensity of the initial system. Such results are somewhat data analyses. Thanks are also given to the Central different from the previous studies that show weaker Weather Bureau for providing the observational data. typhoons having more chances to induce a secondary The valuable comments raised by two anonymous re- low formed on the lee side (Chang 1982). However, viewers are highly appreciated. This research was sup- further studies are still needed to clarify this issue. ported by the National Science Council of the Republic Through the course of landfall, Mindulle itself did of China (Taiwan) under Grants NSC 94-2625-Z-002- not create excessive damage from winds and precipita- 015, NSC 95-2625-Z-002-005, NSC 94-2111-M-002-019- tion. However, the strong mesoscale convective systems AP1, and NSC 94-2111-M-003-001-AP2. Our gratitude that formed after Mindulle made landfall brought ex- also goes to the Academic Paper Editing Clinic, NTNU. tremely heavy rainfall over the southern and central parts of the island west of the CMR, resulting in severe REFERENCES flooding and mudslides in many regions. The convection was related to the intensification of the southwest- Bender, M. A., R. E. Tuleya, and Y. Kurihara, 1985: A numerical erly flow (especially on 3 July 2004). However, the role study of the effect of a mountain range on a landfalling tropi- that the secondary low and the typhoon circulation cal cyclone. Mon. Wea. Rev., 113, 567–582. played in the heavy rainfall (on 2 July 2004) should not ——, ——, and ——, 1987: A numerical study of the effect of be neglected. Our study further showed that without island terrain on tropical cyclones. Mon. Wea. Rev., 115, 130– the secondary low and the typhoon circulation, the con- 155. vection near the leading edge of the southwesterly flow Chang, S.-W., 1982: The orographic effects induced by an island mountain range on propagating tropical cyclones. Mon. Wea. was weaker and there was no heavy precipitation oc- Rev., 110, 1255–1270. curring in Taiwan. It is thus clear that the circulation of Grell, G. A., J. Dudhia, and D. R. Stauffer, 1994: A description of Mindulle and the associated secondary low provided the fifth-generation Penn State/NCAR Mesoscale Model extra convergence, resulting in strong west–east-ori- (MM5). NCAR Tech. Note, NCAR/TN-398ϩSTR, 138 pp.

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Hong, S.-Y., and H.-L. Pan, 1996: Nonlocal boundary layer ver- data assimilation in a limited-area mesoscale model. Part I: tical diffusion in a medium-range forecast model. Mon. Wea. Experiments with synoptic-scale data. Mon. Wea. Rev., 118, Rev., 124, 2322–2339. 1250–1277. Huang, C.-Y., and Y.-L. Lin, 1997: The evolution of mesoscale ——, ——, and F. S. Binkowski, 1991: Use of four-dimensional vortex impinging on symmetric topography. Proc. Taiwan data assimilation in a limited-area mesoscale model. Part II: Natl. Sci. Counc., 21A, 285–309. Effects of data assimilation within the planetary boundary Jian, G.-J., C.-S. Lee, and G. T.-J. Chen, 2006: Numerical simu- layer. Mon. Wea. Rev., 119, 734–754. lation of Typhoon Dot (1990) during TCM-90: The discon- Wang, S.-T., 1980: Prediction of the behavior and strength of tinuous track across Taiwan. Terr. Atmos. Oceanic Sci., 17, typhoons in Taiwan and its vicinity. Research Rep. 108, Na- 23–52. tional Science Council, Taipei, Taiwan, 100 pp. Lin, Y.-L., 1993: Orographic effects on airflow and mesoscale Wu, C.-C., 2001: Numerical simulation of Typhoon Gladys (1994) Terr. Atmos. Oceanic Sci., weather systems over Taiwan. 4, and its interaction with Taiwan terrain using the GFDL hur- 381–420. ricane model. Mon. Wea. Rev., 129, 1533–1549. ——, J. Han, D. W. Hamilton, and C.-Y. Huang, 1999: Orographic ——, and Y.-H. Kuo, 1999: Typhoons affecting Taiwan: Current influence on a drifting cyclone. J. Atmos. Sci., 56, 534–562. understanding and future challenges. Bull. Amer. Meteor. ——, D. B. Ensley, S. Chiao, and C.-Y. Huang, 2002: Orographic Soc., 80, 67–80. influences on rainfall and track deflection associated with the passage of a tropical cyclone. Mon. Wea. Rev., 130, 2929– ——, T.-H. Yen, Y.-H. Kuo, and W. Wang, 2002: Rainfall simu- 2950. lation associated with Typhoon Herb (1996) near Taiwan. ——, N. C. Witcraft, and Y.-H. Kuo, 2006: Dynamics of track Part I: The topographic effect. Wea. Forecasting, 17, 1001– deflection associated with the passage of tropical cyclones 1015. over a mesoscale mountain. Mon. Wea. Rev., 134, 3509–3538. Yeh, T.-C., and R. L. Elsberry, 1993a: Interaction of typhoons Reisner, J., R. T. Bruintjes, and R. J. Rasmussen, 1998: Explicit with the Taiwan orography. Part I: Upstream track deflec- forecasting of supercooled water in winter storms using the tions. Mon. Wea. Rev., 121, 3193–3212. MM5 mesoscale model. Quart. J. Roy. Meteor. Soc., 124B, ——, and ——, 1993b: Interaction of typhoons with the Taiwan 1071–1107. orography. Part II: Continuous and discontinuous tracks Stauffer, D. R., and N. L. Seaman, 1990: Use of four-dimensional across the island. Mon. Wea. Rev., 121, 3213–3233.

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