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 hurricane, but converged along the band instead of the eyewall (Figs. 2e,f). The strong convergence was responsible for the sustained vertical motion 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 example, 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 actually 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. Assuming the cold pool depths to be 1–3 km, this gave a speed of approximately 10–17 m s–1, which is larger than the cross-band wind shear of 6–8 m s–1 estimated over the same depths, as measured at Fu- zhou (Fig. 4b). Clearly, the cold-pool-induced vorticity is larger than the cross- band wind shear and thus is in the so-called suboptimal state (Rotunno et al. 1988), which implied that Fig. 4. Skew T–logp diagram at Fuzhou at (a) 0000 and (b) 1200 UTC 2 Oct the system tended to slope 2015. rearward instead of having an upright updraft. In ad- POSSIBLE CAUSES FOR THE RAINBAND. dition, the weak shear does not favor the regenera- What is the cause of such a long-lasting and strong tion of strong cells along the outflow boundary and rainband? Radiosonde measurements at Fuzhou thus the maintenance of squall lines (Rotunno et al. captured the thermodynamic and kinematic envi- 1988; Weisman and Rotunno 2004). However, this ronmental characteristics during the evolution of the robust rainband lasted for more than 13 h with strong rainband (Fig. 4). At 0000 UTC before the passage of upright convection located in the inner edge of the the band, the CAPE was only 131 J kg–1. At 1200 UTC rainband, in contrast to most relatively short-lived around the time of the passage of the rainband over squall lines having the leading convective line (e.g., Fuzhou, the CAPE increased to about 561 J kg–1 with Meng and Zhang 2012; Yu and Tsai 2013). The small the level of free convection at about 1,450 m. Although CAPE and suboptimal state implied that squall-line convection and precipitation might have reduced the dynamics alone probably could not maintain such a CAPE at 1200 UTC, it is very difficult to generate and long-lived rainband. sustain such strong convection and updrafts solely by Regarding the possible wave dynamics for this the release of CAPE. This is part of the reason why we rainband, we noted that the wave dynamics in this think there are other types of forcing, possibly wave band is different from that in Yu and Tsai (2010, 2013). forcing and frictional contrast across the coastline as For example, the temperature and pressure perturba- well, rather than thermodynamic instability alone, tions were 90° different across the band studied in Yu for this band. and Tsai (2010, cf. their Fig. 12), which is consistent Similar to Yu and Tsai (2013), the possibility of cold with the basic features of gravity waves. In contrast, pool dynamics was examined first. There was an obvi- the temperature and pressure perturbations were ap- ous cold pool around 3 K associated with the passage proximately out of phase (180° difference) at several of the rainband (Fig. 3). Based on the strength of the stations we studied that were passed by the rainband cold pool, we estimated the moving speed of the cold (Fig. 5). This feature is consistent with Rossby-type pool approximately as wave characteristics. Note that caution should be paid since convection itself can significantly influence , temperature and pressure perturbations. The vortex Rossby wave (VRW) has been pro- where g is the gravitational acceleration, H is the cold posed to explain the formation of typhoon rainbands

pool depth, θυ0 denotes initial the environmental (Montgomery and Kallenbach 1997). To determine

virtual potential temperature, and ∆θυ is the virtual the propagation of the rainband, the isochrones of potential temperature deficit influenced by the cold the area of strong convection with radar reflectiv-

pool. For the present case, ∆θυ is about 3.1 K and θυ0 ity greater than 45 dBZ in the band were tracked

1130 | JUNE 2018 Unauthenticated | Downloaded 10/03/21 07:59 PM UTC rather steady with a speed of about 4.67 m s–1 anticyclonically (Fig. 6b), which is inconsistent with the cyclonically propagating speed noted for VRWs (e.g., Wang 2002; Corbosiero et al. 2006). A careful wave decompo- sition analysis following Corbosiero et al. (2006) using the CWB radar images indicated that the convective segment used for the previous tracking is actually only a part of a longer rainband, more specifically, the tail of a long spiral rainband (Fig. 7). The evolution of the wavenumber-2 com- ponent corresponded well with that of the major body of the northwestern rainband (cf. Fig. 1). Note that wavenumber-2 analysis is fairly robust and does not strongly depend on the position of the storm center (Fig. ES3; Reasor et al. 2000; Corbosiero et al. 2006). The major body of the rainband identified by the wave analysis actually propagated cyclonically with an azimuthal phase speed of about 12 m s–1 (Fig. 7g). Note that the ambi- ent 850- and 700-hPa tangential wind speeds near this location of the rainband are about 16.5 and 27.7 m s–1, respectively, based on the sounding from Fuzhou at 1200 UTC, implying that this wave propagated against the Fig. 5. Time series of pressure (red line) and temperature (green tangential flow of the typhoon. The line) perturbations over a 2-h time period covering the passage azimuthal propagation against the of the rainband at the six stations labeled in Fig. 6a. A 2-h running tangential flow together with the slow average was used to compute the perturbations. The yellow line outward radial propagation from the denoted the time of the maximum precipitation at each station. typhoon center is consistent with the characteristics of VRWs documented from 0900 to 1756 UTC (Fig. 6a). Surprisingly, the in previous studies (Wang 2002; Corbosiero et al. radial speed of the rainband relative to the typhoon 2006). For example, the mean azimuthal speed was center is rather steady at about 5.78 m s–1 (Fig. 6b). 68% of the local tangential mean flow (Corbosiero Relatively small radial propagation speeds of VRWs et al. 2006). Similar orientation between the rainband have been found in both numerical modeling [e.g., and cyclonic tangential flow also implies that the 4–5 m s–1 for wavenumber-1 VRWs in Wang (2002)] rainband was strongly affected by the dynamics of and observational studies [e.g., 5.2 m s–1 in Corbosiero the typhoon. Note that although the majority of the et al. (2006)]. The small radial propagation speed of band propagated cyclonically, the convective sector this band is thus consistent with VRWs rather than of interest was actually moving anticyclonically rela- the high-frequency gravity waves [e.g., greater than tive to the storm center and was always located near 15 m s–1 in Diercks and Anthes (1976)]. However, the the coast (Fig. 1). One possible reason for the strong azimuthal propagation speed determined based on convection near the coast is mainly due to the strong the direct tracking of the strong convective sector is low-level moisture convergence enhanced by surface

AMERICAN METEOROLOGICAL SOCIETY JUNE 2018 | 1131 Unauthenticated | Downloaded 10/03/21 07:59 PM UTC Fig. 6. (a) The isochrones of the rainband and the track of the typhoon with a mean time interval of 1 h (rep- resented by different colors), and the position of the typhoon center at 1-h intervals was interpolated from the International Best Track Archive for Climate Stewardship (IBTrACS) dataset. The rainband is portrayed as the connection of the average position of the points on each longitude with a reflectivity greater than 45 dBZ from the 1.45°-elevation PPI scans of the Changle Doppler radar. The red dots are those surface stations that measured an instantaneous precipitation rate greater than 40 mm h–1, which are marked from 1 to 6 for PT, CL, Fuzhou (FZ), Minhou (MH), LY, and ND, respectively. (b) The solid and dashed lines denote the temporal variations of the radial and tangential distances (calculated as aR, in which a is the slant angle of the rainband relative to the storm center as shown in Fig. 2c and R is the average radial distance of the rainband to the center of the storm) of the rainband to the typhoon center, respectively.

Fig. 7. (a)–(f) Wavenumber-2 asymmetry of radar reflectivity of Longwang from CWB from 0900 to1400 UTC 2 Oct 2005, with the purple dashed line showing the +0.5-dBZ perturbation overlaid with green contours showing the rainband of interest with radar reflectivities greater than 30 dBZ. Black circles are the 50-, 150-, and 250-km radii from the typhoon center. (g) Azimuth–time Hovmöller diagram of the wavenumber-2 asymmetry at a radius of 170 km denoted by the black dashed circles in (a)–(f).

1132 | JUNE 2018 Unauthenticated | Downloaded 10/03/21 07:59 PM UTC Fig. 8. (a)–(c) Model 850-hPa reflectivity (dBZ, color shading) overlaid with 850-hPa wavenumber-2 asymmetry of potential vorticity (solid line for 0.1 PVU and dashed line for 1 PVU, where 1 PVU = 10−6 K kg–1 m2 s–1) at 0700, 0800, and 0900 UTC 2 Oct 2005. (d)–(f) As in (a)–(c), but overlaid with the 850-hPa wavenumber-2 vertical ve- locity (solid line, 0.05 m s–1; dashed line, 0.2 m s–1). Purple circles are the 50-, 100-, 150-, and 200-km radii from the typhoon center marked using the typhoon symbol.

friction over land, as we can see in the European Cen- maintained by the wave dynamics, but reinforced by tre for Medium-Range Weather Forecasts (ECMWF) the differential friction along the coast. Overall, it is interim reanalysis (ERA-Interim; not shown). In con- suggested that the interplay between VRWs and the trast, the rainband is much weaker in the western part coastal convergence is the most likely reason for this of the storm, likely because of unfavorable moisture long-lasting rainband. More details about the simula- supply although the wave signal is still present. tion results will be reported upon in a separate study. In addition, we have performed a Weather Research Considering the small radial propagation speed and Forecasting (WRF) Model simulation of the storm and other features noted above, we think that this with 2-km grid spacing and found a similar rainband rainband is probably related to VRW dynamics in the simulation (Fig. 8). Similar to Chen and Yau although other mechanisms cannot be completely (2001), we found that the model reflectivity corre- ruled out at this stage. Further investigations, includ- sponded well with the wavenumber-2 potential vor- ing numerical simulations, are needed for a better ticity and vertical motion (Fig. 8). Of course, the wave understanding of the trigger and maintenance of mechanism noted in the simulation may not be the this rainband. same as that in the observed rainband. Nevertheless, the results indicate the possible contributions by VRWs ACKNOWLEDGMENTS. The authors thank the to the triggering and maintenance of the rainband. two anonymous reviewers for their thoughtful comments. Surprisingly, the strong convection tends to move with The radar images used in this study were provided by the the main body of the wave farther inland in another Taiwan Central Weather Bureau. This work was supported WRF simulation with the terrain height reduced by by the outreach project of the State Key Laboratory of half (Fig. ES4) instead of remaining stationary near Severe Weather, Chinese Academy of Meteorological Sci- the coast in the control simulation. This indicated ences (2016LASW-B02). Y. Wang was supported in part that the strong convection is probably triggered and by the National Basic Research and Development Project

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