GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L07807, doi:10.1029/2007GL032773, 2008 Click Here for Full Article

Single Doppler radar observation of the concentric eyewall in Typhoon Saomai, 2006, near landfall Kun Zhao,1,2 Wen-Chau Lee,3 and Ben Jong-Dao Jou4 Received 27 November 2007; revised 30 January 2008; accepted 25 February 2008; published 3 April 2008.

[1] Landfalling Typhoon Saomai (2006) was observed by associated with a locally enhanced vorticity field embed- the CINRAD WSR-98D radar on the southeast coast of ded in the outer ring. In contrast, the moat region is China. This study documents the formation and evolution of characterized by downward air motion and lower vorticity a concentric eyewall episode using the axisymmetric [e.g., Houze et al., 2007]. The eyewall replacement cycle circulation derived from the ground-based velocity track is often occurred in intense TCs (Cat 3 and above). TCs display technique. Saomai’s outer eyewall formed after with concentric eyewalls often undergo a replacement reaching its peak intensity, 5 hours before landfall. cycle in which the inner eyewall is replaced by the outer Updraft, tangential wind maximum and shallow low-level eyewall, coinciding with a temporary decrease in storm inflow coincided with the high reflectivity and voriticity intensity. The TCs eventually intensify should the eyewall ring in both inner and outer eyewalls, surrounding a moat replacement cycle be completed. region characterized by weak downward motion and lower [3] The aforementioned airborne datasets were limited in reflectivity. The subsidence and rain-free moat region spatial and especially in temporal resolutions, typically with between the two eyewalls was filled with rain and upward only several snap shots. In recent years, the coastal Doppler motion prior to landfall, indicating a breakdown in the outer weather radar network in the United States, China, Taiwan, eyewall which was a barrier to radial inflow. Meanwhile, the Korea and Japan have provided a growing number of high outer vorticity maximum flattened and the central pressure temporal (6 minutes) and spatial (1 km) resolution dropped 9 hPa. The eyewall replacement cycle didn’t observations of double eyewalls in landfalling TCs (e.g., complete probably due to the landfall. Citation: Zhao, K., Danny (1997), Bilis (2000), Lekima (2001), Dujuan (2003), W.-C. Lee, and B. J.-D. Jou (2008), Single Doppler radar Maemi (2003), Haitang (2005), Saomai (2006)). Blackwell observation of the concentric eyewall in Typhoon Saomai, 2006, [2000] documented the concentric eyewall structure of near landfall, Geophys. Res. Lett., 35, L07807, doi:10.1029/ Danny at landfall near Mobile, Alabama, using reflectivity 2007GL032773. and Doppler velocity data. Hong and Chang [2005] docu- mented the double eyewall structure of Dujuan by using 1. Introduction reflectivity and Doppler velocity but the double eyewall in Dujuan was not concentric (where the trajectory of the inner [2] Intense, highly symmetric tropical cyclones (TCs) eyewall possessed a helical pattern). Therefore, the ground- sometimes exhibit concentric eyewall radar reflectivity based velocity track display (GBVTD) derived 3-D struc- patterns [Willoughby et al., 1982; Willoughby, 1990; tures in typhoon Saomai is the first study to document the Willoughby and Black, 1996]. Only a very limited number evolution and structure of a concentric eyewall in a land- of TCs with concentric eyewalls have been documented by in falling TC. situ aircraft observations [e.g., Jorgensen, 1984; Willoughby [4] The Super Typhoon Saomai, the eighth typhoon to hit et al., 1982; Willoughby, 1990; Willoughby and Black, 1996]. China in 2006 formed over the Western Pacific Ocean on Airborne pseudo-Doppler radar analyses have revealed detail 6 August 2006 and then moved northwestward toward three-dimensional kinematic structures of concentric eye- southeastern China. While Saomai entered the East China walls in hurricane Gilbert (1988) and Rita (2005) [Dodge et Sea north of Taiwan, it strengthened rapidly to a Catagory 5 al., 1999; Houze et al., 2007]. Deep convection within the storm, reaching its peak intensity with a central pressure of inner eyewall is surrounded by a near echo-free moat, 898 hPa at 1200 UTC 9 August. The storm began to decay which in turn is surrounded by an outer ring of convec- steadily after 0000 UTC 10 August, about 9 hours prior to tion. Both convective regions typically are accompanied its landfall on Zhejiang, China at 0925 UTC 10 August. by well-defined local tangential wind maxima and up- During this 9-hour period, Saomai possessed a concentric draft. The maximum vorticity usually resides inside the eyewall. The intensity of Saomai decayed rapidly after inner eyewall, while the outer wind maximum is usually landfall while moving northwestward across Zhejiang. Floods, mudslides and landslides occurred along Saomai’s 1Key Laboratory of Mesoscale Severe Weather, Ministry of Education path in a total of seven provinces, including Zhejiang and of China, Nanjing, China. Fujian. Saomai was the strongest typhoon made landfall in 2Department of Atmospheric Sciences, Nanjing University, Nanjing, China since 1949. China. 5 3 [ ] As Saomai approached landfall, it was continuously Earth Observing Laboratory, National Center for Atmospheric monitored by China’s next radar 1998 weather surveillance Research, Boulder, Colorado, USA. 4Department of Atmospheric Sciences, National Taiwan University, Doppler (CINRAD WSR-98D) located at Wenzhou (WZRD) Taipei, Taiwan. during an 12-hour period (00 to 12 UTC, 10 August 2006) with unprecedented 6-minute volume resolution (Figure 1a) Copyright 2008 by the American Geophysical Union. to document the formation and evolution of Saomai’s con- 0094-8276/08/2007GL032773$05.00

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Figure 1. (a) Track and radar coverage of Super Typhoon Saomai from 0000 UTC 10 August to 1227 UTC 10 August 2006. Range rings indicate maximum Doppler coverage (150 km) for Wenzhou (WZRD) CINRAD WSR-98D radars. Colors indicate JTWC best track Saffir-Simpson scale intensity. (b–g) Constant altitude PPI reflectivity at 2 km at six different analysis times. centric eyewall. It was also the first concentric eyewall TC contrast, the outer rainbands intensified and formed an outer captured by a CINRAD WSR-98D. This preliminary study eyewall, creating a moat region. The outer eyewall intensi- examines the kinematic and dynamic structures of Saomai’s fied at 0655 UTC (Figure 1e) with high reflectivity concentric eyewall at five time periods using the GBVTD (> 40 dBZ) and began to contract. The concentric eyewall technique [Lee et al., 1999] to derive axisymmetric kine- structure broke down at 0850 UTC (Figure 1f ), about matic and dynamic structures from the WZRD CINRAD 0.5 hour prior to the landfall while the inner eyewall became WSR-98D data. increasingly asymmetric with an enhanced convection on its southeast side. Saomai’s eye could still be identified one 2. Radar Analysis hour after it made landfall at 1033 UTC (Figures 1g) while the moat region was filled with precipitation (> 40 dBZ). [6] The domain of the GBVTD analyses extends from the [8] The evolution of Saomai is illustrated using GBVTD- center of the TC to 70 km radius and from 1 km to 12 km in derived radial profiles of the axisymmetric tangential winds the vertical. Grid spacing in both the radial and vertical at z = 2 km at five different times; 0355, 0537, 0655, 0850, directions is 1 km, consistent with the radar sampling and 1033 UTC (Figure 2). At 0355 UTC, the most intense resolution. The TC center is first determined at each altitude mean reflectivity was in the eyewall (R = 20 km) associated as a point that yields the maximum circulation enclosed by with a maximum mean tangential wind of 53 m sÀ1. the radius of maximum wind (RMW) using the GBVTD- Beyond R = 38 km, there was an outer ring of high simplex algorithm [Lee and Marks, 2000]. Then, the quan- reflectivity, but no accompanying secondary wind maxi- tities, including the along-beam (connecting the radar and mum. At 0537 UTC, a secondary wind maximum of TC center) component of the mean wind, axisymmetric 49 m sÀ1 appeared at R = 48 km near the outer ring of tangential and radial winds, and the asymmetric tangential high reflectivity (Figure 1). Such wind maximum formed winds, are deduced by the GBVTD analysis. Once the near the outer convective ring but lagged behind the azimuthal mean tangential and radial winds are obtained reflectivity maximum, implying a forced response of tan- at each radius and height, dynamic quantities, such as the gential wind to a heat source [Shapiro and Willoughby, vertical velocity, vorticity, angular momentum and pressure 1982]. At 0655 UTC, the maxima winds at the inner and deficit, are computed from the GBVTD-derived axisym- outer eyewalls increased to 55 m sÀ1 and 50 m sÀ1 metric circulations [Lee et al., 2000; Lee and Bell, 2007]. respectively, accompanied with a slightly decrease of mean The vertical velocity was calculated from the radial conver- reflectivity in the inner eyewall. Between 06550850 UTC, gence field using the kinematic method [Armijo, 1969]. the RMW of outer eyewall contracted rapidly from R = [7] The low-level reflectivity structures, illustrated using 45 km to R = 34 km (Figure 2a) concurrent with the radius 2 km constant altitude PPI (CAPPI), of Saomai at six time of maximum reflectivity contracting to 42 km and the periods (from 0231 UTC to 1033 UTC 10 August 2006) are tangential wind maximum increasing to 58 m sÀ1.In portrayed in Figures 1b–1g. At 0231UTC (Figure 1b), the contrast, the maximum wind in the inner eyewall increased eyewall reflectivity exhibited distinct asymmetric structure slightly to 60 m sÀ1 with the change of RMW less than accompanied by several spiral rainbands outside the eye- 1 km. It was noted that the tangential wind profile near the wall. Between 03550537 UTC (Figures 1c and 1d), the outer eyewall flattened so that no clear secondary maximum inner eyewall weakened, but became nearly symmetric. In tangential wind was observed at 0850 UTC. As suggested

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Figure 2. Radial profiles at z = 2 km from five analysis times (0355, 0537, 0655, 0850, and 1033 UTC). Different line styles/colors represent analysis times: (a) mean tangential wind (m sÀ1), (b) mean vertical vorticity (sÀ1*10À3), (c) mean reflectivity (dBZ), and (d) perturbation pressure deficit (hPa) assuming zero at 70 km radius. Analysis domain extends to 70 km radius. by Kossin et al. [2000], the short distance between outer complete symmetric vorticity profile in Saomai’s inner core eyewall and inner eyewall might flatten the tangential wind using Doppler data alone, the barotropic instability mecha- near the outer eyewall. The reflectivity profile shows the nism [Schubert et al., 1999] may be responsible for the moat region was filled with hydrometeors (> 40 dBZ), which breakdown of the inner eyewall between 05370655 UTC, implies the collapse of the moat region. After landfall, the where the ring vorticity pattern evolved into a monotonic maximum axisymmetric tangential wind speed decreased pattern. In contrast, the vorticity ring in the outer eyewall rapidly to 45 m sÀ1 at 1033 UTC, but the maximum initially was broader and weaker before 0655 UTC. It tangential wind evolved into a more uniform profile, consis- steadily intensified and became narrower until 0850 UTC tent with a near uniform reflectivity field (Figure 2c). The with a peak vorticity of 0.002 sÀ1 at R = 31 km, then usual eyewall replacement cycle [Willoughby, 1990] was became broader and weaker after landfall at 1033 UTC. The probably disrupted by Saomai’s landfall. evolution of the radial profiles of the axisymmetric tangential [9] The radial vorticity patterns are very similar to those wind (Figure 2a) and vorticity (Figure 2b) between 0655 and observed in classic concentric eyewalls with the large 1033 UTC is similar to the numerical simulations presented vorticity just inside the inner eyewall and an enhanced by Kossin et al. [2000], who suggested that the nonlinear vorticity in the outer eyewall at 0537, 0655 and 0850 mixing from instability across the moat will perturb the UTC [Kossin et al., 2000]. The vorticity ring in the inner vortex and weaken the vorticity of outer ring, when the radial eyewall, which satisfies the necessary condition for com- extent of the moat is sufficiently narrow. Through this case, a bined barotropic-baroclinic instability [Schubert et al., forecaster may have been able to monitor the vorticity profile 1999] existed in 0537 UTC with a peak vorticity of changes with the high temporal Doppler radar data, and 0.01 sÀ1 at R = 12 km. Subsequently, it rapidly evolved nowcast the evolution of hurricane structure and intensity. into a near monotonic pattern at 0655 UTC with a peak [10] The evolution of Saomai’s intensity was illustrated vorticity of 0.009 sÀ1 at R = 7 km, and re-evolved into a by central pressure deficit, which was estimated from ring pattern near landfall at 0850 UTC. Although the lack of GBVTD-derived winds (The symmetric radial gradient wind scatterers within the eye prohibits us from obtaining the equation was integrated inward, assuming the total pressure

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and the outer wind maximum tilted about 60° from the zenith. This result is consistent with the azimuthal aver- ages of reflectivity field, i.e., smaller vertical tilt in the inner eyewall and larger tilt in the outer eyewall. The derived secondary circulation (vectors, Figure 3b) shows an inflow beyond R = 50 km below 2-km height, which turned into an updraft beneath the outer eyewall and sloped outward with height. It is worth to point out that this inflow could be a proxy for the lower inflow pattern, since the strongest low-level inflow can’t be directly observed by a radar at far distance. Two gyres located near the updraft in the outer eyewall with the clockwise (counter-clockwise) gyre outside (inside) of the updraft, which enhanced the low-level outflow inside and the low- level inflow outside of the outer eyewall. These two gyres with the sloping updraft have been deduced in theoretical models [Eliassen, 1952; Shapiro and Willoughby, 1982]. In the inner eyewall, the primary updraft located just inside the reflectivity maximum. Part of this updraft entered the eye, and the rest flowed outward at mid- and high-levels turning into a downdraft. It was also noted that the low-level inflow outside of the inner eyewall, which was almost confined below 1-km height except for a narrow region at R = 30 km, was much weaker and shallower than that outside of outer eyewall. This weak inflow indicates that the high- energy air from the large-scale environment was impeded by the outer eyewall. Between R = 28 km and 38 km, there was a moat region, characterized by low reflectivity (<35 dBZ) and a weak downward air motion. This downward motion (Figure 3b) was partly induced by the secondary circula- tion associated with the outer eyewall, in agreement with other concentric eyewall TCs observations and modeling studies [e.g., Houze et al., 2007; Wang, 2008]. [12] The Saomai’s outer eyewall contracted and the moat region was filled with precipitation between 05370850 UTC (Figures 3a and 3c). The tangential wind Figure 3. Radius-height cross-section of the azimuthal profile indicated the primary wind maximum increased to À1 mean structure at (a, b) 0537 UTC and (c, d) 0850 UTC. more than 60 m s in the inner eyewall and the Color represents reflectivity in dBZ; contours are mean secondary wind maximum associated with the outer eye- tangential wind (Figures 3a and 3c, m sÀ1), and mean radial wall was not clear due to the increase of wind speed in the wind (Figure 3b and 3d, m sÀ1). Vectors in Figures 3b and moat (see Figure 3c). Figure 3d indicates the low-level 3d indicate mean secondary circulation. inflow outside the outer eyewall at 0537 UTC had intensified and penetrated through the outer eyewall to produce the at R = 70 km remained steady.) [Lee et al., 2000; Lee and weak updraft in moat region. Such enhanced inflow could Bell, 2007]. Saomai’s center pressure deficit changed slightly partly be explained by the response to increasing latent heat from 0355 to 0537 UTC, then rapidly dropped about 9 hPa release associated with the enhanced convection in the moat between 0537  0850 UTC. The rapid intensification near or and the enhanced surface friction over land [Lee et al., at landfall was also observed during the landfall typhoon 2000]. The improvement of the lowest level analysis could Herb (1996) by Taiwan Doppler radar [Chang, 2000]. Such a be another important factor as the storm moved closer to the sudden intensification of the storm during its landfall period radar. The enhanced inflow, in conjunction with rain-filled posed challenges for the local forecasters to forecast very moat region with upward motion also indicated a breakdown short-term typhoon intensity changes before landfall. After in the outer eye as a barrier to radial inward flow. Apparently, Saomai’s landfall at 1033 UTC, it rapidly weakened and its the influence of landfall may be responsible for the break- central pressure filled 12 hPa in one hour. down of the outer eyewall. It is also noted that the inner [11] Figure 3 shows the vertical cross sections of Saomai’s eyewall was associated with weaker updraft and enhanced axisymmetric structure at 0537 UTC and 0850 UTC. At low-level outflow, which implied the inner eyewall was in 0537 UTC, the tangential wind speed (solid lines) and the dissipating stage. reflectivity (in color) indicate that the maximum axisymmet- ric tangential wind at the inner and outer eyewalls exceeded À1 À1 3. Concluding Remarks 52 m s and 48 m s , respectively at z = 1 km, and the wind speeds decreased with height (Figure 3a). The inner [13] In this study, the kinematic and dynamic structures wind maximum tilted about 20° from zenith in the vertical of the concentric eyewall in Super Typhoon Saomai during

4of5 L07807 ZHAO ET AL.: RADAR OBSERVATION OF TYPHOON SAOMEI L07807 landfall were examined at five different times by the Chang, P. L. (1996), Circulation change analysis on landfalling typhoon: A case study Herb (1996), Ph.D. dissertation, 158 pp., Natl. Taiwan Univ., GBVTD analysis using the coastal CINRAD WSR-98D Taipei, Taiwan. data in China. The GBVTD-derived tangential wind, vor- Dodge, P., R. W. Burpee, and F. D. Marks Jr. (1999), The kinematic struc- ticity and perturbation pressure deficit reveal that Saomai ture of a hurricane with sea level pressure less than 900mb, Mon. Weather Rev., 127, 987–1004. formed an outer eyewall about 5 hours prior to landfall Eliassen, A. (1952), Slow thermally or frictionally controlled meridional when the strong updraft and shallow inflow collaborated circulation in a circular vortex, Astrophys. Norv., 5, 19–60. with the high reflectivity in both inner and outer eyewalls, Hong, J.-S., and P.-L. Chang (2005), The trochoid-like track in Typhoon while the moat region between the two eyewalls was Dujuan (2003), Geophys. Res. Lett., 32, L16801, doi:10.1029/ 2005GL023387. characterized by a weak downward motion and a lower Houze, R. A., Jr., S. S. Chen, B. F. Smull, W. C. Lee, and M. M. Bell reflectivity. The outer eyewall then strengthened and con- (2007), Hurricane intensity and eyewall replacement, Science, 315, tracted, while the inner eyewall broke down and weakened. 1235–1239. Jorgensen, D. F. (1984), Mesoscale and convective-scale characteristics of The rain-free moat region was filled with high reflectivity mature hurricanes, part I: General observations by research aircraft, (>40 dBZ) and weak updraft prior to landfall. Meanwhile, a J. Atmos. Sci., 41, 1268–1286. broad region of high reflectivity and a flat tangential wind Kossin, J. P., W. H. Schubert, and M. T. Montgomery (2000), Unstable interactions between a hurricane’s primary eyewall and a secondary ring profile formed outside the inner eyewall, accompanied with of enhanced vorticity, J. Atmos. Sci., 57, 3893–3917. steadily enhanced tangential wind, low-level radial inflow, Lee, W.-C., and M. M. Bell (2007), Rapid intensification, eyewall contrac- and a 9 hPa drop of the center pressure. It was noted that the tion, and breakdown of Hurricane Charley (2004) near landfall, Geophys. eyewall replacement cycle didn’t complete due to the Res. Lett., 34, L02802, doi:10.1029/2006GL027889. Lee, W.-C., and F. D. Marks Jr. (2000), Tropical cyclone kinematic struc- landfall. ture retrieved from single Doppler radar observations. part II: The [14] In future study, we propose to (1) analyze the GBVTD-simplex center finding algorithm, Mon. Weather Rev., 128, evolution and three-dimensional structures of Saomai’s 1925–1936. Lee, W.-C., J.-D. Jou, P.-L. Chang, and S.-M. Deng (1999), Tropical concentric eyewall in six-minute intervals over a six-hour cyclone kinematic structure retrieved from single Doppler radar ob- period during landfall, (2) deduce the asymmetric structure servations. part I: Interpretation of Doppler velocity patterns and the of Saomai, and (3) investigate the effect of landfall on the GBVTD technique, Mon. Weather Rev., 127, 2419–2439. eyewall structure by analyzing the symmetric tangential Lee, W.-C., J.-D. Jou, P.-L. Chang, and F. D. Marks (2000), Tropical cyclone kinematic structure retrieved from single-Doppler radar observa- wind and high-resolution numerical model simulations. This tions. part III: Evolution and structure of Typhoon Alex (1987), Mon. data set can be used for testing the assimilation of Doppler Weather Rev., 128, 3892–4001. radar data in hurricane models like HWRF. Schubert, W. H., M. T. Montgomery, R. K. Taft, T. A. Guinn, S. R. Fulton, J. P. Kossin, and J. P. Edwards (1999), Polygonal eyewalls, asymmetric eye contraction, and potential vorticity mixing in hurricanes, J. Atmos. [15] Acknowledgments. We would like to thank F. Zhang for his Sci., 56, 1197–1223. comments on the analysis. Internal reviews provided by Y. Wang and Shapiro, L. J., and H. E. Willoughby (1982), The response of balanced M. Bell greatly improved this manuscript. We appreciate the comments hurricanes to local sources of heat and momentum, J. Atmos. Sci., 39, and suggestions provided by two anonymous reviewers that significantly 378–394. improved the content and clarity of this manuscript. We would also like Wang, Y. (2008), Rapid filamentation zone in a numerically simulated to acknowledge the China Meteorological Administration for collecting tropical cyclone, J. Atmos. Sci., in press. and archiving the radar data used in this study. This study is supported Willoughby, H. E. (1990), Temporal changes in the primary circulation in in whole or in part by the National Natural Science Foundation of tropical cyclones, J. Atmos. Sci., 47, 242–264. China (grants 40505004, 40405012, and 40333025), the National Grand Willoughby, H. E., and P. G. Black (1996), Hurricane Andrew in Florida: Fundamental Research 973 Program of China (973; 2004CB418301), Dynamics of a disaster, Bull. Am. Meteorol. Soc., 77, 543–652. and the NMC-TIGGE Program GYHY (QX) 2007-232 6-1. The first Willoughby, H. E., J. A. Clos, and M. G. Shoreibah (1982), Concentric author would like to thank NTU for a postdoctorship in 2005 and 2006. eyewalls, secondary wind maxima, and the evolution of the hurricane vortex, J. Atmos. Sci., 39, 395–411. References ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ Armijo, L. (1969), A theory for the determination of wind and precipitation B. J.-D. Jou, Department of Atmospheric Sciences, National Taiwan velocities with Doppler radars, J. Atmos. Sci., 26, 570–573. University, Taipei, 10617, Taiwan. Blackwell, K. G. (2000), The evolution of Hurricane Danny (1997) at W.-C. Lee, Earth Observing Laboratory, National Center for Atmospheric landfall: Doppler-observed eyewall replacement, vortex contraction/ Research, Boulder, CO 80307, USA. intensification, and low-level wind maxima, Mon. Weather Rev., 128, K. Zhao, Department of Atmospheric Sciences, Nanjing University, 4002–4016. Nanjing 210093, China. ([email protected])

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