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Outer Tropical Associated with Matmo (2014)

CHENG-KU YU,LIN-WEN CHENG,CHUN-CHIEH WU, AND CHIA-LUN TSAI Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

(Manuscript received 18 February 2020, in final form 14 April 2020)

ABSTRACT

On 23 July 2014, a commercial aircraft (GE222) crashed near the Ma-Gong Airport on Penghu Island off the southwestern coast of Taiwan as it struggled to land in the stormy weather that was caused by the outer rainbands (OTCRs) of Typhoon Matmo. This study aims to document the detailed aspects of airflow and of OTCRs through high-resolution radar and surface observations and to identify how these observed structures contribute to aviation weather hazards. Analyses indicate that the weather at the airport was significantly influenced by the passage of three OTCRs (R1, R2, and R3), and these rainbands share common characteristics of squall-line-like airflow and precipitation structures. As GE222 descended to approach the runway and flew immediately behind and roughly parallel to the leading edge of R3, the aircraft encountered the heaviest precipitation of the and the prominent crosswind that was a manifestation of the rear-to-front flow generated locally by the rainband. The heavy –induced poor visibility and the occurrence of strong crosswinds were primary weather hazards affecting this flight event. Momentum budget analyses suggest that the frontward pressure gradient force provided by the near-surface, convectively gen- erated mesohigh played a major role in driving the low-level rear-to-front flow inside the band. The results from the present study imply that closely monitoring convective activities in the outer regions of tropical and their potential transformation into squall-line-like is crucial to complement the routine aviation alert of severe weather under the influence of tropical cyclones.

1. Introduction traffic and airport operation in areas where TCs pass by (Breslin 2016; Goodman and Small Griswold 2019). A The impact of weather on the operation and safety of mature TC is an approximately circular, strong cyclonic aviation is a worldwide issue (Humphreys 1930; WMO vortex, and its associated precipitation is typically 1989). Although the direct causes of accidents are most characterized by an organized, banded feature called commonly related to human error, weather is often a pri- ‘‘rainbands’’ or ‘‘spiral bands’’ (i.e., tropical cyclone mary contributing factor for aviation accidents (Helmreich rainbands, TCRs) (Senn and Hiser 1959; Willoughby 1997; Kulesa 2003). A number of weather phenomena, et al. 1984; Marks 2003; Yu and Chen 2011). It is well such as mountain waves and , have been recognized that the inner core, which is approximately shown to produce hazardous circumstances that may lead within 100–200 km or 2–3 times the radius of maximum to fatal aircraft accidents (Wurtele 1970; Fujita and Byers wind (RMW) from the TC center, is the most hazardous 1977; Wilson et al. 1984; Smith 1986; Haddad and Park region for TCs because it contains the most intense 2010; Keller et al. 2015). In particular, weather-induced swirling winds and eyewall convection (Anthes 1982; intense rainfall, high wind shear or strong downdrafts at Willoughby 1988; Rozoff et al. 2006; Wang 2009; Houze low altitudes near airports can significantly affect the air- 2010). Currently, both TC location and movement can craft takeoff/landing safety (Kessler 1985). The effective be monitored and predicted effectively by Doppler ra- documentation and prediction of aviation weather hazards dar and satellite observation systems and numerical are thus critically important for the prevention of aviation models. It is thus practically possible for meteorological accidents (Chun et al. 2017). forecasters to issue an appropriate, lead-time warning Tropical cyclones (TCs) are not only one of the most for the approach of hazardous, inner-core circulations of life-threatening and destructive natural phenomena on TCs. This weather alert usually allows aviation con- Earth but are also well known to strongly affect air trollers to direct aircraft to a safer space or hold air- planes on the ground over a sufficient time period Corresponding author: Cheng-Ku Yu, [email protected] beforehand (Goodman and Small Griswold 2019).

DOI: 10.1175/MWR-D-20-0054.1 Ó 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 09/24/21 06:41 PM UTC 2936 MONTHLY WEATHER REVIEW VOLUME 148

In contrast to the inner core of TCs, both convective On 23 July 2014, a commercial aircraft (model: phenomena in the outer region of TCs and their po- ATR72–212A) with 2 pilots, 2 cabin crew, and 54 pas- tential impacts on aviation activities have not received sengers on board executed regular public transport service much attention. The outer vicinity of TCs exhibits (flight number GE222) from Kaohsiung (KH) International weaker swirling winds, and the moist convection in this Airport located in southern Taiwan to Ma-Gong (MG) region is not significantly filamented or constrained by Airport on Penghu Island off the southwestern coast of the inner-core vortex (Rozoff et al. 2006). However, the Taiwan (Fig. 1). The aircraft crashed near MG Airport outer region of TCs tends to possess larger convective as it struggled to land in the stormy weather caused by available potential energy (CAPE) and lower humidity the OTCRs of Typhoon Matmo (2014). Unfortunately, 4 than the inner-core environment (Frank 1977; Bogner flight crew and 44 passengers were killed in this airplane et al. 2000; Yu and Chen 2011; Molinari et al. 2012). accident. As noted in the investigation report of this These environmental conditions facilitate intense con- accident by the Taiwan Aviation Safety Council (TASC vection, making the structural characteristics of outer 2016), during the landing of the aircraft, meteorological TCRs (OTCRs) resemble severe thunderstorms such as conditions near MG Airport, including squall lines (Houze 2010; Eastin et al. 2012; Yu and activities, heavy rain and significant changes in visibility, Chen 2011; Yu and Tsai 2013; Tang et al. 2014; Moon wind direction and speed, were among the contributing and Nolan 2015). More recently, a comprehensive in- factors for the cause of the aircraft incident. The ob- vestigation of OTCRs by Yu et al. (2018, hereafter jective of this study is to use various available observa- YU18) analyzed a large set of 50 rainband cases tions to document the detailed aspects of the airflow and through dual-Doppler observations and identified a precipitation of OTCRs related to this accident and to frequent similarity (58%, 29 rainband cases) between identify how these mesoscale structural characteristics OTCRs and squall lines. These squall-line-like OTCRs contribute to the occurrence of hazardous weather are generally characterized by convective precipita- conditions that may impact aviation safety. Over the tion, an obvious convergence zone between the band- accident area, there is relatively good, persistent cov- relative rear-to-front flow and front-to-rear flow at low erage of temporal and spatial high-resolution measure- levels and a surface cold pool signature. The processes ments from two ground-based Doppler radars at Chi-Gu responsible for the initiation of OTCRs have been (CG) and MG Airport (see Fig. 1). These Doppler radar partially addressed in the literature. Limited research observations provide an unparalleled depiction of the evidence suggests that the origin of OTCRs is probably finescale rainband features of the OTCRs and their re- related to different scenarios and forcings, such as the lationship with the aerial incident. outer propagation of inner-core convective activities, the intensification of convectively generated cold pools 2. Data and the potential interaction of inner-core vortex cir- culation with its outer environmental flow (Willoughby As described in the Introduction section, the primary et al. 1984; Yu and Cheng 2014; Li et al. 2017; Yu et al. datasets used to document the detailed features of the 2019; Li et al. 2019). airflow and precipitation of Matmo’s OTCRs and their Unlike the inner-core, hazardous region with a connection to the occurrence of aviation weather haz- quasi-circular geometry that is well recognized and ards are provided by two ground-based Doppler radars can be appropriately located given a known TC cen- available in the surrounding area of the flight accident ter, our awareness and understanding of aviation (locations in Fig. 1). One is the S-band (10 cm) opera- weather hazards caused by OTCRs is relatively less tional Doppler radar of the Central Weather Bureau at adequate. In particular, the detailed aspects of OTCR- CG located at the coast of southwestern Taiwan, ap- produced severe weather conditions and how they af- proximately 65 km southeast of Penghu Island (Fig. 1a). fect aircraft safety have been neither described nor The other is the C-band (5 cm) operational Doppler elaborated in the literature. It should be noted that radar of the Weather Wing of the Chinese Air force at turbulence associated with OTCRs can lead to very MG Airport. As indicated in Fig. 1b, this radar site is rough flights, which are well recognized by pilots and located ;1 km immediately adjacent to the eastern flank scientists who flew into TCs (Chapter 10, Houze 2014). of the runway of MG Airport. MG Airport has a single Schaefer et al. (1992) noted that moderate turbulence runway oriented north-northeast to south-southwest, is frequently located in transverse waves emanating designated R20 and R02, respectively. The altitudes of from the OTCRs. It is possible that the impacts on the CG and MG radar sites are 38 and 48 m MSL, re- aviation for both the inner and outer regions of TCs spectively. The CG (MG) radar is operated with a would be equally important. temporal interval of 7.5(10) min between each volume

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and a maximum observational range of 188 (120) km. The observational range of the non-Doppler scanning mode for the CG radar can be extended to ;460 km. The detailed characteristics of the CG and MG radars are described in Yu and Cheng (2013) and YU18. These two Doppler radars provide a continuous and compre- hensive view of precipitation and airflow information over the coastal area of southwestern Taiwan during the occurrence of the aircraft accident. Other data sources used in this study include 1-min temporal resolution surface observations from the Penghu (PH) station located on the western coast of Penghu Island and the automated weather observing system (AWOS) station located close to the northern runway threshold (see Fig. 1b) and 1–4-s-temporal-resolution flight-level aircraft data retrieved from the flight data recorder (FDR) of GE222. The FDR data used for analysis herein include altitude, longitude, latitude, and horizontal wind information. In addition, the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data (0.5830.58) (Saha et al. 2010) are also analyzed to provide a general context of large-scale, environmental flow for this par- ticular event.

3. Flight overview and Matmo’s rainbands Flight GE222 departed KH International Airport and headed for Penghu Island at 0945 UTC 23 July 2014. Shortly after ;1.3 h, the aircraft failed to land at MG Airport and crashed into the residential area approxi- mately 625 m east-northeast of the runway threshold at 1106 UTC. The entire flight track for GE222 (in red) is indicated in Fig. 1a. During the flight period, the cyclone center of Typhoon Matmo had already made landfall on the southeastern coast of China (Fig. 1a), and its inten- sity continued to weaken, with a maximum wind speed 2 less than 33 m s 1 (i.e., a weak typhoon). FIG. 1. (a) Best track of Typhoon Matmo (2014) from the The low-level plan position indicator (PPI) scans of Central Weather Bureau of Taiwan. The position of the typhoon center is indicated by solid gray circles every 3 h, with dark (light) radar reflectivity from the CG radar valid at 0945 UTC shading indicating moderate (weak) intensity of the typhoon. The (i.e., the take-off time) and 1108 UTC (i.e., close to the entire flight track for GE222 is indicated by the red curve. The accident time) indicate a highly asymmetric pattern of locations of the Chi-Gu (CG) Doppler radar and Kaohsiung precipitation with prominent rainbands confined to the (KH) International Airport are denoted by the triangle and the southeastern quadrant and outer region of the typhoon solid circle, respectively. The CG radar is located at (x, y) 5 (0, 0). The arrow highlights the location of Penghu Island. (b) Enlarged (Fig. 2). The observed asymmetry in the precipitation map of Penghu Island. The runway of Ma-Gong Airport is indi- field has been recognized as a common rainfall distri- cated by the thick black line. The track of GE222 within the do- bution for a westward-moving typhoon passing over main and the crash location of the aircraft are indicated by the red Taiwan, as its outer circulation interacts with the sum- curve and red solid star, respectively. The location of the Ma- mer southwesterly monsoon active over the South China Gong (MG) Doppler radar is indicated by the triangle, and the locations of the surface station at Penghu (PH) and the auto- Sea (Yu and Cheng 2013, 2014). Consistent with this mated weather observing system (AWOS) station are indicated scenario, the large-scale environment for the present by squares. case was characterized by strong southwesterly mon- 2 soonal flow (;10–20 m s 1) prevailing at low levels over

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the South China Sea, as revealed by the wind analysis of the NCEP-CFSR data valid at 1200 UTC 23 July 2014 (Fig. 3a). A flow confluence between the outer circula- tion of Matmo and the southwesterly monsoon was ev- ident over the oceanic region near Penghu Island and off the southwestern coast of Taiwan, where low-level convergence prevailed (Fig. 3b). The presence of this larger-scale confluent zone corresponded to the vigor- ous activities of the OTCRs observed there, as shown in Fig. 2. Three OTCRs (highlighted by R1, R2, and R3 in Fig. 2a) located ;200–300 km from the typhoon center were well identified and exhibited highly or- ganized, elongated features of high reflectivities (.45 dBZ). These rainbands were oriented at some small angles to the circle about the TC center, similar to the typical geometry of OTCRs observed previously (Anthes 1982; Yu et al. 2019). Over the duration of 5.5 h from 0700 to 1230 UTC 23 July encompassing the flight period of GE222 (0945–1106 UTC), the weather on Penghu Island was significantly influenced by the passage of these three rainbands as they propagated northeast- ward from the offshore region west of Penghu Island to the western/northwestern coast of Taiwan. This scenario can be best depicted by the time–distance section of low-level reflectivities taken from a band- normal segment (marked in Fig. 2b) from the CG radar, as shown in Fig. 4. MG Airport (location highlighted by the vertical solid line in Fig. 4) clearly experienced an intermittent, heavy precipitation caused by R1, R2, and R3, with relatively weaker rainfall in between. According to the flight arrival information provided from MG Airport, seven other commercial aircraft successfully landed during this period. Unlike GE222, these flights during their landing effectively avoided the hazardous timing of the most intense precipitation ob- served within R1–3 (Fig. 4). Three of these flights (FE081, FE031, and B7647) landed near the leading edge of R1–3 but they actually experienced less threat- ening wind conditions compared to GE222, as will be FIG. 2. The low-level PPI scan (0.58 elevation) of radar re- flectivity (dBZ, color shading) from the non-Doppler scanning elaborated in section 4. mode of the CG radar at (a) 0945 and (b) 1108 UTC. The locations At the take-off time of GE222 (i.e., 0945 UTC), R1 of outer tropical cyclone rainbands (OTCRs) associated with nearly made landfall on the northwestern coast of Matmo are marked with R1, R2, and R3. The corresponding ty- Taiwan, while the leading edge of R2 just passed over phoon center located ;50–80 km inland of southeastern China is Penghu Island (Figs. 2a and 4). As noted in the investi- also indicated. Range rings (km) with respect to the typhoon center are also indicated. The location of the CG radar is indicated by the gation report of GE222, bad weather observed at MG triangle. In (b), the thick dashed line indicates the band-normal Airport during this time was actually beyond the landing segment for calculating the time–distance sections of radar re- circumstances for an aircraft (TASC 2016), indicating flectivity and radial velocity from the CG radar shown in Figs. 4 the significant impact of the intense convection of R2 on and 10. aviation safety. With time, weather conditions near MG Airport improved temporarily as R2 soon moved away from Penghu Island. At 1106 UTC, R2 already touched the western coast of Taiwan (Fig. 2b); however, Penghu

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FIG. 4. Time–distance section of low-level PPI scan (0.58 eleva- tion) of reflectivities (dBZ, color shading) from the CG radar along the band-normal segment indicated in Fig. 2b during 0700 to 1230 UTC 23 Jul. Three observed OTCRs are marked with R1, R2, and R3. The vertical solid line indicates the position of the runway (R20) of MG Airport. The horizontal dashed line highlights the accident time of GE222. The arrival times of several commercial aircraft that landed at MG Airport during the analysis period are marked by hollow circles with flight numbers.

For reference, the time series of the FDR-recorded flight altitudes and corresponding along-track reflectivity values extracted from the low-level PPI scans (0.58 elevation) of the CG radar are also shown in Fig. 5e. Note that the along-track reflectivity values should be reasonably representative of the precipitation in- tensities encountered by GE222 because the heights of radar beams for the extracted reflectivities (0.4– 1.3 km MSL) are well within the range of flight alti- tudes (0–2 km MSL). After take-off, GE222 flew northbound and remained at altitudes of 1.8–2.1 km MSL heading toward the oceanic area of Penghu Island (Figs. 5a,e). When the aircraft initially approached Penghu Island, it passed through the southern end of R2 (Fig. 5a), where a local maximum of the along-track reflectivity (;40 dBZ)at FIG. 3. The large-scale wind distributions at 1 km MSL from the ; NCEP-CFSR data valid at 1200 UTC 23 Jul 2014. (a) Wind speed 1007 UTC was also evident, as shown in Fig. 5e. After 2 2 2 (m s 1) and (b) convergence (310 4 s 1) are indicated by color the transverse, GE222 flew with a circle-like, holding shading. flight pattern in regions ;30 km northeast of MG Airport and waited for approach clearance for runway Island at this time experienced the arrival of intense R20 (Fig. 5a), due to the low visibility of ;800 m at the precipitation associated with R3, the target rainband airport during the passage of R2. Note that because influencing the flight GE222 (Figs. 2b and 4). of different landing assistance systems, runway R20 Figures 5a–d illustrate more details about the flight (R02) required a higher (lower) visibility limitation of trajectory of GE222 and its relative location to the ob- 1600 (800) m for aircraft to land; see TASC (2016). served R2 and R3 during which the aircraft approached Shortly after, the pilots of GE222 then chose to exe- Penghu Island and attempted to land at MG Airport. cute an R02 approach, and the aircraft started to track

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FIG. 5. The flight track of GE222 (thick solid curve) superposed on the low-level PPI scan (0.58 elevation) of radar reflectivity (dBZ, color shading) from the CG radar at four selected times: (a) 1024, (b) 1039, (c) 1054, and (d) 1109 UTC. In (a)–(d), the positions of GE222 over the duration from approximately 1000 UTC to the analysis time of each panel are also highlighted by solid gray circles every 5 min. The observed OTCRs are marked (i.e., R2 and R3), and the location of the CG radar is denoted by the triangle. (e) The time series of the FDR-recorded flight altitudes (solid curve) and corresponding along-track reflectivity values (dashed curve) extracted from the low-level PPI scans (0.58 elevation) of the CG radar. For reference, the analysis times of (a)–(c) and the crash time (i.e., 1106 UTC) are marked by red arrows.

Unauthenticated | Downloaded 09/24/21 06:41 PM UTC JULY 2020 Y U E T A L . 2941 southbound after 1030 UTC over regions of relatively weak echoes (;25–30 dBZ) ahead of R3 (Figs. 5b,e). At pretty much the same time, weather conditions and visibility improved at MG Airport, as it was just located in the gap region of precipitation between R2 and R3 (Figs. 5b,c). Subsequently, the flight crew of GE222 decided to take an R20 approach again for landing, so the aircraft turned back to the north with decreasing altitude and arrived near the northern end of R3 (Figs. 5c,e). During the final stage of the flight, GE222 headed southwestward (i.e., roughly parallel to the orientation of runway R20, Fig. 1b) and kept descending imme- diately along the rear flank of the leading edge of R3, where strong reflectivities (.40–45 dBZ) prevailed (Fig. 5d). A fatal deviation of the flight track to the east from the runway (Fig. 1b) was recorded by the FDR as GE222 arrived at the northern shore of Penghu Island and flew into the zone of the most in- FIG. 6. Flight-level winds recorded by the FDR as GE222 flew tense precipitation (;50 dBZ) observed in the middle from the prerainband vicinity of R3 to the region to the rear of the leading edge of R3. The low-level PPI scan (0.58 elevation) of radar segment of R3 (Figs. 5d and 1b). As will be demon- reflectivity (dBZ) from the CG radar valid at 1109 UTC is indicated 2 strated later, the eastward deviation of the flight track by gray shading. Wind flags correspond to 25 m s 1, full wind barbs 2 2 was consistent with the presence of strong westerly correspond to 5 m s 1, and half barbs correspond to 2.5 m s 1. windsproducedlocallybyR3.Atalmostthesame Line segment A–B marks the location of the vertical cross time, the visibility near the runway, based on the sections shown in Fig. 9. AWOS measurements, was reduced very rapidly to ;250 m due to the arrival of the heavy rainfall of R3. by the FDR of GE222 further show a prominent low- The flight crew of GE222 was unable to visually locate level wind alternation across the leading edge of the the runway within the extremely low visibility environ- rainband, as shown in Fig. 6. Strong south-southwesterly 2 ment before the aircraft descended to the minimum flow (20–25 m s 1) prevailed in the prerainband region, descent altitude1 (MDA, 300 ft; ;90 m) for runway R20 which was associated with environmental, monsoonal (TASC 2016). In the last minute, the pilots attempted to southwesterly flow originating over the South China execute a go-around procedure until the aircraft hit the Sea, as shown in Fig. 3. The low-level winds became 2 ground. At the accident time (1106 UTC), the along- more westerly (;18–20 m s 1) within the rainband, track reflectivity was also observed to reach a peak value particularly inside the zone of the most intense pre- of ;48 dBZ (Fig. 5e). It is clear that R3, one of the cipitation (maximum . 50 dBZ) near Penghu Island. OTCRs from Matmo, represents a critical, hazardous The cross-band wind shift was also evident but rela- precipitation system during the occurrence of this un- tively gentle over the northern segment of R3 with fortunate flight accident. discrete reflectivity elements (.40 dBZ). Considering the runway oriented roughly parallel to the prerainband south-southwesterly flow, the existence of the west- 4. Finescale structures of OTCRs erly flow observed immediately to the rear of the This section focuses primarily on the finescale struc- leading edge of R3 is expected to yield a significant tural analysis of R3, followed by a brief description of wind component perpendicular to the runway (i.e., the other two OTCRs (i.e., R1 and R2). As shown in crosswind). The intensity of the calculated crosswind 2 Fig. 5, R3 was characterized not only by very intense could reach a maximum of ;18 m s 1, a magnitude precipitation but also by a sharp gradient of reflectivities close to or slightly beyond the landing crosswind along its leading edge. The flight-level winds recorded limitation of the aircraft model of GE222 [(Avions de Transport Régional) ATR 2010]. The more detailed views of the airflow and precipi- 1 The minimum descent altitude (MDA) is a specific altitude in a tation of R3 near Penghu Island can be further revealed nonprecision approach, in which the descent of the aircraft must by high-resolution observations from the CG and MG not be made without the required visual reference. radars. It is possible to perform the dual-Doppler wind

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FIG. 7. The low-level PPI scan (0.58 elevation) of (a) radar reflectivity (dBZ, color shading) and (b) radial ve- 2 locities (m s 1, color shading) from the CG radar valid at 1101 UTC 23 Jul 2014. Corresponding surface winds measured by the PH and AWOS stations are also shown for reference. The runway of MG Airport is indicated by 2 2 the black thick line. Full wind barbs correspond to 5 m s 1; half barbs correspond to 2.5 m s 1. The azimuth (8) and radial distance (km) from the CG radar are indicated by thin solid lines and dashed lines, respectively. (c),(d) As in (a) and (b), but for analyses valid at 1109 UTC 23 Jul 2014. synthesis using the CG and MG radar observations for MG radar at 1057 UTC is also shown in Fig. 8 for ref- the present study. However, based on the consideration erence. The heights of the radar beams from the 0.58 PPI of synthesized geometries, the cross-beam angles be- scan of the CG (MG) radar calculated to be ;850 tween the two radars over the region of interest (i.e., (;100) m MSL over regions of Penghu Island are low near Penghu Island) are very small (Fig. 1), which is enough to provide useful information on the low-level expected to produce large uncertainties and errors in the flow encountered by GE222. dual-Doppler-derived winds. In view of this, this study A discontinuity in radial velocities from positive to focuses only on single Doppler radar analysis. Figure 7 negative values was observed to be coincident with the shows the low-level PPI scans (0.58 elevation) of re- leading edge of the rainband that was marked by a very flectivity and radial velocity from the CG radar at two sharp horizontal gradient of reflectivities (Figs. 7a–d). consecutive times (1101 and 1109 UTC) encompassing The aforementioned westerly flow inside the rainband the accident time of GE222 (i.e., 1106 UTC). The low- was evidently highlighted by a local area of enhanced level PPI scan (0.58 elevation) of radial velocity from the negative radial velocities (i.e., approaching the radar)

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FIG.8.AsinFig. 7b, but for the low-level PPI scan (0.58 eleva- 2 tion) of radial velocity (m s 1) from the MG radar valid at 1057 UTC 23 Jul 2014. The leading edge of the rainband is indi- cated by the thick dashed line. The location of the MG radar is indicated by the triangle. immediately behind the leading edge of the rainband (Fig. 7b). The area of enhanced negative radial ve- locities (i.e., westerlies) moved rapidly eastward with the leading edge of the rainband and soon arrived over the runway during the landing of GE222 (Fig. 7d). The peak intensity of the negative radial velocities found 2 at 1101 UTC could have exceeded 18 m s 1 but slightly 2 weakened (16–18 m s 1) immediately after the flight accident at 1109 UTC. Generally, positive but weaker 2 radial velocities (,10 m s 1) observed in the prerainband region were consistent with the environmental south- southwesterly flow prevailing there, as shown in Fig. 6. The velocity patterns of the MG radar seen in Fig. 8 consistently indicated the development of rainband-induced westerly flow that was character- ized by a local area of pronounced negative radial FIG. 9. (a) Vertical cross section of radar reflectivity (dBZ, color velocity. With the advantage of the observational shading) and ground-relative radial velocities (contours with an 2 geometry of the MG radar relative to the runway, the interval of 3 m s 1) from the CG radar along A–B in Fig. 6 valid at intensity of the low-level crosswind associated with 1101 UTC 23 Jul 2014. Negative values of radial velocities are the westerly flow could be appropriately estimated hatched, and zero radial velocities are highlighted with thick solid from the values of radial velocity along the 2908 curves. The location of the runway (R20) is also indicated. (b) As in (a), but for band-relative radial velocities. azimuth angle (i.e., perpendicular to the runway), 2 which was found to be ;16 m s 1 (cf. Fig. 8). This magnitude was comparable to that of the FDR A range height indicator (RHI) vertical cross section of measurements described earlier. The westerly flow the CG radar perpendicular to the orientation of R3 and inside the rainband was also evident at the ground passing through the location of the runway at 1101 UTC level, as observed by the PH and AWOS stations (i.e., 5 min prior to the flight accident) was used to illus- (Figs. 7 and 9), although its intensity decreased to trate the vertical distributions of rainband precipitation 2 ;6–8 m s 1, presumably due to the significant influ- and airflow (line A–B, Fig. 6). To obtain a better ap- ence of surface friction. proximation of horizontal winds by radial Doppler

Unauthenticated | Downloaded 09/24/21 06:41 PM UTC 2944 MONTHLY WEATHER REVIEW VOLUME 148 velocities in these RHI analyses, the velocity component of the particle terminal velocity projected onto the radar beam was removed from the raw radial velocities (Yu and Jou 2005; Yu and Hsieh 2009). In addition to the ground-relative radial velocities, band-relative radial velocities are also calculated and shown in Fig. 9b for reference because of the propagating nature of the rainband. The band-relative field was obtained by 2 subtracting a band-normal propagation speed of 5.5 m s 1 from the observed radial velocities based on the propaga- 2 tion speed and direction of R3 at 18.1 m s 1 and 658,re- spectively. In the context of the observed propagating characteristics, the eastern (western) flank of R3 may refer to the front (rear) side of the propagating convective system. The RHI vertical cross section indicates the highly convective characteristic of R3 precipitation, with the 40-dBZ contour exceeding 5 km MSL. A narrow zone of heaviest precipitation coincided with enhanced conver- FIG. 10. As in Fig. 4, but for the ground-relative radial velocities 2 gence between positive and negative radial velocities at (m s 1, color shading) from the CG radar. Positive (negative) the leading edge of the rainband, with the strongest re- values represent velocities away from (toward) the radar. flectivities (.50 dBZ) confined to the lowest 2.5 km MSL (Fig. 9a). The area of the enhanced westerly flow, of squall-line-like OTCRs described in YU18. In addi- namely, strongly negative radial velocities (.12– tion, the RHI analyses, together with the results from 2 15 m s 1), extended vertically up to 4.5 km MSL within Figs. 6–8, strongly suggest that the occurrence of haz- the primary region of the leading heavy precipitation, and ardous crosswind (i.e., the westerly flow) encountered relatively weaker intensities of negative radial velocities by GE222 was exactly a manifestation of band-relative (i.e., weaker westerly flow) were observed behind. As rear-to-front flow generated locally inside the rainband. indicated in the band-relative field (Fig. 9b), a deep layer Similar radar analyses performed for R1 and R2 in- of south-southwesterly inflow (i.e., positive band-relative dicate that these two OTCRs also exhibited squall-line- radial velocities) was present in the prerainband region. like structures, such as the highly convective nature of 2 The inflow had stronger intensities (;6–12 m s 1) precipitation, a deep layer of prerainband inflow and a below 3 km MSL and was characterized by a front- low-level convergence zone between the band-relative ward vertical shear of the horizontal wind component rear-to-front flow and front-to-rear flow near the leading perpendicular to the rainband. The magnitude of the edge of the rainbands (not shown). Despite some calculated low-level cross-band vertical shear was differences in the details of these structures among 2 2 approximately 3.6 m s 1 km 1. This strength of the the three rainbands (R1–3), they share common charac- cross-band vertical shear may be considered moderate teristics of precipitation and airflow patterns. Accordingly, compared to those of previously documented OTCRs the time–distance section of low-level radial velocities (Yu and Tsai 2013; Yu et al. 2019) but was comparable from the CG radar over a longer time window, cor- to that of the prerainband environment of tropical responding to that of Fig. 4, shows the repetitive ap- squall lines (Barnes and Sieckman 1984). The low- pearance of negative radial velocities, namely, the level inflow fed the rainband and extended rearward westerly flow or band-relative rear-to-front flow in- (westward) and upward to higher altitudes (above side R1–3, near runway R20 of MG Airport (Fig. 10). 6 km MSL) immediately ahead of the leading edge of Over the prerainband regions or in the precipitation the low-level rear-to-front flow, consistent with the gap regions between the rainbands, positive radial upward transport of horizontal momentum by leading velocities generally prevailed, reflecting the presence convective updrafts (cf. Fig. 9b). The maximum in- of environmental south-southwesterly flow. Note that tensities of the low-level rear-to-front flow inside the GE222 encountered the strongest precipitation and 2 rainband could reach 6–9 m s 1, which was somewhat westerly flow (i.e., crosswind) observed inside R3 stronger than those of the radar-derived rear-to-front during its landing (Fig. 10). Fortunately, all other 2 flow documented previously within OTCRs (3–6 m s 1) commercial aircraft landed on the runway in time ei- (Yu and Tsai 2013). The band-relative airflow structures ther prior to the arrival of the heaviest precipitation shown in Fig. 9b were quite similar to the airflow patterns and strong crosswind associated with R1–3 or just

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FIG. 11. (top) Time–height cross section of radar reflectivity (dBZ, color shading) at the PH station observed by the CG radar. (middle),(bottom) Time series of p0 (mb; 1 mb 5 1 hPa), 21 21 T (8C), Td (8C), ue (K), Wd (8), Ws (m s ), and rainfall intensity (mm h ) at 1-min temporal resolution observed from the PH station (location shown in Fig. 1) from 0700 to 1230 UTC 23 Jul 2014. Note that time increases to the left so the rainband geometry (i.e., rear/front side) is consistent with that of the vertical cross section shown in Fig. 9. Ground-relative winds are 2 also indicated by wind flags with full wind barbs corresponding to 5 m s 1 and half barbs 2 corresponding to 2.5 m s 1. The duration encompassing the passage of the observed OTCRs (R1, R2, and R3) is highlighted by gray shading. The accident time of GE222 is marked by the red solid star. after the passage of R3. These analyses provide im- fluctuations caused by the passage of the rainbands, the portant evidence that squall-line-like OTCRs repre- perturbation pressure was calculated by subtracting the sent a potentially threatening weather phenomenon cyclone-scale pressure tendency (i.e., 1-h running mean hazardous to aviation safety. of surface pressure) from the pressure values recorded at a given time (Yu and Tsai 2010; Yu and Chen 2011). The time–height cross section of elevated radar re- 5. Surface characteristics of OTCRs flectivity observed at the position of the PH station by To further evaluate the potential similarity of the the CG radar is also shown in Fig. 11 to provide a better observed OTCRs with squall lines, observations from context of precipitation information for locating the the surface stations located on Penghu Island as the rainbands. rainbands passed by were analyzed. Figure 11 shows the The results indicate quite similar surface fluctuations time series analyses of 1-min-temporal-resolution mea- during the passage of the three observed rainbands surements recorded from the PH station (location in (R1–3). The leading edges of the rainbands were high- Fig. 1b) within a time window corresponding to that of lighted by sharp changes in wind direction from south- Fig. 4. Seven surface meteorological quantities, namely, southwesterly flow to westerly flow, as we have seen temperature (T), dewpoint temperature (Td), equiva- from the FDR and radar observations (Figs. 6–8). The 21 lent potential temperature (ue), perturbation pressure winds became stronger (8–11 m s ) in the region of 0 (p ), wind speed (Ws), wind direction (Wd), and rainfall most intense rainfall and gradually returned to the en- rate (RR), are presented. To better isolate pressure vironmental south-southwesterly flow toward the rear

Unauthenticated | Downloaded 09/24/21 06:41 PM UTC 2946 MONTHLY WEATHER REVIEW VOLUME 148 edges of the rainbands. These wind alternations strongly failure of GE222. Time series analyses of the visibility support that the westerly flow (i.e., crosswind or band- data recorded by the AWOS station located close to the relative rear-to-front flow) was produced locally and runway (location in Fig. 1b) indicate an extremely sharp convectively inside the rainbands. and dramatic change in visibility from values well above

A prominent drop in T (;2.58–38C) and ue (;15 K) 2 km in the prerainband region to only 200–400 m inside across the leading edge of the rainbands was evident the rainbands (Fig. 12). In addition to R3, both R1 and R2 (Fig. 11). This cold pool signature remained within the similarly produced poor visibility due to their intense rainbands, but the temperature tended to recover to rainfall, which was well beyond the visibility limitation some degree shortly after the passage of the primary of 1.6 km for landing on runway R20, as described in precipitation of the rainbands. Much lower equivalent section 3. GE222 encountered the most hazardous timing 2 potential temperature could be attributed to the pres- of the heaviest precipitation (.100 mm h 1) and lowest ence of colder and less-moist air (i.e., lower T and Td) visibility (;200 m) caused by the arrival of R3 just after inside the rainbands. The observed reductions in both 1100 UTC.

T and Td cannot be explained simply by the evaporative effect of precipitation that is also expected to be rela- 6. Forcing mechanism for the rear-to-front flow tively minor under nearly saturated ambient conditions

(T ’ Td) for the present case. This thermodynamic The low-level rear-to-front flow (i.e., ground-relative feature has been shown to commonly occur for TCRs or westerly flow) generated locally within R3 not only is tropical deep convection due to the downward transport responsible for strong crosswinds but is also one of the of low-ue air originating from higher altitudes by con- primary kinematic signatures for OTCRs documented vectively induced downdrafts (Barnes et al. 1983; Skwira previously (Yu and Tsai 2013; YU18). This kind of low- et al. 2005; Tompkins 2001; Yu and Chen 2011). level airflow pattern has been frequently observed in the The perturbation pressure was observed to rise nota- convective region of a mature (e.g., Roux bly during and after the passage of the leading edge. The et al. 1984; Smull and Houze 1985; Roux 1988; Houze maximum pressure perturbations (;0.8–1 mb) tended et al. 1989; Wang et al. 1990; Jorgensen et al. 1997; to coincide with the region of the heaviest precipitation Houze 2004). In fact, the low-level band-relative rear-to- and prominent temperature deficits, suggesting that the front flow, which implies that wind speeds within the observed positive perturbation pressure (i.e., mesohigh) layer of cold air are substantially faster than the prop- was primarily caused by the negative buoyancy associ- agating speed of the cold-air leading edge, also repre- ated with the cold pool and water loading. The hori- sents one of the key features of atmospheric gust fronts zontal extent of the mesohigh associated with R3 in the or laboratory density currents (e.g., Charba 1974; Goff cross-band direction was estimated to be ;13 km, based 1976; Simpson and Britter 1980). Although the existence on the band-normal propagation speed of this rainband of the low-level rear-to-front flow has been commonly 2 (5.5 m s 1) and the duration of its associated positive considered to be related to the convectively generated pressure perturbations (;40 min, cf. Figure 11). The outflow from the existing convective cells (e.g., Wakimoto important role played by the mesohigh observed on the 1982), the forcing mechanism of the low-level rear-to- front side of R3 will be further elaborated in the next front flow for OTCRs and squall lines remains ambigu- section. On the rear side of or outside the rainbands, ous in the literature. For squall-line systems, the elevated the perturbation pressure generally had near-zero or rear-to-front flow, known as the so-called ‘‘rear inflow,’’ is slightly negative values. These thermodynamic charac- usually present in the low- to midtroposphere (or teristics observed for the present rainbands are funda- above the layer of the cold pool) behind the leading mentally similar to those of squall-line-like OTCRs convection or over the trailing stratiform region. The documented previously (Yu and Tsai 2013; YU18; Yu forcing mechanism for the elevated rear-to-front flow et al. 2019). has been shown to be closely related to the buoyancy- The precipitation associated with R1–3 was quite in- induced low pressure underneath the warm updrafts tense, and their maximum rainfall rates could reach that slopes over the low-level cold pool (LeMone et al. 2 above 80 mm h 1, as expected from the highly convec- 1984; Lafore and Moncrieff 1989; Klimowski 1994). tive nature of radar echoes with a pronounced horizontal The resulting pressure difference between the back and gradient and a significant vertical extent of reflectivities leading portions of the convective system may act to shown in Figs. 11 and 9. As described in section 3,rapidly accelerate the midlevel air from the rear to the front. decreasing visibility at MG Airport due to the arrival of However, whether this forcing mechanism can sub- R3 heavy precipitation was one of the striking and haz- stantially contribute to the development of low-level ardous weather conditions contributing to the landing rear-to-front flow is uncertain because the elevated

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FIG. 12. (top) Time–height cross section of radar reflectivity (dBZ, color shading) at the AWOS station observed by the CG radar. (middle),(bottom) Time series of rainfall intensity 2 (mm h 1) and visibility (km) at 1-min temporal resolution observed from the AWOS station (location shown in Fig. 1) from 0700 to 1230 UTC 23 Jul 2014. The duration encompassing the passage of the observed OTCRs (R1, R2, and R3) is highlighted by gray shading. The accident time of GE222 is marked by the red solid star. rear inflow and the low-level rear-to-front flow would rainbands, such as that evident in the middle segment probably belong to a different circulation system within of R3 (cf. Figs. 6–9). squall lines. Since the low-level rear-to-front flow observed in the Another potential forcing mechanism for the low- present case was mostly westerly, its forcing mechanisms level rear-to-front flow is related to mesoscale vortices, may be appropriately evaluated by the horizontal mo- if any, developing along convective rainbands, which mentum equation in the east–west direction, which can have been found to reinforce the rear-to-front flow be written as locally to produce arc-shaped or bow-shaped radar echoes (Weisman 1993; Weisman and Davis 1998; Yu ›u ›u ›u ›u 1 ›p ›(u0w0) 52u 2 y 2 w 2 1 f y 2 , (1) and Tsai 2013; Wakimoto et al. 2015). For the present ›t ›x ›y ›z r ›x ›z case, the velocity signatures seen from radar observa- tions did indicate evidence of mesoscale vortices lo- where u, y, and w are the east–west, north–south, and cated near the northern end of R3 and the southern end vertical wind components, respectively; r is the air of R2 (figure not shown). Nevertheless, these line-end density; p is the pressure; f is the Coriolis parameter; and vortices were small in nature (;10 km in radius) and u0w0 is the turbulent momentum flux. The term on the did not appear directly relevant to the overall devel- left-hand side of (1) represents local acceleration. The opment of the low-level rear-to-front flow along the firstthreetermsontheright-handsideof(1) represent

Unauthenticated | Downloaded 09/24/21 06:41 PM UTC 2948 MONTHLY WEATHER REVIEW VOLUME 148 advective acceleration, and the remaining terms repre- sent the horizontal pressure gradient force, Coriolis ac- celeration and frictional effect. As described in section 5,a clear signature of the rear-to-front flow was also evi- dent near the ground level (cf. Fig. 11), so the diagnosis of the momentum equation may be made possible with surface observations available on Penghu Island. To a first approximation, the north-south variation in u and the near-surface vertical velocities are both considered small so the second and third terms on the right-hand side of (1) can be negligible. Assuming that the magni- tude of the turbulent momentum flux decreases linearly with height and has a zero value at the top of the boundary layer (Pennell and LeMone 1974; Fankhauser et al. 1992), the vertical flux divergence may be esti- 0 0 mated by the surface momentum flux [(u w )s] divided by the depth of the boundary layer (H). The surface flux can be represented by a bulk aerodynamic formula as 0 0 52 (u w )s CdWsu, where Cd is the drag coefficient and FIG. 13. (a) Time series of the east–west wind component (u) Ws is the wind speed. Considering the smaller island measured by the PH station from 1020 to 1220 UTC 23 Jul 2014. environment such as Penghu, the representative mag- (b) Corresponding local acceleration calculated based on u com- nitude of Cd would most likely lie between the land and ponents in (a). (c) Time series of magnitudes of various terms oceanic characteristics of roughness (Stull 1988) and is (A–E) in the horizontal momentum [Eq. (2); see details in the text]. 2 thus set to be a moderate, constant value (;5 3 10 3). The duration encompassing the passage of R3 is highlighted by With these approximations above, (1) can be further gray shading and the accident time of GE222 is marked by vertical dashed line. expressed as

2 ›u ›u 1 ›p C W u leading edge of R3 and reached a maximum of 9 m s 1 at 52u 2 1 f y 1 d s . (2) › › r › |{z} ;1105 UTC (i.e., close to the accident time of GE222). |{z}t |ffl{zffl}x |ffl{zffl}x |fflfflfflffl{zfflfflfflffl}H Term D Term A Term B Term E After this time, the westerly flow continued to weaken Term C 2 and decrease to a minimum intensity (1 m s 1) near the The PH and AWOS stations, separated by a horizontal rear edge of the band, a magnitude similar to those ob- distance of 7.3 km, are oriented roughly in the east–west served in the prerainband region (Fig. 13a). The corre- direction (cf. Fig. 1b), so their measurements can pro- sponding local acceleration calculated based on Fig. 13a vide reasonable estimates of the spatial gradient for is shown in Fig. 13b, whose temporal trend was generally terms B and C in (2). The MG sounding valid at consistent with that of term A predicted by the sum- 0000 UTC 23 July (not shown) reveals a slightly stable, mation of all terms (B–E) in (2), as shown in Fig. 13c. shallow boundary layer (H ; 100 m), a common vertical The contribution of the Coriolis effect (term D) to flow extent characterizing the coastal atmospheric boundary acceleration was calculated to be quite minor. However, layer (Samelson and Lentz 1994). If all terms on the the pressure gradient force (term C) was found to be a right-hand side of (2) are estimated properly, their dominant forcing contributing to the positive accelera- summation may well predict the local acceleration (i.e., tion. This frontward pressure gradient force was pro- term A). On the other hand, the local acceleration term vided by the convectively generated mesoscale high on can also be calculated directly based on the temporal the front side of the band as described in section 5 (cf. variation in winds measured at a fixed location such Fig. 11). The initial reduction in the u component inside as surface stations. The degree of consistency between the band at approximately 1108 UTC was due to the the predicted and calculated local acceleration may negative effect of both friction (term E) and advective provide a relative sense of reliability for the diagnosis of acceleration (term B) (cf. Fig. 13c); however, the the momentum equation. pressure gradient acceleration became negative after Figure 13a shows the time series of the u component ;1120 UTC, which also contributed partly to the per- measured by the PH station from 1020 to 1220 UTC as sistent decrease in the u component on the rear side R3 passed by. The near-surface westerly flow started to of the band. The appearance of the relatively small, intensify prominently near and after the arrival of the rearward pressure gradient force (i.e., toward the west)

Unauthenticated | Downloaded 09/24/21 06:41 PM UTC JULY 2020 Y U E T A L . 2949 in these regions (or times), where convection-induced (;199 km) recorded by the Joint Typhoon Warning perturbation pressure was much weaker (Fig. 11), Center (JTWC). The OTCRs observed ;250–300 km would mostly reflect the presence of the -scale from the center of Matmo, as described in section 3 low pressure system of Matmo located northwest of (cf. Fig. 2b), were apparently situated in outer regions Penghu Island (cf. Fig. 1a). well beyond the TC size. In fact, the TC warning for MG It is noteworthy that the layer of the rear-to-front flow Airport was officially terminated at 0940 UTC, ;5min associated with R3 extended vertically up to ;4km prior to the take-off of GE222 (TASC 2016)asthecenter MSL (cf. Fig. 9b). In addition to the forcing mechanism of Matmo was located ;250 km north-northwest of MG identified above that played a key role in driving the Airport, a radial distance obviously greater than the low-level rear-to-front flow, we cannot completely rule storm size of the typhoon (cf. Fig. 1). In this context, to- out the possibility of other processes that would also gether with the frequent development of squall-line-like operate and favor the development of the elevated rear- OTCRs (YU18), one may suggest a potential need to to-front flow observed within the rainband. Given that expand the area of routine TC alert, or alternatively, to the airflow structure of R3 seen in Fig. 9b suggests a reasonably postpone the termination of TC warnings. strong, rearward tilting updraft sloping over the low- Such operational work may help pilots and/or ground level cold pool, the buoyancy-induced pressure gradient, controllers maintain a strong lookout for TC-related se- as described earlier, would also be likely to occur in the vere weather and prevent any risky decisions on the flight present case to strengthen the upper-level rear-to-front operations. When considering the threat of TCs on avi- flow. This process may help explain why an additional, ation, closely monitoring convective activities in the outer local maximum of the rear-to-front flow existed between regions of TCs and their potential transformation into 3 and 4 km MSL, as evident in Fig. 9b. Future detailed squall-line-like storms by real-time radar observations kinematic and thermodynamic information collected would also represent a critical task for operational fore- within OTCRs and squall lines will be required to clarify casters to make a timely warning of rapidly changing the relative roles of the buoyancy-induced high pressure weather conditions. (at low levels) and low pressure (at upper levels) in contributing to the development of the rear-to-front 8. Conclusions flow inside the squall-line-like rainbands. Analysis results presented in the previous sections have elaborated that the OTCRs of Typhoon Matmo exhibited 7. Operational implications squall-line-like convective characteristics, which in turn It is well known that the spatial coverage of TC produced critical weather hazards impacting aviation warnings depends closely on the storm size of TCs, safety. Specifically, both the heavy rainfall–induced low which is practically determined by the radius of a certain visibility and strong convectively generated crosswind threshold of near-surface wind speed or by the radial associated with one of the observed OTCRs (i.e., R3), extent of the outermost closed isobar (e.g., Frank and were primary weather hazards affecting the flight GE222. Gray 1980; Merrill 1984; Knaff et al. 2014). Global ob- The structural features of R3 and their relationship with servations have indicated the most typical, median size the hazardous weather conditions encountered by GE222 of TCs at approximately 200 km based on the azimuth- during its final approach to MG Airport are illustrated 2 ally averaged radius of 12 m s 1 (Chavas and Emanuel schematically in Fig. 14. R3 was observed to possess 2010). Nevertheless, in addition to a common charac- squall-line-like structures with a highly convective nature 2 teristic of outward propagation, the OTCRs had been of precipitation (maximum rain rates . 100 mm h 1), observed to be active over extensive outer regions of obvious low-level convergence between the prerainband TCs from the inner-core boundary (;50–100 km) to the south-southwesterly inflow and band-relative rear-to- broad vicinity at large radii (;500–600 km) (Yu and front flow and surface cold pool signatures. The low-level Chen 2011; YU18). A considerable portion of OTCRs inflow fed the rainband and rose rearward immediately occurring in the real atmosphere may be located in or ahead of the leading edge of the low-level rear-to-front propagate into regions outside the operational alert flow. As GE222 descended to approach the runway of area of TCs. MG Airport and flew immediately behind and roughly For the present case, according to the warning report parallel to the leading edge of R3, the aircraft en- of the typhoon issued by CWB, Matmo maintained a countered the heaviest precipitation of the rainband similar size of ;180–200 km (in radius) during the pe- and the prominent crosswind (i.e., westerly flow) that was riod of our primary interest, which was approximately exactly a manifestation of band-relative rear-to-front flow 2 identical to the average radius of 34 kt (1 kt ’ 0.51 m s 1) generated locally by the rainband. The momentum budget

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important but much more challenging compared to the routine operational procedures to issue TC warnings of the inner-core hazardous region with a quasi-circular geome- try that can be practically identified given a predicted or observed TC center.

Acknowledgments. The Doppler radar data and sur- face observations used in this study were provided by both the Taiwan Central Weather Bureau (CWB) and the Weather Wing of the Air Force of Taiwan. The authors thank three anonymous reviewers for providing constructive comments that improved the manuscript. This study was supported by the Ministry of Science and Technology of Taiwan under Research Grants MOST106-2111-M-002-002-MY3 and MOST106-2111- M-002-013-MY3.

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