JULY 2020 Y U E T A L . 2935 Outer Tropical Cyclone Rainbands Associated with Typhoon 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 tropical cyclone rainbands (OTCRs) of Typhoon Matmo. This study aims to document the detailed aspects of airflow and precipitation 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 rainband and the prominent crosswind that was a manifestation of the rear-to-front flow generated locally by the rainband. The heavy rain–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 cyclones and their potential transformation into squall-line-like storms 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 thunderstorms, 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 thunderstorm 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.