The Inland Eyewall Reintensification of Typhoon Fanapi

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The Inland Eyewall Reintensiﬁcation of Typhoon Fanapi (2010) Documented from an Observational Perspective Using Multiple-Doppler Radar and Surface Measurements

YU-CHIENG LIOU,TAI-CHI CHEN WANG, AND PEI-YU HUANG Department of Atmospheric Sciences, National Central University, Jhongli, Tao-Yuan City, Taiwan

(Manuscript received 4 April 2015, in ﬁnal form 27 September 2015)

ABSTRACT

This study documents observational changes in the eyewall of Typhoon Fanapi (2010) after landfall in Taiwan. The observations indicate that Fanapi’s eye and eyewall disappeared on the eastern side of Taiwan’s Central Mountain Range (CMR) after landfall, but reemerged on the western side of CMR. The cyclonic circulation, increasing wind speed, a low-level low pressure and high temperature zone, the associated updrafts and downdrafts, and surface pressure and rainfall measurements all support the existence of a reintensified eyewall. The storm slowed down during the redeveloping stage, thus prolonging the rainfall duration over Taiwan. On the western side of CMR a northwest–southeast-oriented rainband formed at an earlier stage, possibly due to the large-scale interaction between Fanapi’s remnant flow and the environment. However, the subsequent reintensification might be attributed to the interaction between the circulation and topography. This is supported by the finding that adjacent to CMR, strong wind develops vertically from lower levels, indicating that the reintensification appears to be initiated through a bottom-up process. A vorticity budget analysis shows that at lower layers the stretching mechanism plays a leading role in increasing positive vorticity, followed by the contributions from tilting and horizontal advection. The horizontal advection plays a comparable role to the vertical advection in increasing low- to midlevel vorticity. The vertical advection aloft is responsible for transporting the vorticity upward. Finally, this research provides a relatively rare documentation of the vortical hot towers (VHTs) over terrain using ground-based radars, in contrast to most previous studies focusing on maritime VHTs using simulations or aircraft measurements.

1. Introduction core (Shea and Gray 1973; Gray and Shea 1973); the effects of eye formation in stabilizing the vortex (Schubert The eyewall of a tropical cyclone (TC) contains tow- and Hack 1982); the storm intensification associated ering convection, strong wind, and torrential rain. The with a wavelike inflow surge to the storm core (Molinari investigation of the eyewall’s formation, structure, and and Skubis 1985); the observational interpretation of TC evolution is an important research topic, since the vari- eye thermodynamics (Willoughby 1998); the dynamical ability of the eyewall is closely linked to the TC intensity mechanism for the formation of a polygonal eyewall, changes. It is well known that there has been only very asymmetric eye contraction, and vorticity redistribution limited progress in the prediction of the TC intensity, (Schubert et al. 1999); the transitions of the kinematic and compared with the significant improvement in TC track thermodynamic distributions within the hurricane eye forecasts in recent decades. Numerous studies have been and eyewall through horizontal vorticity mixing (Kossin conducted to better understand the behaviors of the eye and Eastin 2001); and the concentric eyewall replacement and eyewall of the typhoon focusing on different aspects. cycle (Willoughby et al. 1982; Black and Willoughby Some examples are the description of the structural 1992; Houze et al. 2007; Judt and Chen 2010). characteristics and variability of the hurricane’s inner Most of the above-mentioned articles focused on studying the eye and eyewall over a flat surface such as the ocean. However, when TCs make landfall on an Corresponding author address: Dr. Tai-Chi Chen Wang, Department of Atmospheric Sciences, National Central University, No. 300, Jhongda island, a different evolution is often observed due to the Road, Jhongli, Tao-Yuan City, 320 Taiwan. interaction between the TC circulation and the terrain. E-mail: [email protected] Wu et al. (2003) and Wu et al. (2009) used satellite data

DOI: 10.1175/MWR-D-15-0136.1

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examine the rainfall distribution, track continuity, and deflection induced by orographic effects as a typhoon passed over the CMR or a similar island mountain range. Using a series of numerical simulations to investigate the looping motion of Typhoon Krosa (2007), Huang et al. (2011) found that the terrain height of Taiwan played the most important role. Strong channel winds enhanced between the storm and the terrain when deflection oc- curred. By analyzing 84 typhoons that reached the eastern coast of Taiwan, Hsu et al. (2013) reported that the terrain-induced track deflection can be explained by the asymmetry of convectively generated potential vorticity. Peng et al. (2012) reviewed 131 westbound cross-Taiwan TCs between 1897 and 2009, and found that the continuity of the TC track depended on the landfall location, the approaching direction, and the maximum wind speed of the cyclone. From numerical simulations, Wang et al. (2012) and Wang et al. (2013) demonstrated the sensi- FIG. 1. Locations of the five weather radars (RCHL, RCMK, tivity of typhoon tracks to asymmetric latent heating/ RCCG, RCKT, and TEAM-R) (triangles), the Samhua station (square), the Gunsan station (star), and the disdrometer (circle). rainfall induced by Taiwan’s topography during Ty- Terrain height is denoted by shading with an interval of 0.25 phoons Morakot (2009) and Fanapi (2010), respectively. (1.0) km below (above) 1.0-km height. The inner domain denotes Their research reveals that the reduction of the storm’s the wind analysis area. Red and blue solid lines mark the location speed can be attributed to the asymmetric latent heating of 08 and 2108 RHI scan of TEAM-R. Meteorological and terrain rather than the environmental flow. data in boxes U and H are used to estimate the Froude number, respectively. The coordinates are the distance (in km) to Typhoon Fanapi made landfall on Taiwan at 0040 UTC RCCG radar. 19 September 2010. Its eyewall disappeared after landfall, but went through a reintensification process over land. and numerical simulation to illustrate that when Typhoon During this reintensification process, a record-breaking Zeb (1998) made landfall at Luzon (island in the Philip- 600 mm of rainfall within 7 hours was observed over the pines), the eyewall experienced contraction before land- southwestern plain of the island. This study utilizes a newly fall, weakening and/or breakdown during landfall, and designed advanced multiple-Doppler wind synthesis reformation after moving offshore. By examining 23 ty- method, high-resolution observed and retrieved products phoons crossing the Philippines and 6 typhoons crossing from five Doppler radars, as well as surface observations to Taiwan, Chou et al. (2011) reported the statistical char- document the reintensification process. The investigation acteristics of the eyewall evolution in terms of its expan- is presented from an observational perspective to examine sion, reorganization, and contraction. They also pointed the eyewall structure of Typhoon Fanapi during the entire out that small vertical wind shear, high low-level relative landfall period. Possible mechanisms responsible for this humidity, and sea surface temperature were important inland reintensification are also proposed. factors for the eyewall reorganization and the following The rest of this article is arranged as follows. Section 2 contraction when the typhoon moved offshore. introduces the locations of the instruments and the The topography of Taiwan is dominated by the north– method for retrieving the three-dimensional wind fields. south-oriented Central Mountain Range (CMR; see In section 3 the eyewall structures before and after the Fig. 1), which has peaks reaching almost 4000 m (MSL) passage of Typhoon Fanapi over the CMR are com- and stretches a distance of about 300 km. The CMR can pared. Section 4 highlights the role played by the local substantially alter the structure and motion of a typhoon topography by showing the vertical development of a that crosses the island. In idealized numerical experi- band of strong wind through a bottom-up process. Sec- ments by Chang (1982), the original low-level TC center tion 5 presents more evidence to support the existence failed to pass the mountain range, a secondary low-level of a reintensified eyewall on the western side of the circulation center formed in the induced lee trough. CMR before moving offshore. The signature of vortical Bender et al. (1987) also showed the development of hot towers developing over land, a possible mechanism secondary lows at low levels in the lee of the mountain in for the eyewall reintensification, is documented in sec- Taiwan. Yeh and Elsberry (1993a,b), Lin et al. (2005), tion 6, followed by conclusions and future work in and Jian and Wu (2008) utilized numerical simulations to section 7.

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FIG. 2. Track of Typhoon Fanapi (2010). The landfall time is 0040 UTC 19 Sep. The typhoon symbol represents the location of the storm, marked every 6 hours.

2. Locations of instruments and methodology for an advanced algorithm designed by Liou and Chang three-dimensional wind analysis (2009) and Liou et al. (2012), named the Wind Synthesis System using Doppler Measurements (WISSDOM), is Figure 1 depicts the topography of southern Taiwan, employed. This method has several advantages over other the analysis domain (approximately 104 km2), as well as commonly used traditional approaches. For example, it is the locations of the observational instruments. They in- able to recover the wind field along the radar baseline and clude three S-band Doppler radars (RCHL, RCCG, and immediately above the mountain slopes. The latter is ac- RCKT), one C-band dual-polarimetric radar (RCMK), complished by employing the so-called immersed bound- one X-band dual-polarimetric mobile radar [the Taiwan ary method (Tseng and Ferziger 2003), so that the Experimental Atmospheric Mobile-Radar (TEAM-R)], topographic forcing can be taken into account during the two surface stations (Samhua and Gunsan), and one wind retrievals. The retrieved three-dimensional winds disdrometer. Note that the RCHL data are not utilized satisfy the vertical vorticity equation, thus a direct vorticity for the multiple-Doppler wind synthesis. budget analysis can be performed without producing an A weather radar is capable of probing the structure of extra residual term. Furthermore, the resulting wind fields the precipitating systems with high temporal (,10 min) can be readily applied to derive the thermodynamic and spatial (;1 km) resolution. A Doppler radar detects structure of the weather system with a higher accuracy the reflectivity and radial winds (V ). The latter is a r (Protat and Zawadzki 2000). To the authors’ best knowl- projection of the three-dimensional wind field (u, y, w) edge, among all the existing multiple-Doppler radar wind along the radar beam, and can be expressed as synthesis methods, WISSDOM is the only algorithm that 1 can achieve all the goals mentioned above. Although V 5 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi [xu 1 yy 1 z(w 1 W )], (1) r x2 1 y2 1 z2 T the design of this synthesis method allows it to have the flexibility to use data from any number of radars, the where (x, y, z) are the coordinates of the observed point current study is the first attempt to apply this method relative to the radar, and WT is the terminal velocity of using data simultaneously from four Doppler radars. the hydrometeors, which can be estimated empirically from the radar reflectivity (Sun and Crook 1997). 3. Analysis results Using the radial winds measured by more than one a. The eyewall evolution on the eastern side of CMR radar, techniques have been developed to reconstruct the three-dimensional flow field (e.g., Armijo 1969; Protat and Figure 2 shows that Typhoon Fanapi (2010) formed at Zawadzki 1999; Chong and Bousquet 2001). In this study, 1800 UTC 14 September 2010 over the western North

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FIG. 3. Multifunctional Transport Satellite (MTSAT) infrared (IR) imagery at (a) 2000 UTC 18 Sep, and (b) 0100, (c) 0800, and (d) 1400 UTC 19 Sep for Typhoon Fanapi (2010).

Pacific Ocean near 19.08N, 129.58E, and then moved 0100 UTC 19 September, only 20 min after landfall, steadily westward to make landfall on the eastern coast of Fig. 3b shows that the eye of Fanapi disappears from the Taiwan at 0040 UTC 19 September. This path is classified satellite imagery. At 0800 and 1400 UTC 19 September as the ‘‘type 3 track’’ by the Central Weather Bureau (Figs. 3c and 3d), the main body of Fanapi is located on (CWB) of Taiwan. According to statistics released by the the western side of the island. The satellite imagery CWB, from 1949 to 2010, about 13.4% of all 172 west- shows a more compact cloud pattern at 1400 UTC than bound typhoons, or 23 typhoons made landfall on Taiwan that at 0800 UTC, implying that the storm’s structure following this track type. The track of Fanapi shown in becomes more organized. Fig. 2 also reveals that the storm substantially slowed on Since a radar is capable of probing the internal structure the western side of CMR. of a precipitating system, the behaviors of the typhoon The Multifunctional Transport Satellite (MTSAT) In- eyewall before and after the landfall are further infrared (IR) imagery at 2000 UTC 18 September illus- vestigated by displaying the composite maximum radar trated in Fig. 3a indicates that 4 hours prior to landfall, the reflectivity from 1800 UTC 18 September to 0600 UTC storm center can be clearly identified over the ocean. At 19 September in Fig. 4. Basically, similar features revealed

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FIG. 4. Composite maximum radar reﬂectivity (5-dBZ interval) at (a) 1800, (b) 2000, and (c) 2200 UTC 18 Sep; and (d) 0000, (e) 0200, (f) 0300, (g) 0400, (h) 0500, and (i) 0600 UTC 19 Sep 2010 for Typhoon Fanapi. The coordinates show the distance (in km) relative to RCCG. by the satellite data regarding the evolution of Fanapi’s at a proper position to observe this typhoon. Although eyewall are also detected by the radar observations. The the Doppler radial wind only reveals the wind compo- structure of the eye and eyewall broke down after Fa- nent along the direction parallel to the radar beam, it napi made landfall (Figs. 4a–f). The remnant rainband can still be used to explore the changes in TC intensity, (Figs. 4g–i) did not exhibit any similarity to the cyclonic as long as the radar is not located right inside the eye- typhoon circulation. wall. Since RCHL is located to the north of the landfall It should be pointed out that before Fanapi makes spot, we select a 1.398 low elevation angle plan position landfall on the eastern coast of Taiwan, RCHL (see indicator (PPI) scan by RCHL, and examine the radial Fig. 1 for RCHL’s location) is the only radar that resides wind within a quadrant from east to southwest of the

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FIG. 5. Magnitude of the maximum radial wind detected by RCHL from a 1.398 PPI scan from 0000 to 0500 UTC 19 Sep. The radar scan covered an area with azimuthal angles from 908 to 2008 relative to RCHL. radar site (i.e., from 908 to 2008 azimuthal angles relative (Figs. 6i–l), a complete but asymmetric eyewall gradu- to RCHL). The result depicted in Fig. 5 indicates that in ally formed over southwestern Taiwan. At 1200 UTC a this region where the eyewall is covered, the maximum near-circular pattern was observed with a radius of 2 radial wind speed decreases from 35 m s 1 at 0100 UTC 50 km (Fig. 6j). From 1300 to 1400 UTC (Figs. 6k–l), the 2 19 September (20 min after landfall) to 22 m s 1 at eyewall extended to approximately 75 km in radius, and 0200 UTC 19 September (80 min after landfall), suggesting moved northwestward toward the ocean. Figure 7 de- a rapid and significant weakening of the typhoon in- picts the time series of the hourly rainfall rate and ac- tensity immediately after landfall. cumulated precipitation at the Gunsan site (see Fig. 1 for Overall, evidence obtained from both the satellite and its location) on 19 September. It can be seen that after radar observations show that the typical structure of the 0500 UTC, or approximately 4 hours after the landfall eyewall is absent after Fanapi makes landfall on the on the eastern coast at 0040 UTC, the Gunsan station 2 eastern coast of Taiwan. begins to record heavy rainfall (.15 mm h 1) for 17 consecutive hours. The accumulated rainfall at this city b. The evolution of the reintensified eyewall on the reaches more than 900 mm in a single day. This suggests western side of CMR that the rainfall duration is prolonged by the reemerged The RCCG radar is located at a favorable place (Fig. 1) TC eyewall, leading to the persistent torrential rainfall for observing Fanapi when the storm is on the western over the southwestern plain of Taiwan. side of CWB. Thus, the radar reflectivity fields from a c. The wind fields of the reintensified eyewall retrieved 1.398 PPI scan made by RCCG from 0300 to 1400 UTC by Doppler radars 19 September are displayed in Fig. 6. Note that the times of Figs. 6a–d are overlapped with Figs. 4f–i for a contin- Unlike the situation on the eastern side of CMR, three uous description of the storm’s evolution. It should be more radars, including RCKT, RCMK, and TEAM-R, pointed out that at this elevation angle the radar beam are available on the western side of CMR. This allows us would reach about Z 5 2.43 km on the eastern side of to conduct a multiple-Doppler three-dimensional wind CMR. From 0300 to 0600 UTC (Figs. 6a and 6d), a synthesis for this area. The size of the analysis domain is northwest–southeast-oriented line-shaped rainband 100 3 100 3 15 km3, and the synthesized flow fields have a gradually formed on the western side of CMR, possibly horizontal and vertical resolution of 1.0 and 0.5 km, due to the large-scale interaction between the remnant respectively. storm with the environmental flow, but it did not exhibit To better describe the reemerged eyewall structure, any signature that was similar to a typical TC eyewall. In the retrieved low-level horizontal wind fields using four contrast, from 0600 to 1400 UTC (Figs. 6d–l), this con- radars (i.e., RCCG, RCMK, RCKT, and TEAM-R) are vective rainband gradually gained curvature on the presented. From 0600 to 0700 UTC (Figs. 8a,b), the western side of CMR, and eventually evolved into a spiral northerly airflow mainly came from the remnant of the shape. The eastern edge of the rainband was deflected weakened storm on the eastern side of the CMR. It toward the north, and became nearly parallel to the local converged with the northwesterly flow, helping to north–south-oriented topography. From 1100 to 1400 UTC maintain an east–west-oriented rainband. The direction

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FIG. 6. Radar reﬂectivity (5-dBZ intervals) observed by RCCG at an elevation angle of 1.398 at (a) 0300, (b) 0400, (c) 0500, (d) 0600, (e) 0700, (f) 0800, (g) 0900, (h) 1000, (i) 1100, (j) 1200, (k) 1300, and (l) 1400 UTC 19 Sep 2010 for Typhoon Fanapi. The thick and thin solid lines denote the coastline of Taiwan and the Z 5 500-m contour line, respectively. The box enclosed by the dashed line in (h) is the region where the retrieved wind ﬁelds are displayed in Fig. 8. The triangle in (h) marks the location of Gun-San surface station.

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FIG. 7. The hourly (histogram) and accumulated (solid line) rainfall at Gunsan station from 0100 to 2400 UTC 19 Sep 2010. of the prevailing wind immediately along the western that starting from 0800 UTC, the wind speed in the edge of CMR was either northerly or northwesterly. eyewall at Z 5 2.0 km gradually increases with time, Thus, the wind component deflected toward north was indicating the reintensification of the storm occurring at not obvious during this period of time. In contrast, from low levels. The area with the maximum wind speed 0800 to 1000 UTC (Figs. 8c–e), a closed cyclonic circu- moves counterclockwise as the time evolves from 0900 lation began to reemerge. Starting from 0900 UTC to 1200 UTC. (Fig. 8d), the storm center can be clearly identified by Based on a 20-yr (1971–90) dataset from 22 surface examining the streamlines (not shown). The wind speed stations, Chang et al. (1993) showed that a leeside sec- 2 at this time reaches 35 m s 1, equivalent to the TC in- ondary low could develop on the west coast of Taiwan tensity on the eastern side of CMR before landfall. It is when a typhoon center was located within a special re- worth mentioning that Brand and Blelloch (1974) ex- gion. Their research suggests that the observed scale of amined 22 typhoons (1960–72) that did not intensify this low is much smaller than that produced by the in- when crossing Taiwan. They found on average, the interaction between the mean flow and the CMR, and tensity of these typhoons decreased by 41% from 6 should be triggered by a localized factor related to the hours prior to hitting the island to 6 hours after leaving, against-mountain return flow. Nevertheless, the distri- but their moving speed remained almost unchanged bution of their data is not dense enough to reliably lo- after landfall. Thus, in this study, the recovery of the cate the circulation center. It is demonstrated from Fig. 8 tangential wind speed along the eyewall and the slow- that this difficulty has been resolved in our study. Note down of the storm’s translation speed (see Fig. 2) on the that the successful retrievals of the complete lee vortex western side of CMR constitute the evidence suggesting (Figs. 8d,e) are made possible due to the capability of that a reintensification process of the eyewall has been the WISSDOM method, which can simultaneously uti- taking place over the land. lize high-resolution data from any number of radars, and Near the CMR, the wind coming mainly from the west the availability of four radars near the analysis area. In is deflected instead of ascending over the mountains, addition, the deployment of the mobile radar TEAM-R apparently due to the blockage by the local north–south- at a critical location deep inside the mountainous area oriented topography. This can be confirmed by esti- provided crucial observations near the reorganized mating the Froude number Fr 5 U/(NmH), where U is typhoon center. the upstream wind speed, H is the mountain height, and Chou et al. (2011) utilized the Hovmöller display of

Nm is the saturated Brunt–Väisälä frequency defined in radar reflectivity to show that Typhoon Fanapi’s eyewall Durran and Klemp (1982). Computation is conducted reorganized after landfall. The results discussed in this using data collected at 0900 UTC near CMR (see Fig. 1). section, especially the well-retrieved leeside vortex cir- 2 After substituting appropriate values (U ; 2.3 m s 1, culation, provide convincing evidence to illustrate an 22 21 H ; 1265 m, Nm ; 2.43 3 10 s ), we have Fr ;0.4, inland reintensification process, which may still be at its indicating that the wind tends to flow around the terrain. early stage when compared with the description of Chou At 1200 UTC, the vortex center moved offshore et al. (2011). (Fig. 8f). The retrieved flow field from west, southwest, Figure 9 illustrates the radar reflectivity, vertical mo- and south basically exhibits a pattern typically found in tion, and horizontal divergence fields at 1000 UTC the southeast quadrant of a cyclonic circulation. Figure 8 along a vertical cross section depicted by the solid line in also depicts the evolution of a low-level isotach over a Fig. 8e. This was the time when a complete cyclonic flow period of 6 hours from 0600 to 1200 UTC. It can be seen already formed. The plot extends from the center of the

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FIG. 8. Multiple-Doppler radar synthesized horizontal ﬂow ﬁelds (wind barbs) at Z 5 2 km MSL at (a) 0600, (b) 0700, (c) 0800, (d) 0900, (e) 1000, and (f) 1200 UTC 19 Sep 2010. The thick line denotes the coastline of Taiwan. 2 The color shading indicates the isotach (5 m s 1 intervals). The blue solid lines denote the TEAM-R RHI scanning at 08 and 2108 azimuthal angles. The red dashed line in (e) denotes the location of the vertical cross section shown in Fig. 9. The small dot in (c)–(f) indicates the location of TEAM-R.

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21 21 FIG. 9. Multiple-radar synthesized vertical wind field (contours in m s with 1.0 m s interval) superimposed by (a) radar reflectivity (color shading in dBZ), and (b) horizontal di- 2 2 vergence (310 3, color shading in s 1) over a vertical cross section denoted by the solid line in Fig. 8e. Updrafts and downdrafts are plotted using white solid lines and black dashed lines, respectively. circulation to the southeast, where the eyewall cloud is structure of the eyewall obtained in our analysis is in evident as revealed by high radar reflectivity. It can be good agreement with the conceptual model proposed in clearly seen in Fig. 9a that updrafts of about 3.0– Willoughby’s research. 2 5.0 m s 1 are associated with the outward tilted eyewall cloud, which is characterized by strong radar reflectivity 4. The upward development of a band of strong reaching more than 45 dBZ at x ; 60 km. The updrafts wind speed near CMR are associated with a low-level convergence zone and a divergence zone aloft, as illustrated in Fig. 9b. On the Figure 10 depicts the difference of the horizontal wind other hand, Fig. 9a also shows downdrafts of about 21.0 speed between a layer (Z 5 2.0 km) lower than CMR 2 to 23.0 m s 1 along the inner edge of the eyewall. Ac- and a layer (Z 5 5.0 km) higher than CMR from 0800 to cording to Willoughby (1998), this descending flow may 1000 UTC. Inside a very small region near the southeast also be attributed to the evaporation effect. The corner of the domain and adjacent to the CMR, the wind

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FIG. 10. The difference of the horizontal wind speed between the layer at Z 5 2.0 km and Z 5 5.0 km at (a) 0800, (b) 0900, and (c) 1000 UTC 19 Sep 2010. Positive (negative) value indicates the wind speed at lower (higher) layer is 2 stronger. Contours highlight the area where the difference is above 10 m s 1. The storm center in (b) and (c) is marked by a typhoon symbol. speed difference is negative, implying that the horizon- one can identify a region at x 5 30 to 50 km, y 5260 tal wind speed at the lower layer is persistently smaller to 250 km where the wind speed at Z 5 2.0 km turns out than that at the higher layer. The reason is because at to be significantly greater than that at Z 5 5.0 km. This Z 5 2.0 km, the prevailing wind in this area is from al- area expands to a larger northeast–southwest-oriented most due west. It is slowed down when approaching the band on the western side of CMR at 0900 UTC north–south-oriented local topography. On the other (Fig. 10b), and becomes north–south oriented adjacent to hand, the wind field at Z 5 5.0 km, due to its higher al- the CMR at 1000 UTC (Fig. 10c). Within this band, the titude, is not affected too seriously by the terrains. incoming air flows at Z 5 2.0 km are westerly, but turn to Nevertheless, it can be seen that in general, the area with southwesterly or southerly when approaching the north– stronger low-level wind occupies most of the analysis south-oriented mountain (Fig. 8d). Although these azi- domain, and expands in scope with time. From Fig. 10a, muthal changes of the wind direction are consistent with

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FIG. 11. Radar radial wind observed by two TEAM-R RHI scans at (a),(e) 0806; (b),(f) 0905; (c),(g) 1002; and (d),(f) 1207 UTC 19 Sep 2010. Panels (a)–(d) are for 2108 azimuthal angle, and (e)–(h) are for 08 azimuthal angle. The abscissa is the distance (in km) to TEAM-R.

Unauthenticated | Downloaded 10/03/21 08:27 PM UTC JANUARY 2016 L I O U E T A L . 253 the cyclonic flow of a TC, it should be pointed that since this area is immediately along the western edge of the north–south-oriented CMR, this is also an indication that at 2.0-km height the role played by topography in deflecting the wind northward should also be taken into account. At 1000 UTC, the complete cyclonic circulation in the eyewall has already been established (Fig. 8e), and extended from Z 5 2.0 to 5.0 km (not shown). The aforementioned band with positive wind speed difference also exhibits a curved structure similar to that possessed by the eyewall (see Fig. 10c). To demonstrate that the positive difference of the horizontal wind speed displayed in Fig. 10 is not caused by the reduction of winds at higher altitude, Fig. 11 illustrates a series of radial wind observations at 0806, 0905, 1002, and 1207 UTC obtained from two range– height indicator (RHI) scans made by TEAM-R at FIG. 12. The vertical profile of the horizontal wind speed aver- 08 and 2108 azimuthal angles (see Fig. 1 for the directions aged over each horizontal layer at 0800 (dotted line), 0900 (dashed of these two RHI scans). Note that the cross section line), 1000 (solid line), and 1200 UTC (dotted–dashed line). The from the 08 RHI scan is immediately along the western average is made over these grid points where the radar reflectivity border of CMR, and is nearly parallel to the local north– is greater than 30 dBZ, and the wind speed ranks the top 30%. south-oriented topography. During this period of time, the center of the storm was situated generally to the west reintensification of Fanapi on the western side of CMR of TEAM-R (see Figs. 8d–f). Therefore, the TEAM-R- starts from lower levels, which might be attributed to the observed positive (negative) radial velocity at the interaction between the small-scale convective activities 08 (2108) azimuthal angle indicates southerly flow, and and the local topography. can be used as an index to describe the intensity change of the flow. From Fig. 11, it can be clearly seen that as the 5. Other evidence supporting the existence of the time evolves, an area of strong wind develops vertically reintensified eyewall from the lower to higher layers. Within this area, the wind speed recovers with time, and eventually reaches The thermodynamic retrieval technique proposed by 2 35 m s 1, which is also the magnitude found in Fanapi Gal-Chen (1978) is applied to derive the deviations before it makes landfall on the eastern side of CMR. The (from horizontal average) of the pressure and potential depth of this strong wind band increases approximately temperature perturbations from 0900 to 1000 UTC from 2.0 to 6.5 km within 3 hours. It is also found that the 19 September using the radar-synthesized wind infor- low-level strong wind region is shallower at the edge of mation. In Fig. 13, a near-surface (z 5 2 km) low pres- the eyewall, and deeper in the middle of the eyewall, as sure and high temperature region usually accompanying illustrated in Fig. 11. the TC eye can be identified. This feature is not tran- Furthermore, we compute the averaged horizontal sient, as it lasts for at least 1 hour. In Fig. 13c, the wind speed over each horizontal plane. The average is pressure depression and temperature rise over a dis- made over these grid points where the radar reflectivity tance of approximately 60 km from the periphery to the is greater than 30 dBZ, indicating that the focus is on the storm center can reach 24.0 hPa and 68C, respectively. evolution of the wind speed in the rainband/eyewall area. The hourly precipitation is recorded by a Joss– Furthermore, to highlight the growth of the strongest Waldvogel (JW) disdrometer deployed at the location of winds, the average is also made over points where the the TEAM-R site. As indicated in Fig. 14, the rainfall wind speed ranks in the top 30%. The result is displayed ceases in that area for about 3 hours from 0600 UTC, but Fig. 12. It can be seen that as time evolves from 0800 to resumes from 0900 UTC, which is also the time a closed 1200 UTC, the wind speed does increase at almost all cyclonic circulation is formed as revealed in Fig. 8d. altitudes. At 0900 and 1000 UTC, the maximum wind Furthermore, Fig. 14 also depicts that the surface pres- 2 speed reaching above 28 m s 1 occurs at about Z ; sure measured at the Samhua station (see Fig. 1 for its 2.0 km, and decreases with heights. At 1200 UTC, the location) steadily decreases by 7–8 hPa during a period of location of the maximum wind speed rises to Z ; 3.0 km. 10 hours, reaching a minimum at about 1000 UTC, fol- The plots in Figs. 11 and 12 appear to show that the lowed by a steady increase of 18 hPa over the next 12

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FIG. 13. Retrieved deviation of pressure (hPa, color shading with 0.5-hPa intervals) and potential temperature perturbations (K, contours with 1.0-K intervals) from their horizontal average at (a) 0900, (b) 0930, and (c) 1000 UTC 19 Sep 2010. The height is Z 5 2.0 km. The coordinates are the relative distance (in km) to RCCG. The storm center is marked by a typhoon symbol.

hours. We believe that this constitutes additional evi- 6. Signature of vortical hot towers dence suggesting the existence of a reemerged low pressure center, although its intensity may not have been Vortical hot towers (VHTs) are characterized by deep very strong. convection with collocated updrafts and positive vertical

21 FIG. 14. Hourly rainfall (mm h in histogram) observed by the disdrometer deployed at the TEAM-R site, and surface pressure (hPa in red line) measured at the Samhua station during 19 Sep 2010.

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23 21 21 FIG. 15. Retrieved vertical vorticity (310 in s ) and vertical velocity (in m s ) at (a),(b) 0800; (c),(d) 0900; (e),(f) 1000; and (g),(h) 1200 UTC 19 Sep 2010 at Z 5 2.0 km. The ﬁgures in (a),(c),(e), and (g) are for the vertical vorticity, and ﬁgures in (b),(d),(f), and (h) are for vertical velocity. The thick black line is the coastline of Taiwan. The storm center in (d),(f), and (h) is marked by a typhoon symbol.

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extremely tall convective towers existed in the eyewall of a tropical cyclone. Using airborne Doppler radar, Reasor et al. (2005) observed that multiple low- to mid- tropospheric mesoscale cyclonic circulations were co- incident with deep, vertically penetrating cumulonimbus convection within the pre-TC disturbance prior to the genesis of Hurricane Dolly (1996). The observational study conducted by Sippel et al. (2006) of Tropical Storm Allison (2001) indicated that the formation of multiple vortices was a key component of cyclogenesis through 23 21 FIG. 16. Vertical vorticity (310 , color shading in s ) and several scales. In Allison, the meso-g-scale vortices were 2 vertical velocity (contours in 1.0 m s 1) fields along the vertical likely associated with the hot towers. cross section marked by the thick solid line in Fig. 15g. Solid To the authors’ best knowledge, most of the previous (dashed) line represents the updraft (downdraft). observational studies of VHTs were conducted using air/ space-borne instruments, and focusing on the tropical cy- vorticity. These tall cumulonimbus towers can develop clones while they were still over oceans. In this research, it upward, and reach or even penetrate the tropopause. is attempted to provide a relatively rare documentation of Numerical simulations (e.g., Hendricks et al. 2004; the VHTs over terrain using ground-based radars. Montgomery et al. 2006) suggest that VHTs play an One of the advantages possessed by the multiple- important role in an upscale transfer mechanism by Doppler wind synthesis method (WISSDOM) employed which convective-scale vorticity anomalies can act in this study is that the retrieved three-dimensional wind through a collective manner to convert a tropical de- field is enforced to satisfy a simplified vertical vorticity pression into a cyclone. The VHTs overcome the det- equation in a least squares sense, as expressed in the rimental thermodynamic and dynamic effects of following: downdrafts by consuming convective available potential energy, moistening the troposphere, and merging Dz ›z 5 1 vort_adv 52(vort_div 1 vort_til), (2a) vortices with nearby hot towers. The net convergence Dt ›t of the angular momentum produced by small-scale ›y ›u z 5 2 , (2b) deep convection can exceed the spindown effect imposed ›x ›y by surface friction. The vortical cores can also support a ›z ›z ›z more efficient conversion of latent heat energy into rota- vort_adv 5 u 1 y 1 w , (2c) ›x ›y ›z tional kinetic energy of the horizontal winds, as suggested by Hack and Schubert (1986) using an axisymmetric ›u ›y vort_div 5 (z 1 f) 1 , and (2d) primitive equation tropical cyclone model. ›x ›y On the other hand, the observational studies have also ›w ›y ›w ›u vort_til 5 2 . (2e) provided evidence to show the role played by hot towers ›x ›z ›y ›z in the formation of tropical cyclones. Malkus et al. (1961) constructed the maps of clouds in Hurricane Daisy (1958) In (2a)–(2e), f is the Coriolis parameter, and z is the using aerial photographs made by aircrafts. Their results vertical vorticity. The advection term, divergence term, showed the persistence of the cloud patterns throughout a and tilting term of the vorticity equation are denoted by 3-day period. In addition, their estimation indicated that vort_adv, vort_div, and vort_til, respectively. Note that the ratio of the rain area covered by hot towers was about the advection term has been shifted to merge with ›z/›t 1%, 2.5%, and 4% during the formation, deepening, and to form the material rate of change of the vorticity maturity stages. The analysis, also using aircraft data, (Dz/Dt). Shapiro et al. (2009) pointed out that the baro- performed by Simpson et al. (1998) of Hurricane Daisy clinic vector serves primarily to generate horizontal (1958) in the Atlantic and Tropical Cyclone Oliver (1993) vorticity, thus it can be neglected in (2a). As a result, in the Coral Sea advanced the link between the hot when the retrieved three-dimensional wind is substi- towers and TC genesis. They found that in the nascent tuted to estimate each term in (2a), no residual term will eyewall, hot towers were able to provide an upflux of high be produced. The contributions from each term on the ue subcloud air during the formation stage. Kelley et al. right-hand side of (2a) to the temporal change of the (2004) analyzed the data from the Tropical Rainfall vorticity can be examined accurately. Equation (2a) Measuring Mission (TRMM) Precipitation Radar, and offers an additional constraint to the recovery of the found that the chance of intensification increased when wind fields, and helps to improve the precision of the

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FIG. 17. The distribution of the VHTs, defined in the manuscript, shown by blue color shading. Figures are shown for (a) 0800, (b) 0900, (c) 1000, and (d) 1200 UTC 19 Sep 2010. The coordinates are the relative distance (in km) from RCCG. The storm center in (b)–(d) is marked by a typhoon symbol. retrieved first-order derivative products such as the during Hurricane Ophelia (2005), the VHTs presented vorticity fields used in this session. in this research developed over complex terrain, with 2 Figure 15 shows that from 0800 to 1200 UTC, a sub- relatively weaker updraft (2.0–3.0 m s 1), smaller width stantial portion of the analysis area is indeed occupied by (;5 km), and shallower depth (;10 km). However, it positive z (Figs. 15a,c,e,g), whose distribution generally should be pointed out that with 1.0-km grid resolution, follows the curved eyewall, and significantly correlates with the multiple-radar-synthesized vertical wind speed the updrafts (Figs. 15b,d,f,h). Inside the eyewall, downward could be underestimated. In addition, more studies are 2 motions with maximum amplitude reaching 22.0 m s 1 can needed before one can reach conclusions regarding the be identified. One example of the VHT is presented in differences in the scale and intensity between maritime Fig. 16, showing the z and w fields over the vertical cross and inland VHTs. section denoted by P–Q in Fig. 15g. It can be seen that To obtain an overall evaluation of the area occupied in some regions (e.g., y 5240 to 245 km) within the by the VHTs, we examine each air column, and compute eyewall, the updraft and positive z can be recognized as the mean vertical velocity w and vorticity z averaged being nearly inside the same column. Compared with from the ground to Z 5 10 km. When z is positive and w 2 the maritime VHTs reported by Houze et al. (2009) is greater than 0.5 m s 1, this air column is categorized

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22 6 FIG. 18. The material time rate of change of vertical vorticity (Dz/Dt; color shading in s and amplified by 10 )at Z 5 2.0 km for (a) 0800, (b) 0900, (c) 1000, and (d) 1200 UTC 19 Sep 2010. The coordinates show the relative distance (in km) from RCCG. The storm center in (b)–(d) is marked by a typhoon symbol. as a VHT, meaning it is occupied by coexisting updrafts To identify which processes could be contributing to and positive vorticity. Figure 17 depicts the area cov- the strengthening of the vortex at the early stage of the ered by VHTs from 0800 to 1200 UTC. A quantitative reintensification, Fig. 19 first displays the vertical profile computation reveals that during this 4-h period of time, of the averaged (over each horizontal layer) vertical the ratio of the rain area covered by VHTs increases vorticity from 0800 to 1000 UTC. It can be seen that the from approximately 17.4% to 25.3% of the total anal- magnitude of the positive vorticity is generally in- ysis domain. From 0800 to 1200 UTC, the spatial dis- creasing with time, particularly at the midlevels. This is tribution of VHTs evolves from a less-organized an indication of the TC reintensification, as demon- structure (Figs. 17a–c)toapatterncloselyfollowing strated by a growth of cyclonic motion. Since the re- the curved eyewall (Fig. 17d). Figure 18 depicts the intensification starts approximately from 0800 UTC, a material time rate of change in the vorticity (i.e., vorticity budget diagnosis at this time level is conduct- Dz/Dt) from 0800 to 1200 UTC at Z 5 2.0 km. It can be ed and depicted in Fig. 20. It is shown that at lower seen that as the time progresses, the area containing layers from Z ; 1.5 km to approximately Z ; 2.5 km, the notable variation in vorticity gradually widens, and the stretching mechanism plays a leading role in increasing spatial distribution matches the structure of the curved positive vorticity, followed by the contributions from eyewall quite well. tilting and horizontal advection. The horizontal advection

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FIG. 19. The vertical profiles of the vertical vorticity averaged FIG. 20. The vertical profiles of the terms related to vertical over each horizontal layer at 0800 (dotted line), 0900 (dashed line), vorticity budget, averaged over each horizontal layer. The hori- and 1000 UTC (solid line). zontal advection, vertical advection, stretching, and tilting terms are denoted by ADVH (dotted line), ADVV (solid line), STRE (dashed line), and TILT (dotted–dashed line), respectively. with positive value extends up to Z ; 4.0 km. The horizontal advection plays a comparable role to the vertical speed, the recovery of the wind speed (reaching the advection in increasing low- to midlevel vorticity from magnitude before landfall), the derived (from the approximately Z 5 2.0 to 4.0 km. The vertical advection information of winds) high temperature and low becomes positive above Z ; 2.0 km, implying its role in pressure center, the associated updrafts and down- transporting the vorticity upward. The total vorticity drafts, as well as the in situ pressure and precipitation tendency estimated from Fig. 20 is consistent with the measurements on the ground, all support the exis- vorticity evolution shown in Fig. 19. tence of a reintensified eyewall on the western side of These VHTs and their collective forcing are believed the CMR. to be a possible factor in the reintensification of the 2) At an earlier stage, a northwest–southeast-oriented eyewall of Fanapi. However, it should be pointed out line-shaped rainband formed on the western side of that this section only provides observational documen- CMR, and might be attributed to the large-scale in- tation to support the existence of VHTs over Taiwan’s teraction between the remnant storm and environmen- terrain during the reintensification stage of Fanapi’s tal flow. However, evidence is collected to demonstrate eyewall. The detailed connection between the VHTs that the subsequent reintensification of Typhoon and the evolution of the eyewall needs to be investigated Fanapi appears to be initiated through a bottom-up by other means such as numerical model simulations. rather than a top-down process. This is revealed by the finding that the horizontally averaged wind speed 7. Conclusions and future work possesses a maximum at lower layers, and increases with time at almost all altitudes. Furthermore, adja- In this study we utilize observational data to docu- cent to CMR, a region of strong wind speed develops ment in detail the eyewall reintensification process from lower altitudes, then gradually grows to upper taking place over terrain when Typhoon Fanapi struck layers, indicating the role played by the local topog- Taiwan. The current investigation is made possible be- raphy to initiate the reintensification should not be cause of the use of Doppler radar data, as well as a newly excluded. designed analysis method (WISSDOM), which is capa- 3) Using ground-based radars, a relatively rare docu- ble of retrieving high-resolution three-dimensional wind mentation of VHTs over terrain, also a possible fields over complex topography. The major conclusions contributor to the TC reintensification, is conducted are as follows: in this research. The size of the area occupied by 1) A leeside vortex circulation can be clearly retrieved VHTs expands with time. A vorticity budget analysis using WISSDOM and data collected by four Doppler indicates that positive vertical vorticity increases radars. The slowdown of the storm’s translation with time, and can be attributed to the stretching

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mechanism at lower layers, followed by the contri- Chong, M., and O. Bousquet, 2001: On the application of MUSCAT butions from tilting and horizontal advection pro- to a ground-based dual-Doppler radar system. Meteor. Atmos. cesses. The horizontal advection plays a comparable Phys., 78, 133–139, doi:10.1007/s007030170011. Chou, K.-H., C.-C. Wu, and Y. Wang, 2011: Eyewall evolution of role to the vertical advection in increasing low- to typhoons crossing the Philippines and Taiwan: An observational midlevel vorticity. The vertical advection aloft is study. Terr. Atmos. Oceanic Sci., 22, 535–548, doi:10.3319/ responsible for transporting the vorticity upward. TAO.2011.05.10.01(TM). Durran, D. R., and J. B. Klemp, 1982: On the effects of moisture on The detailed description presented in this article shows the Brunt–Väisälä frequency. J. Atmos. 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