OCTOBER 2019 Y U E T A L . 3267 Tracking a Long-Lasting Outer Tropical Cyclone Rainband: Origin and Convective Transformation CHENG-KU YU,CHE-YU LIN, AND JHANG-SHUO LUO Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan (Manuscript received 8 May 2019, in final form 7 August 2019) ABSTRACT This study used radar and surface observations to track a long-lasting outer tropical cyclone rainband (TCR) of Typhoon Jangmi (2008) over a considerable period of time (;10 h) from its formative to mature stage. Detailed analyses of these unique observations indicate that the TCR was initiated on the eastern side of the typhoon at a radial distance of ;190 km as it detached from the upwind segment of a stratiform rainband located close to the inner-core boundary. The outer rainband, as it propagated cyclonically outward, underwent a prominent convective transformation from generally stratiform precipitation during the earlier period to highly organized, convective precipitation during its mature stage. The transformation was ac- companied by a clear trend of surface kinematics and thermodynamics toward squall-line-like features. The observed intensification of the rainband was not simply related to the spatial variation of the ambient CAPE or potential instability; instead, the dynamical interaction between the prerainband vertical shear and cold pools, with progression toward increasingly optimal conditions over time, provides a reasonable explanation for the temporal alternation of the precipitation intensity. The increasing intensity of cold pools was suggested to play an essential role in the convective transformation for the rainband. The propagation characteristics of the studied TCR were distinctly different from those of wave disturbances frequently documented within the cores of tropical cyclones; however, they were consistent with the theoretically predicted propagation of convectively generated cold pools. The convective transformation, as documented in the present case, is anticipated to be one of the fundamental processes determining the evolving and structural nature of outer TCRs. 1. Introduction from satellite and radar observations, usually exhibit higher asymmetry as opposed to the quasi-circular ge- Tropical cyclone rainbands (TCRs) are the most ometry of the inner TCRs (Willoughby et al. 1984; striking and persistent feature of tropical cyclones (TCs) Houze 2010). Moreover, the outer areas of TCs tend to (Senn and Hiser 1959; Anthes 1982; Willoughby et al. possess larger convective available potential energy 1984; Marks 2003; Houze 2010; Yu and Chen 2011). (CAPE) than the inner-core environment (Frank 1977; Despite the high variability in convective characteristics Bogner et al. 2000; Yu and Chen 2011; Molinari et al. and organization for TCRs, they are conveniently clas- 2012; Yu and Tsai 2013), which supports intense con- sified into inner and outer rainbands based on the degree vection and potentially threatening and severe weather to which convection is influenced by the inner-core conditions (Houze 2010; Yu and Tsai 2013). The im- vortex circulation. Around 100–200 km or approxi- portance of both inner and outer TCRs on the devel- mately 2–3 times the radius of maximum wind (RMW) opment of TCs has also been well acknowledged is a common, approximate threshold of radial distance (Shapiro and Willoughby 1982; Willoughby et al. 1982; to distinguish these two distinct rainbands (Willoughby Barnes et al. 1983; Willoughby 1990; May and Holland 1988; Wang 2009). The moist convection of the inner 1999; Houze et al. 2006; Wang 2009; Riemer et al. 2010). TCRs is strongly constrained by the inner-core vortex Theoretically, the appearance of TCRs has long been and is rapidly filamented (Rozoff et al. 2006). The fila- recognized as a consequence of atmospheric waves ini- mentation effect associated with the outer TCRs is rel- tiated near the eyewall or close to the TC center atively weak, and their precipitation patterns, as seen (Macdonald 1968; Diercks and Anthes 1976; Kurihara 1976; Willoughby 1977, 1978; Guinn and Schubert 1993; Corresponding author: Cheng-Ku Yu, [email protected] Montgomery and Kallenbach 1997; Gall et al. 1998; DOI: 10.1175/JAS-D-19-0126.1 Ó 2019 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 09/26/21 12:31 PM UTC 3268 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76 Chen and Yau 2001; Wang 2002; Corbosiero et al. 2006; Hence and Houze 2008; Didlake and Houze 2009; Yu and Tsai 2010). Inner TCRs have been thought to Tang et al. 2014). However, most of these studies dealt be more probably related to vortex Rossby waves with the so-called principal band, a well-known rain- (Montgomery and Kallenbach 1997; Corbosiero et al. band type that is typically located near the bound- 2006), although a consensus for this wave interpretation ary between the inner core and outer region in TCs has not been completely reached (e.g., Moon and Nolan (Willoughby et al. 1984; Marks 2003; Houze 2010). 2015). Compared to the inner TCRs, explanations for These aircraft investigations are mainly confined to the origin of the outer TCRs remain more controversial. the inner-core vicinity and thus are unable to address For example, although the outer TCRs are traditionally the structures and dynamics of the outer TCRs. It is considered as a manifestation of inertia–gravity waves also practically difficult for aircraft observations to (Diercks and Anthes 1976; Kurihara 1976; Willoughby document the evolving aspects of TCRs over a long 1977; Chow et al. 2002), the outward propagation of the period of time because of typically only a few hours observed outer TCRs (several meters per second; Yu for a given flight mission (e.g., Tang et al. 2018). and Tsai 2010) seems much slower than the typical As a matter of fact, a considerable number of outer outward speed of inertia–gravity waves (several tens of TCRs have been studied and reported in the literature meters per second) (Chow et al. 2002; Sawada and from the observational perspective. Earlier studies of Iwasaki 2010; Li and Wang 2012; Nolan and Zhang the outer TCRs focus mostly on the gross characteristics 2017). Theoretically, the inward propagation of inertia– of surface fluctuations as the rainbands passed by (Ligda gravity waves excited at the storm periphery is also 1955; Ushijima 1958; Hamuro et al. 1969; Skwira et al. possible (Willoughby 1977). On the other hand, the in- 2005). With high-resolution Doppler radar measure- creasing evidence from observational and modeling ments, several recent studies have depicted the detailed studies reveals the important effect of convectively aspects of airflow and precipitation associated with the generated cold pools, instead of wave forcings, on the outer TCRs as they approached the coastal area or made triggering and maintenance of moist convection associ- landfall (Yu and Tsai 2010, 2013, 2017). One of the most ated with outer TCRs (Eastin et al. 2012; Yu and Tsai interesting findings from these radar examinations is 2013; Moon and Nolan 2015). Moreover, results from a that the outer TCRs can sometimes exhibit structural few recent numerical studies of the outer TCRs show and surface characteristics similar to ordinary convec- that the outer rainband formation would be probably tive systems such as squall lines. More recently, Yu et al. linked to the preexisting activity of the inner rainbands (2018, hereafter YU18) explore the degree of preva- as they move radially outward to the outer region of TCs lence for this potential similarity by analyzing long-term with decreased filamentation and stabilization (Li and dual-Doppler radar and surface observations from a Wang 2012; Li et al. 2017). Complicated interactions large set of 50 outer TCRs within 22 TCs as they ap- between inner-core vortex circulation and its outer en- proach the Taiwan area. The study documents not only a vironmental flow represent another potential factor that frequent similarity between the outer TCRs and squall would contribute to the initiation of outer TCRs and/or lines (;58%) but also some variability in structural fea- the occurrence of heavy rainfall in the outer region of tures of the observed outer TCRs. However, owing to the TCs (Willoughby et al. 1984; Wu et al. 2009; Akter and inherent limitation of observations in both temporal and Tsuboki 2012; Chen et al. 2014). The formative pro- spatial coverage, investigations for all of these aforemen- cesses of the outer TCRs are possibly diverse in nature, tioned studies are primarily confined to the mature stage but it is clear that the interplay of atmospheric waves, of the rainband’s lifetime so that little is learned about the cold pool dynamics, vortex–environment interactions, initiation and/or evolving scenarios of the outer TCRs. and various ambient conditions in influencing outer The primary objective of this study is to use radar rainband formation is still poorly understood and de- observations to document a long-lasting outer TCR of serves further clarification. Typhoon Jangmi (2008) as it propagated northwestward Because of a general lack of detailed observations from ;400 km east of Taiwan to the offshore area north over the open ocean, aircraft observations including of Taiwan. A unique aspect of this particular case is that flight-level in situ and airborne radar measurements the combination of the Taiwan Doppler radar observing have served as a critical way to investigate the kine- network and a Japanese radar at Ishigaki located matic, thermodynamic, and precipitation features of ;250 km east of Taiwan (Fig. 1) is able to track the TCRs. These aircraft studies have considerably ad- temporal alternations of the TCR’s precipitation over a vanced our knowledge of various mesoscale aspects of considerable period of time (;10 h) from its formative TCRs (e.g., Barnes et al.