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A Long-Lasting Vortex Rossby Wave–Induced Rainband of Typhoon Longwang (2005)

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A Long-Lasting Vortex Rossby Wave–Induced Rainband of Typhoon Longwang (2005) A Long-Lasting Vortex Rossby Wave–Induced Rainband of Typhoon Longwang (2005) YANLUAN LIN, YUANLONG LI, QINGSHAN LI, MINYAN CHEN, FANGHUA XU, YUQING WANG, AND BIN HUANG n 2 October 2005, a record-breaking rainfall event Tsai 2013). As Typhoon Longwang approached the with 152 mm of rainfall in an hour occurred as coast of Fujian Province at 0800 UTC, one type of this OTyphoon Longwang approached Fujian Province, transient rainband in the northeast sector started to China. The severe rainfall was unexpected and signifi- weaken and dissipate (Fig. 1b). At the same time, the cantly underpredicted by the local weather forecasters eyewall underwent an asymmetry transformation and caused a total of 96 deaths. Because of the severe accompanied by a bended convection pattern in the damage over Taiwan and mainland China, the name north (Fig. 1b). The bended convection transformed of Longwang, which means a dragon in charge of into a strong convective band along the eyewall to the rainfall in Chinese, was removed from the name list north and moved outward relative to the storm center for future typhoons. (Fig. 1c). The convective band continued to intensify with a sharp inner edge (Fig. 1d). An hour later, the EVOLUTION AND BASIC FEATURES OF convective band achieved its maximum intensity with THE RAINBAND. The formation and evolution a large area of stratiform precipitation outward and of the rainband associated with the rainfall event was downstream (Fig. 1e). At this time, cloud brightness captured by the radar mosaic produced by the Central temperatures as low as −80°C were measured by a Weather Bureau (CWB) of Taiwan (Fig. 1). Longwang Geostationary Operational Environmental Satellite had frequent rainband activity as it made landfall and (GOES; see Fig. ES1 in the online supplement to this passed over Taiwan Island (Yu and Tsai 2010, 2013). article: https://doi.org/10.1175/BAMS-D-17-0122.2) Rainbands during this period were more transient and hourly precipitation of 152 mm was measured and had the features of squall-line dynamics (Yu and at Changle under this band. As the convective band detached from the original eyewall and propagated outward relative to the storm center, a new weak AFFILIATIONS: LIN, Y. LI, Q. LI, AND XU—Key Laboratory for eyewall started to form and gradually intensified Earth System Modeling, Ministry of Education, and Department as the storm center moved over the Taiwan Strait of Earth System Science, and Joint Center for Global Change (Figs. 1e–h). Before Longwang made landfall near Studies, Tsinghua University, Beijing, China; CHEN—Weather Quanzhou around 1500 UTC (Fig. 1i), the rainband Bureau of Fujian Province, Fuzhou, Fujian, China; WANG—State persisted with strong intensity. Finally, after the sec- Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China, and International Pacific ond landfall of Longwang over mainland China, the Research Center, and Department of Atmospheric Sciences, rainband slowly weakened (Figs. 1j–o), but still had School of Ocean and Earth Science and Technology, University a striking line form with large reflectivities until it of Hawai‘i at Maˉnoa, Honolulu, Hawaii; HUANG—National dissipated completely after 2300 UTC 2 October. The Meteorological Center, Beijing, China rainband that produced the heavy rainfall lasted for CORRESPONDING AUTHOR: Yanluan Lin, at least 13 h. The triggering and maintenance of this [email protected] long-duration strong rainband are worthy of a detailed DOI:10.1175/BAMS-D-17-0122.1 investigation. A supplement to this article is available online (10.1175/BAMS-D-17-0122.2) The Doppler radar at Changle captured a more ©2018 American Meteorological Society detailed view of the evolution of the rainband (Fig. 2). For information regarding reuse of this content and general copyright The sharp inner edge of the band, which was sustained information, consult the AMS Copyright Policy. and fueled by the strong low-level convergence, was distinct during the whole lifespan of the rainband. AMERICAN METEOROLOGICAL SOCIETY JUNE 2018 | 1127 Unauthenticated | Downloaded 10/03/21 07:59 PM UTC This can be inferred from the radial wind pattern recorded by the radar (Figs. 2e–h). There is a sharp gradient of cross-band winds along the inner edge of the band with low-level outward flow inside of the band and inward flow outside of the band, espe- cially toward the later times. This is similar to the sec- ondary circulation in a hur- ricane, but converged along the band instead of the eye- wall (Figs. 2e,f). The strong convergence was responsible for the sustained vertical mo- tion extending to the upper troposphere, as indicated by reflectivities of up to 40 dBZ at an altitude of ~9 km MSL as measured by the radar at Xiamen (Fig. ES2). Another feature to note is the very sharp tip or tail of the band (Figs. 2a–d), which was pref- FIG. 1. Doppler radar reflectivity mosaic images at 1-h interval (unit: dBZ) erentially located near the from the CWB, from 0700 to 2100 UTC 2 Oct 2005. The rainband of interest coast during the whole life is denoted using a purple arrow. duration of the band. The inner edge was very sharp along a narrow zone with Unprecedented heavy rainfall due to this band an arc-shaped radar echo over 50 dBZ (Figs. 2a,b). This occurred over the coastal area of Fujian Province feature is in contrast to that of typical squall lines, before and during the landfall of Typhoon Longwang. which generally have a broad area of stratiform pre- More than 10 stations recorded hourly precipitation cipitation located behind the leading-edge convection greater than 40 mm in a short period during 2–3 (Yu and Tsai 2013; Meng and Zhang 2012). Instead, October (Fig. 6a), with the maximum precipitation of some typical inner rainband-like characteristics are 152 mm h–1 (minute rainfall up to 5.5 mm) recorded evident. For example, the band propagated outward at Changle between 1100 and 1200 UTC 2 October (Willoughby et al. 1984), with an along-band jet (Fig. 3). The 3-h accumulated rainfall is over 300 mm (>30 m s–1; Figs. 2e,f) on the radially outward side of over a wide area swept by the rainband (not shown). the convective cells (Houze 2010). The band contin- The temporal evolution of rainfall at a few ued to move outward and approached Changle radar representative stations (Fig. 3) clearly showed the station at 1139 UTC (Fig. 2b). Although the Doppler propagation of rainfall associated with the move- radar at Changle was shut down for protection from ment of the rainband. At each station, heavy rainfall 1200 to 1400 UTC, it still captured the evolution and lasted for 1–2 h. The maximum precipitation rates fine structures of the rainband. Reflectivities up to at the six representative stations were 77, 152, 111, 50 dBZ were measured along a ~200-km-long band 84, 108, and 43 mm h–1, and occurred at 0900–1000, extending from the ocean toward the inland area of 1100–1300, 1200–1400, 1200–1400, 1500–1700, and Fujian Province. No hail was recorded at the surface 1600–1800 UTC, respectively (Fig. 3). Associated during the passage of the band. Thus, reflectivities of with the passage of the rainband over these stations, 50 dBZ implied a very large number concentration of the surface pressure experienced a ~3-hPa increase raindrops and/or large sizes of raindrops. accompanied by a temperature drop of ~3 K (Fig. 3). 1128 | JUNE 2018 Unauthenticated | Downloaded 10/03/21 07:59 PM UTC Va ᵅ Vc FIG. 2. The 1.45°-elevation plan position indicator (PPI) scans of the (a)–(d) reflectivities (dBZ) and (e)–(h) de- aliased radial velocities (m s–1) from the Changle Doppler radar at four representative times. The four selected times approximately correspond to the passage of the rainband over Putian (PT), Changle (CL), Luoyuan (LY), and Ningde (ND), as denoted using the black dot. The black star marks the position of the radar. The geomet- ric definitions of the Va and Vc wind components are shown in (c), with arrows denoting the positive direction. Another prominent feature is the abrupt change of winds following the passage of the rainband. We decomposed the 10-m winds into their along- (Va) and cross-band (Vc) components. There is a distinct peak of Va with a sharp reduction of Vc associated with the passage of the rainband (Fig. 3). For ex- ample, Va increased from 0 to ~12 m s–1 with the Vc being reduced from 15 m s–1 to near 0 when the band passed over Changle (Fig. 3). There was also a clear reversal of Vc before and after the passage of the band with positive Vc (inward flow) before the passage and negative Vc (outward flow) after the passage as the rainband propagated farther away from the center of the storm (e.g., Luoyuan in Fig. 3). This implied that the band was actu- ally induced and accompanied by strong low-level cross-band convergence. FIG. 3. Time series of minute precipitation rate (PR), temperature (T), surface pressure (P), and Va and Vc wind components at 1-min temporal resolution at the six coastal sites (as shown in Fig. 6a) from 0800 to 1800 UTC 2 Oct 2005. The translucent rectangles denote the arrival of the rainband based on the start of precipitation associated with the rainband at each station, showing the propagation of the rainband. AMERICAN METEOROLOGICAL SOCIETY JUNE 2018 | 1129 Unauthenticated | Downloaded 10/03/21 07:59 PM UTC is estimated to be about 305 K.
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