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A Numerical Study of Typhoon Megi (2010). Part I: Rapid Intensification

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A Numerical Study of Typhoon Megi (2010). Part I: Rapid Intensification JANUARY 2014 W A N G A N D W A N G 29 A Numerical Study of Typhoon Megi (2010). Part I: Rapid Intensification HUI WANG Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, Jiangsu, China, and International Pacific Research Center and Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Manoa, Honolulu, Hawaii YUQING WANG International Pacific Research Center and Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Manoa, Honolulu, Hawaii, and Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, Jiangsu, China (Manuscript received 21 February 2013, in final form 14 August 2013) ABSTRACT Typhoon Megi (15W) was the most powerful and longest-lived tropical cyclone (TC) over the western North Pacific during 2010. While it shared many common features of TCs that crossed Luzon Island in the northern Philippines, Megi experienced unique intensity and structural changes, which were reproduced reasonably well in a simulation using the Advanced Research Weather Research and Forecasting Model (ARW-WRF) with both dynamical initialization and large-scale spectral nudging. In this paper processes responsible for the rapid intensification (RI) of the modeled Megi before it made landfall over Luzon Island were analyzed. The results show that Megi experienced RI over the warm ocean with high ocean heat content and decreasing environmental vertical shear. The onset of RI was triggered by convective bursts (CBs), which penetrate into the upper troposphere, leading to the upper-tropospheric warming and the formation of the upper-level warm core. In turn, CBs with their roots inside of the eyewall in the boundary layer were buoyantly triggered/supported by slantwise convective available potential energy (SCAPE) accumulated in the eye region. During RI, convective area coverage in the inner-core region was increasing while the updraft velocity in the upper troposphere and the number of CBs were both decreasing. Different from the majority of TCs that experience RI with a significant eyewall contraction, the simulated Megi, as the observed, rapidly intensified without an eyewall contraction. This is attributed to diabatic heating in active spiral rainbands, a process previously proposed to explain the inner-core size increase, enhanced by the interaction of the typhoon vortex with a low-level synoptic depression in which Megi was embedded. 1. Introduction and JTWC upgraded it to a category-1 typhoon. On 15 October, JMA upgraded Megi to a typhoon. Megi (15W) was first identified as a tropical distur- As shown in Fig. 1, Megi initially moved northwest- bance over the western North Pacific (WNP) by the ward and then turned west-southwestward. It experi- Joint Typhoon Warning Center (JTWC) on 12 October enced two periods of intensification before it made 2010. The Japan Meteorological Agency (JMA) and landfall at Luzon Island in the northern Philippines. The JTWC began to monitor the low pressure circulation as first intensification occurred from 1200 UTC 12 October a tropical depression (TD). The TD further intensified to 0000 UTC 15 October during which the maximum into a tropical storm (TS), named Megi by JMA at 21 1200 UTC on 12 October. Later on 14 October, the eye 10-m wind speed increased by 35 m s and the central of the storm could be clearly seen from satellite image sea level pressure (SLP) dropped by 45 hPa. The second and JMA thus upgraded Megi to a severe tropical storm intensification occurred from 0000 UTC 16 October to 1200 UTC 17 October. During this 36-h period the 2 maximum 10-m wind speed also increased by 35 m s 1 Corresponding author address: Dr. Yuqing Wang, IPRC/SOEST, while the central SLP dropped by 52 hPa. In the end, Rm. 409G, POST Bldg., University of Hawai‘i at Manoa, 1680 East–West Rd., Honolulu, HI 96822. Megi attained its peak intensity with a central SLP of 2 E-mail: [email protected] 905 hPa and a maximum 10-m wind speed of 80 m s 1, DOI: 10.1175/MWR-D-13-00070.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/03/21 09:59 PM UTC 30 MONTHLY WEATHER REVIEW VOLUME 142 FIG. 1. (a) Track of Typhoon Megi (2010) with every 6-h position indicated by solid circles with different colors for different categories 2 in storm intensity, (b) the storm’s central sea level pressure (hPa), and (c) the maximum sustained 10-m wind speed (m s 1)from0000UTC 12 Oct to 0600 UTC 24 Oct 2010 based on the JTWC best track data. The modeled track, central sea level pressure, and maximum 10-m wind speed from 0000 UTC 15 Oct to 0000 UTC 22 Oct are also shown in red. the most powerful supertyphoon over the WNP and Province, China, on 23 October and finally became a TD South China Sea (SCS) in 2010. Based on the definition and dissipated gradually on the next day. of rapid intensification (RI) by Holliday and Thompson While Megi shared many common features of TCs (1979) for WNP tropical cyclones (TCs) and Kaplan and that crossed Luzon Island (Chou et al. 2011), it also DeMaria (2003) for Atlantic TCs,1 the first intensifica- experienced some unique intensity, structural, and track tion was not rapid. However, the second intensification changes. In addition to the subtle track and intensity can be classified as an RI case according to the definition changes (Fig. 1), Megi also experienced interesting proposed by Kaplan and DeMaria (2003). In this study, structural changes (Fig. 3). For example, deep convec- we will focus on the second intensification period, namely tion in the eyewall was widening without any signal of an the RI phase of Typhoon Megi. During the RI period, eyewall contraction during RI. This is different from the Typhoon Megi moved west-southwestward east of the majority of TCs experiencing RI. The RI ended as a Philippines over the WNP with high sea surface temper- concentric eyewall signal appeared before it made land- ature (SST) and high upper-ocean heat content (OHC) as fall over Luzon Island (Fig. 3), a not uncommon process shown in Fig. 2, both of which are favorable ocean con- at the end of an RI event (e.g., Kossin and Sitkowski ditions for RI of a TC (Lin et al. 2008). 2009), but with the concentric eyewall cycle being in- Megi made landfall over Luzon Island at around terrupted by landfall for the Megi case. The storm ex- 0330 UTC on 18 October. It weakened to a category-2 perienced an eyewall breakdown when it crossed Luzon typhoon immediately after its landfall. After crossing Island, and later on, a new outer eyewall formed at a Luzon Island, Megi entered the SCS and turned north- larger radius as a result of the axisymmetrization of outer westward and then suddenly north-northeastward on spiral rainbands after Megi entered the SCS (Fig. 3). Soon 20 October. During its northwest-to-north turning after, a small inner eyewall, which could have been the motion over the SCS on 19 October, Megi slowed down redevelopment of its original eyewall, appeared for sev- as it reintensified from category 2 to category 4 with a eral hours when it moved over the SCS. This could be the central SLP of 935 hPa and a maximum 10-m wind speed first double-eyewall structure observed to date as a result 2 of 57 m s 1. Early on 20 October, Megi turned north- of the reappearance of the original eyewall within a newly northeastward. It then weakened to a tropical storm, formed outer eyewall. Compared with the well-studied and made its second landfall at Zhangpu in Fujian Typhoon Zeb of 1998 (Wu et al. 2003, 2009), Megi ex- perienced much richer structural changes, such as the lack of the eyewall contraction during RI before landfall and the development of the concentric eyewall structure, as 1 RI was defined as the deepening rate of greater than 2 42 hPa day 1 in the central SLP by Holliday and Thompson (1979) well as a reintensification as it entered the SCS. 2 2 for western Pacific TCs and as 15 m s 1 day 1 in the maximum 10-m In this study, based on a reasonable, week-long con- wind speed by Kaplan and DeMaria (2003) for Atlantic TCs. trol simulation of Typhoon Megi in Wang et al. (2013), Unauthenticated | Downloaded 10/03/21 09:59 PM UTC JANUARY 2014 W A N G A N D W A N G 31 22 FIG. 2. (a) Satellite altimetry SST (8C) and (b) the upper OHC (kJ cm ) on 17 Oct 2010 (shadings and white contours), overlapped with the track of Typhoon Megi (2010) from JTWC, with the storm track overlaid with the colored circles, indicating the intensity of the storm according to the Saffir–Simpson scale, which were produced by the Remote Sensing Laboratory at National Taiwan University and can be accessed online (http://data.eol.ucar.edu/codiac/dss/id5209.027). we focus on understanding some unique features of control simulation. The RI processes of the simulated Megi, including its RI with no eyewall contraction, its Megi are analyzed in section 3. Our major results are structural changes during its landfall over Luzon Island, summarized and discussed in the final section. and its reintensification after it entered the SCS. In this paper, after a brief introduction of the high-resolution 2. Model setup, dynamical TC initialization, and control simulation of Megi, we will present the analyses verification of simulation of RI before Megi made landfall over Luzon Island.
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