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

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The Secondary Low and Heavy Rainfall Associated with Typhoon Mindulle (2004) 1260 MONTHLY WEATHER REVIEW VOLUME 136 The Secondary Low and Heavy Rainfall Associated with Typhoon Mindulle (2004) 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 © 2008 American Meteorological Society Unauthenticated | Downloaded 09/23/21 05:38 PM UTC MWR2069 APRIL 2008 LEEETAL. 1261 (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. Unauthenticated | Downloaded 09/23/21 05:38 PM UTC 1262 MONTHLY WEATHER REVIEW VOLUME 136 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.
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