Tracking a Long-Lasting Outer Tropical Cyclone Rainband: Origin and Convective Transformation

Home , Rainband, Typhoon Hagupit (2014), Typhoon Jangmi (2008)

OCTOBER 2019 Y U E T A L . 3267

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 accompanied 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. 1983; Jorgensen 1984; Marks and to mature stage. Such continuous tracking is usually not Houze 1987; Powell 1990a,b; Samsury and Zipser 1995; possible because a long-lasting TCR embedded within the

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3269

period of primary interest are also indicated in Fig. 1.The TCR was one of the major outer rainbands observed inside Typhoon Jangmi, when the typhoon’s center approached the eastern coast of Taiwan on 27–28 September 2008. In this study, the rainband’s precipitation was documented by five coastal (island) ground-based operational radars, which were the Central Weather Bureau (CWB) of Taiwan S-band (10 cm) Doppler radars at Wu-Fen-San (WFS), Hua-Lien (HL), and Ken-Ting (KT), the Civil Aeronautics Administration of Taiwan C-band (5 cm) Doppler radar (CAA) at Taoyuan International Airport, and the Japan Meteorological Agency C-band radar at Ishigaki (IG) (see the locations in Fig. 1).TheWFS,CAA,HL,andKT Doppler radars were located near the northern coast, the northwestern coast, the eastern coast, and the southern tip of Taiwan, respectively, and were operated with a temporal interval of 5–10 min between each volume and a maximum observational range of 150–460 km. These four coastal radars could provide a comprehensive view of precipitation FIG. 1. Observations used in this study and isochrones (solid information as the TCR approached the oceanic and lines) of the studied TCR as defined by the outer edge of the nearshore area of eastern and northern Taiwan. The de- 30-dBZ-reflectivity contour from 2100 UTC 27 Sep to 0600 UTC tailed characteristics of these coastal Doppler radars were 28 Sep 2008. The locations of four Taiwan Doppler radars at Wu- described in YU18. Fen-San (WFS), Hua-Lien (HL), Ken-Ting (KT), and Taoyuan ; International Airport of Civil Aeronautics Administration (CAA) The IG radar was located offshore, 250 km east of and one Japan radar at Ishigaki (IG) are denoted by triangles. The Taiwan, and provided volumetric distributions of re- locations of selected surface stations at Pengjiayu (PJY), Keelung flectivity measurements with 19 elevations of 20.48–29.98,a (KL), Hateruma (HTM), Ishigaki (IG), Ibaruma (IBM) and Nakasuji 400-km observational range, and a temporal resolution of (NS) are denoted by solid circles. The hourly locations of Typhoon 10 min. The location of the TCR during its formative stage Jangmi (2008) identified from radar images during the period of primary interest are also indicated. (2100–2300 UTC 27 September) was over the open ocean, approximately ;200–450 km off the eastern coast of TC circulation typically can travel several hundred ki- Taiwan (cf. Fig. 1). The availability of the IG radar, located lometers after its initiation, a considerable distance well near the northern segment of the TCR during these earlier beyond the maximum observational range of a given periods, is critical to complement the inadequate data meteorological radar. The outer TCR presented herein coverage of the coastal radars and to provide a more during its mature stage was also noted in YU18 and had complete depiction of low-level precipitation for the been classified as a squall-line-like rainband (labeled as rainband. To construct the low-level precipitation field, TCR33inTable2ofYU18) because of its substantial reflectivity measurements from the low plan position similarity to squall lines in terms of airflow pattern and cold indicator (PPI) scan of each radar (20.28–0.48 eleva- pool signature. As elaborated in YU18, squall-line-like tions) were first interpolated to Cartesian coordinates structures are one of the most common mesoscale charac- with a horizontal grid spacing of 1 km over a large do- teristics for outer TCRs. The present study, particularly main of 1100 3 1100 km2. When reflectivity data from with the availability of radar measurements encompassing different radars were available at a given horizontal grid the earlier stage of the rainband’s development, allows point, the maximum reflectivity value was chosen to clarification of whether the studied TCR underwent any mitigate the effects of attenuation. prominent convective transformation prior to the mature, In addition to these Doppler radar measurements, sur- squall-line-like phase and to provide important insight into face observations from a few Taiwanese and Japanese the possible origin of a squall-line-like outer TCR. meteorological stations located near northern Taiwan and the surrounding oceanic area of Ishigaki Island (locations in Fig. 1) were used to investigate the near-surface charac- 2. Data teristics of kinematics and thermodynamics of the TCR. As The datasets used to investigate the studied TCR are indicated in Fig. 1, the rainband passed over these surface summarized in Fig. 1. For reference, the isochrones of (island) stations in a transverse direction so surface obser- the TCR and the locations of the typhoon center during the vations from these stations were helpful to reveal changes,

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC 3270 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76

FIG. 2. Hourly low-level reflectivity composite (dBZ) associated with Typhoon Jangmi during the study period from 2100 UTC 27 Sep to 0600 UTC 28 Sep. The thick arrows highlight the location of the studied TCR. The inset box (40 3 80 km2) indicates the location for calculating the mean vertical cross sections shown in Fig. 5. The inset boxes are chosen to be located near the central segment of the rainband and to have a better radar sampling of low-level reflectivities along the rainband. During the initiation of the TCR, it detached from an upwind segment of a stratiform rainband that is marked by SR in (a). if any, in surface fluctuations of the rainband as it evolved period of primary interest is shown in Fig. 2. The mean from the formative to mature stage. The temporal resolution radar reflectivities (dBZ) averaged along the length of of these surface observations was 1 or 10 min. The National the TCR at different times are also shown in Fig. 3 for Centers for Environmental Prediction (NCEP) Climate reference. According to the warning report of the ty- Forecast System Reanalysis (CFSR) data (0.5830.58)with phoon issued by CWB, Jangmi was classified as an temporal resolution of 6-h (Saha et al. 2010) are also used to intense typhoon during the study period, with a max- 2 analyze the environmental conditions for the TCR. imum wind speed of ;53 m s 1 and a minimum sea level pressure of 925 hPa. Jangmi moved northwest- 2 ward (;3328) at a mean speed of 25 km h 1 and 3. Precipitation evolution and propagation exhibited a well-identified eye and approximately characteristics symmetric eyewall (Figs. 1 and 2). The studied TCR, The evolution of low-level hourly radar-observed highlighted by thick arrows in each panel of Fig. 2, precipitation associated with Typhoon Jangmi over the was initiated ;190 km east of the typhoon center and

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3271

scale environment for Typhoon Jangmi was characterized by a relatively weak northerly vertical shear with 2 magnitudes of 4–8 m s 1, as calculated over the layer of 700–200 hPa within a radial distance of 150–500 km from the NCEP/CFSR reanalysis data. Note that the re- liability of the reanalysis data in the low troposphere (below ;3 km MSL) is usually hampered by high topography over Taiwan. To avoid this effect, the 700–200 hPa (instead of 850–200 hPa) layer is chosen for the calculation of vertical shear. Given a northerly shear, the initiation and intensification of the TCR thus took place primarily in the upshear region. This FIG. 3. Temporal variation of the mean radar reflectivity (dBZ) averaged over the regions of primary precipitation (.20 dBZ) observed aspect was different from previous idealized along the studied TCR from 2100 UTC 27 Sep to 0600 UTC simulations of TCs showing that outer TCRs preferably 28 Sep. Shading highlights the mature stage of the rainband. form and develop on the downshear side (e.g., Li et al. 2017). The rainband’s precipitation tended to weaken became well defined at ;2100 UTC 27 September considerably after ;0300 LST, and the banded feature of (Fig. 2a). The TCR originally was an upwind segment heavy precipitation collapsed into multiple, discrete ele- of a stratiform rainband (marked by SR in Fig. 1a) lo- ments during these time periods (Figs. 2h,i and 3). The cated close to the inner-core boundary1 (;150 km). The weakening of the rainband was observed to continue as it higher temporal resolution (10 min) of low-level pre- propagated northwestward out of the observational cov- cipitation available around and prior to 2100 UTC in- erage of coastal radars. dicates that the studied rainband detached from the Consistent with the prominent evolution of the TCR’s parent, stratiform rainband over time to form an isolated, precipitation as described above, a dramatic change in outward-propagating rainband (Fig. 4). The parent rain- its convective characteristics was also evident during the band during this period was observed to weaken rapidly, study period. Mean cross-band vertical sections of radar with relatively disorganized and transient precipitation reflectivity valid at times corresponding to each of the features. In addition, this rainband did not exhibit ob- analysis periods shown in Fig. 2 reveal a generally vious convectively active elements of precipitation on stratiform precipitation prior to and approximately at its upwind side or elsewhere. These observed aspects 0000 UTC 28 September (Figs. 5a–d). As the intensi- did not resemble typical principal band characteristics ties of the rainband’s precipitation increased with time, (Willoughby et al. 1984; Houze 2010). an interesting transition of moist convection from During the initiation stage of the TCR, its pre- stratiform to convective rainfall occurred at approxi- cipitation was relatively weak, with a mean reflectivity mately 0100 UTC (Fig. 5e). During the next 2 h, the of only 28 dBZ (Fig. 3). With time, the precipitation precipitation of the rainband developed a significant along the band intensified persistently and was em- vertical extent with the 40-dBZ contour exceeding 4 km bedded with a few separate elements of enhanced re- (MSL) and strong horizontal gradients, indicating the flectivities (Figs. 2b–d and 3). An elongated, narrow highly convective nature of the precipitation (Figs. 5f,g). zone of enhanced reflectivities (.40 dBZ) started to Maximum intensities of precipitation at 45–50 dBZ were appear as the rainband propagated cyclonically into the observed in the lowest 3 km (MSL). The dual-Doppler region north of the typhoon center at approximately radar analyses off the northern coast of Taiwan, as 0100 UTC 28 September (Fig. 2e). The TCR reached its reported in YU18, confirm that the rainband during maximum intensity around 0300 LST 28 September this period exhibited squall-line-like features. Based (Figs. 2g and 3). Highly organized features of intense on these observations, the time period from ;0100 to radar reflectivities, with peak (mean) values exceeding ;0330 UTC 28 September, as highlighted by shading in 50 (34) dBZ, were observed during this time. The large- Fig. 3, may be considered as a mature or squall-line-like stage for the TCR. Shortly after, the precipitation intensity of the TCR decreased considerably and continued to weaken during the rest of the radar observations 1 ; Based on the average RMW of Typhoon Jangmi ( 40 km) (Figs. 5h,i). Although the low-level precipitation below provided by the Joint Typhoon Warning Center (JTWC) and the symmetric feature of rainbands largely confined to ;150 km from 2 km (MSL) was not well captured by the coastal radars the typhoon center, the inner-core region is reasonably assumed to due to the inherent limitation of radar scanning, hori- be within a radial distance of ;120–150 km for the present case. zontally uniform reflectivities and a brightband signature

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC 3272 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76

FIG.4.AsinFig. 2, but showing the low-level reflectivity composite (dBZ) with a high temporal resolution of 10 min during the detached period of the studied rainband: (a) 2040, (b) 2050, (c) 2100, and (d) 2110 UTC. observed at ;4 km (MSL) close to the melting level both slightly at different times to encompass the central seg- indicate the characteristics of stratiform precipitation ment of the rainband. In contrast to the quasi-circular over these late periods. Combination of Figs. 2 and 5 geometry of the inner TCRs, the outer TCRs typically not only supports that the outer TCR underwent an in- are oriented at some angles to circles about the TC triguing convective transformation but also implies that a center, just like the present case (cf. Fig. 1). Since the squall-line-like outer TCR may originate from relatively mean reflectivities in the Hovmöller diagrams are cal- weaker convection observed just outside the inner-core culated along the circles rather than along the TCRs, the boundary of the typhoon circulation. mean radar reflectivities seen from the Hovmöller dia- Other important aspects of the TCR, as suggested by grams cannot be perfectly representative of the actual Fig. 2, are that it tended to move away from the typhoon intensities of precipitation for the TCRs. This is the center and rotate cyclonically. These propagation reason why the peak intensity of the studied rainband at characteristics can be best depicted by the radius–time 0300 UTC (cf. Figs. 3 and 5) was not clearly evident in (azimuth–time) Hovmöller diagram of low-level re- the Hovmöller diagrams shown in Fig. 6. flectivities associated with the TCR, as shown in Fig. 6. The radius–time Hovmöller diagram indicates that Radar reflectivities within a 408 sector from the center the TCR appeared initially at a radial distance of ;190 km and within a 50-km ring band are used to produce the at 2100 UTC 27 September (Fig. 6a). The rainband kept radius–time and azimuth–time Hovmöller diagrams, moving away from the typhoon center and traveled to respectively. The azimuthal location of the sector and farther outer regions at a radial distance of ;290 km near the radial distance of the ring band are both adjusted the end of the study period (i.e., 0600 UTC 28 September).

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3273

FIG. 5. Mean vertical cross sections of reﬂectivity (dBZ; color shading) calculated along inset boxes in Fig. 2 at times corresponding to each of the analysis periods in Fig. 2.

Such an outward propagation has been documented to between ;100 and ;200 km) in the rear side of the TCR be a common characteristic of outer TCRs as described before ;0100 UTC 28 September (Fig. 6a). The possible in YU18. The outward-propagation speed, estimated role of the rear-flank stratiform precipitation in the pres- from Fig. 6a, exhibited some minor fluctuations at dif- ent case will be discussed further in section 5. 2 ferent time periods, ranging from 2.9 to 7.2 m s 1. This A cyclonic rotation of the TCR, from the east to the observed propagation is much slower than the typical northwest of the typhoon, can be clearly seen in the 2 outward speed of inertia–gravity waves (22–60 m s 1) azimuth–time Hovmöller diagram, as shown in Fig. 6b. identified within TCs (Chow et al. 2002; Sawada and A propagation sector of 112.58 (Fig. 6b) and an average Iwasaki 2010; Li and Wang 2012; Nolan and Zhang radius of 240 km (Fig. 6a) over the duration of 4.5 h from 2017), but is similar to previously documented outer 2130 UTC 27 September to 0200 UTC 28 September 2 2 TCRs with outward propagations of 3–5 m s 1 (Yu and yield an azimuthal propagation speed of ;29 m s 1. This Tsai 2010). It is interesting to note that, in addition to the cyclonic propagation is roughly comparable to low-level intense precipitation within the inner core, considerable tangential wind speeds along the TCR that were derived stratiform precipitation continued to exist over a wide from the dual-Doppler observations available off the region across the inner-core boundary (i.e., radial distance northern coast of Taiwan (YU18). Theoretically, vortex

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC 3274 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76

Rossby waves can propagate spatially only in narrow annular waveguides deﬁned by a turning radius close to the storm center and by an outer critical radius where the wave’s energy is absorbed (Cotto et al. 2015; Gonzalez et al. 2015). The outward group propagation of the waves tends to cease at the stagnation radius (;3timestheRMW)(Montgomery and Kallenbach 1997). In addition, the vortex Rossby wave–like rainbands usually propagate cyclonically at a speed much slower than ambient tangential winds (Corbosiero et al. 2006). The TCR observed in the present study propagated outward in the outer region (i.e., beyond the stagnation radius) and did not exhibit upstream propagation relative to the ambient mean ﬂow. These observational aspects are distinctly different from the typical propagating charac- teristicsofvortexRossbywaves.Itshouldbenotedthata relatively slow propagation in both radial and azimuthal directions after 0300 UTC 28 September may be less re- alistic (Figs. 6a,b) because the entity of the studied TCR was only partially observed by the coastal radars during these time periods. Having said so, a lower cyclonic propagation of the rainband in farther outer regions during these periods also seems reasonable, considering that tangential winds typically decrease with radial distance outside RMW.

4. Surface characteristics As described in section 2, the studied TCR passed over several offshore stations located east (north) of Taiwan (Fig. 1). Observations from these island stations provide a unique opportunity to understand surface characteristics of the rainband as its attendant convection intensified from a generally stratiform precipitation at the early stage to a more convective precipitation during the mature stage (cf. Figs. 2, 3,and5). Figure 7 shows the time series analyses of 1-min temporal-resolution measurements recorded from two selected surface stations at IG and PJY (locations in Fig. 1) during the passage of the rainband that were used to illustrate finescale surface fluctuations for these two distinct stages of the evolving rainband. Eight surface meteorological quantities, tem- ö FIG. 6. (a) Radius–time Hovm ller diagram of low-level radar perature T, dewpoint temperature Td, relative humidity reflectivities (dBZ) averaged within a 408 sector encompassing (RH), equivalent potential temperature u , perturbation ö e the central segment of the TCR. (b) Azimuth–time Hovm ller pressure p0, band-relative cross-band and along-band wind diagram of low-level radar reflectivities (dBZ) averaged within a 50-km ring band that roughly moves with the radial location of components Vc and Va and rainfall rate (RR), are the TCR at different times. In (a) and (b), maximum represented herein. To better isolate pressure fluctuations flectivities seen nearby the TCR from each analysis time are caused by the passage of the rainband, the perturbation indicated by thick solid line, highlighting the location of the pressure was calculated by subtracting the storm-scale rainband. pressure tendency (i.e., 1-h running mean of surface pressure) from the pressure values recorded at a given time (Yu and Tsai 2010; Yu and Chen 2011). A reflectivity threshold of 30 dBZ (Yu and Tsai 2010) was

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3275

adopted to delineate the outer (inner) edge of the studied TCR (vertical dashed lines in Fig. 7) that can approximately locate the horizontal extent of primary precipitation of the rainband. Results from surface observations at IG indicate that the temperature decreased gradually from the outer edge to the inner side of the rainband, with an obvious drop (;1.38C) during heavier rainfall at ;44–46 min (Fig. 7a). The lower temperature tended to recover temporarily toward the inner edge of the rainband, where light rain predominated. A moderate decrease in equivalent potential temperature (;7 K), which was accompanied by greater rainfall, was also evident. A generally positive perturbation pressure (a maximum of ;0.7 hPa) was present within the band. Decreases in

both Vc and Va, with a change in wind direction (;228), were observed at approximately 32–34 min. This wind shift occurred in the position close to the leading edge of colder air inside the rainband, where temperature (pressure) started to drop (rise). A typical rainfall rate 2 during this stage is generally less than 10 mm h 1. The cold pool signature observed at IG was less prominent, but a stronger cold pool tended to develop subsequently, as shown in the time series analysis of surface observations at PJY during the passage of the

rainband (Fig. 7b). Sharp decreases in both T and Td (;2.58C) were evident in the outer side, shortly after the arrival of the outer edge. An obvious temperature deﬁcit continued to persist inside the band and beyond the inner edge, implying a wider spatial extent of the convectively generated cold pool during this period. There was a signiﬁcant drop in equivalent potential temperature (;13 K) from ;356 K to a minimum of ;343 K, which was attributed to the presence of colder and

lower-moisture air (i.e., lower T and Td) inside the rainband. The observed reductions in both T and Td cannot be simply explained by the evaporative effect of precipitation, and this thermodynamic feature has been shown to commonly occur for TCRs and tropical deep

convection due to the downward transport of low-ue air originating from higher altitudes by convectively induced downdrafts (Barnes et al. 1983; Skwira et al. 2005; Tompkins 2001; Yu and Chen 2011). Similar characteristics for temperature and moisture perturbations can 0 also be seen in Fig. 7a. FIG. 7. Time series of T, Td, RH, ue, p , Vc, Va and RR with a 1-min temporal resolution observed at (a) the IG surface station Another striking feature of thermodynamics was a from 2250 UTC 27 Sep to 0010 UTC 28 Sep and (b) the PJY surface gradual decrease in p0 immediately before the arrival of station from 0203 to 0307 UTC 28 Sep. For reference, ground- the outer edge, followed by a sharp increase in p0 across relative winds are also indicated by wind flags with full wind barbs 21 21 the outer edge (Fig. 7b). In particular, the observed corresponding to 5 m s and half barbs corresponding to 2.5 m s . pressure jump was accompanied by a prominent wind Negative (positive) values for Vc represent inflow (outflow) relative to the rainband’s motion. Two vertical dashed lines represent shift (;408) from northeasterlies ahead of the outer the outer and inner edges of the rainband. edge to generally easterlies inside the rainband. Note that the temperature stayed nearly constant during the

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC 3276 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76 occurrence of the pressure jump. It is thus more likely that the high pressure was dynamically induced due to the pronounced deceleration in Vc and high rainfall rates (Fig. 7b)(Yu and Tsai 2013). These surface ﬂuctuations are similar to those characterizing the passage of the leading edge of squall lines and gust fronts (Wakimoto 1982; Roux et al. 1984). The minimum p0 observed close to the outer edge was closely collocated with the region of much weaker precipitation and would be probably related to warming by compensating downward motions at upper levels ahead of the leading updrafts (Houze et al. 1989). The rainfall rates of the TCR during this stage were generally very high and had maxima greater 2 than 100 mm h 1, consistent with the highly convective nature of the rainband’s precipitation during this period (cf. Figs. 5f,g).

5. Discussion: Convective transformation a. Environmental CAPE and potential instability

2 22 In this section, we seek possible environmental factors FIG. 8. Mean spatial distribution of ambient CAPE (m s ; color and/or processes that may contribute to the convec- shading) associated with Typhoon Jangmi (2008) during the study tive transformation of the studied TCR. As shown in period. Isochrones (solid lines) of the studied TCR, as shown in the radar analyses, the rainband initially underwent a Fig. 1, are also superposed on the analysis for reference. prominent intensification of precipitation, reached a 2 mature stage at approximately 0100–0330 UTC, and (;200–1600 m2 s 2), presumably due in part to the in- then weakened rapidly afterward as it propagated cy- fluences of persistent inner-core convection (cf. Fig. 2). clonically outward after initiation (cf. Figs. 2–5). Previ- Based on the analysis of Fig. 8, the propagating rainband ous studies of TCs have shown that there were considerable is expected to have experienced a clear transition from a spatial variations of CAPE associated with TC circulations, generally large to small CAPE environment during the with generally larger values in the outer regions than study period. in the inner-core environment (Bogner et al. 2000; To better illustrate the potential connection between Molinari et al. 2012). The observed changes in the the evolving precipitation intensities of the rainband and convective intensity of the studied rainband during its its ambient CAPE, the CAPE values within a band- propagation would be thus probably related to the parallel, elongated zone (25 km wide) ahead of the spatial variations of CAPE in the TC circulation. To rainband are averaged and plotted as a function of time, evaluate this likelihood, the NCEP/CFSR reanalysis as shown in Fig. 9. It should be noted that the temporal data are used to calculate CAPE values over a larger change of the prerainband CAPE is expected to be domain encompassing the cyclonic circulation of Ty- primarily caused by the spatial variation of the CAPE phoon Jangmi. Because the spatial distributions of the as the TCR propagated rapidly from the east to the CAPE at different times remain very similar during northwest of the typhoon (cf. Figs. 1 and 8). Given that the study period, a mean spatial distribution is thus the traveling distance of the TCR during the study pe- calculated, as shown in Fig. 8, and for reference, the riod (;10 h) was more than 600 km (Fig. 1), the tem- isochrones of the rainband are also superposed on this poral variation of the prerainband environment over CAPE analysis. A prominent asymmetric pattern of this large window of space still can be adequately re- ambient CAPE was evident, with larger (much smaller) solved by the reanalysis data with relatively coarse values over the eastern (western) portion of the typhoon spatial resolution. The mean prerainband CAPE re- 2 circulation. The largest CAPE (.2000 m2 s 2) prevailed mained approximately constant with moderate values 2 in farther outer regions over the open ocean, in contrast of ;1000 m2 s 2 during the early period of analysis 2 to quite small CAPE (,200 m2 s 2) over the Taiwan but started to reduce significantly after 0000 UTC Strait and over the landmass of China. Within the inner- 28 September. The environmental CAPE dropped to 2 core region, the CAPE had a wide range of values small values of approximately 200 m2 s 2 or below

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3277

b. Cold pool dynamics Given that one of the most prominent thermodynamic features of the studied TCR was the presence of colder air at low levels, it is worthwhile to evaluate if the cold pool dynamics can provide a reasonable explanation for the convective intensification of the rainband during its early to mature stage. In the context of cold pool dynamics, the presence of suitable ambient vertical shear in the cross-band direction is dynamically critical because it plays an essential role in modulating the con- 2 22 FIG. 9. Temporal variation of the mean CAPE (m s ; solid line) vective intensity forced by the advancing cold pool averaged within a band-parallel, elongated zone (25 km wide) (Rotunno et al. 1988; Parsons 1992). Specifically, ahead of the TCR during 2100 UTC 27 Sep–0600 UTC 28 Sep. The corresponding temporal variation of the mean radar reflectivity whether the horizontal vorticity produced by the en- (dBZ; dashed line), as shown in Fig. 3, is also indicated for com- vironmental cross-band vertical shear can effectively parison. Shading represents the mature stage of the rainband. counteract the cold pool–induced vorticity determines the intensity and tilting nature of the lifted updrafts. As shown in Fig. 10c, the prerainband environment after 0400 UTC 28 September. Small fluctuations was generally characterized by considerable frontward and/or an obvious reduction in the prerainband CAPE vertical shear in the cross-band direction (i.e., decrease are not in agreement with a continuous intensification of in the magnitude of V with height), particularly in the the rainband’s precipitation during its early to mature c lowest 2 km (MSL), making the cold pool–shear inter- stage (i.e., between 2100 UTC 27 September and action possible. Following the vorticity balance theory, 0300 UTC 28 September), although the subsequent as explored by Rotunno et al. (1988), the horizontal weakening of the rainband seems consistent with the vorticity generated by buoyancy gradients across the prominently decreasing CAPE during the late period leading edge of the cold pool may be represented by of analysis. The time–height cross section of relative rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi humidity for the prerainband environment reveals a g C 5 DuH , (1) persistent, moist condition (.90%) in the low to u o midtroposphere (Fig. 10a). The entrainment effects are expected to be relatively minor and cannot reasonably where Du is the cold pool potential temperature deficit explain the inconsistency of temporal trend between the relative to ambient conditions, uo is the prerainband, intensity of the rainband and its ambient CAPE. average potential temperature, and H is the cold pool In addition to CAPE, it is also interesting to examine depth. The horizontal vorticity induced by ambient whether potential instability, one of the important fac- shear can be represented by Du, which is the vertical tors influencing convective motions, was present in the shear over the depth of the cold pool H. The C equal to prerainband environment. The time–height cross sec- Du represents an optimal state; in this situation, the tion of equivalent potential temperature, as shown in convective updrafts at the leading edge of the cold pool Fig. 10b, indicates prominent potential instability (i.e., are upright and the most intense. However, if the cold decrease in ue with height) continued to exist below pool-induced vorticity is not countered by a comparable ;4.5 km (MSL), except at times near the end of the ambient shear (i.e., either C .Du or C ,Du), convec- analysis period (i.e., after ;0400 UTC 28 September). tive intensities would be weaker with slanted updrafts. The prevalence of similar potential instabilities evident In the present case, the evaluation of the vorticity bal- at different time periods appears less relevant to the ance dynamics is presented for two distinct time periods rapid intensification of the rainband’s precipitation as the TCR passed over surface stations at IG and PJY. during its early to mature stages. Note that the layer As evident in the previous sections, the rainband un- of minimum ue (345–346 K) was located near 4.5 km derwent prominent changes in both precipitation and (MSL), and this ue value was quite close to the near- surface characteristics over the duration encompassed surface lowest ue (343–345 K) observed within the region by these two time periods. The temperature deficits of heavy precipitation inside the rainband during its Du associated with the rainband at IG and PJY, calcu- mature stage (Fig. 7b). Assuming the moist adiabatic lated from Fig. 7, are equal to 1.6 and 3.0 K, respectively. processes with little dilution, the low-level cold pool air The uo values at IG and PJY are calculated to be 300.2 would mostly originate from ;4.5 km (MSL) for the and 298.6 K, respectively. Based on the IG sounding represent case. leased at 0000 UTC 28 September (i.e., a time immediately

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC 3278 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76

FIG. 10. Time–height cross sections of the mean (a) RH, (b) ue, and (c) Vc averaged within a band-parallel, elongated zone (25 km wide) ahead of the TCR during the study period from 2100 UTC 27 Sep to 0600 UTC 28 Sep. Shading represents the mature stage of the rainband.

21 after the rainband passed by; cf. Fig. 1), a shallow, el- into (1) yields C 5 10.2 (13.9) m s at IG (PJY). The Vc evated layer of enhanced temperature inversion char- proﬁles in Fig. 10c areusedtocalculatetheambient acterizing the top of the low-level cold pool is found at cross-band vertical shear Du, which is equal to 14.8 2 the height of approximately 2 km (MSL). The cold pool (14.0) m s 1 at IG (PJY). These calculations suggest a depth (H ; 2 km) observed in the present case is close convective transition from a greater-than-optimal state to the typical vertical extent of cold pools documented (i.e., C ,Du) at IG to an approximately optimal state within outer TCRs for northwestern Paciﬁc TCs (Yu (C ;Du) at PJY. Note that given a relatively minor and Tsai 2013; Lin et al. 2018). Substituting these values change in Du, the transition is mostly due to the

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3279

21 increasing cold pool intensity. The progression toward These values yield a Vcp of 12.7 (18.9) m s at IG increasingly optimal conditions over time is consis- (PJY). The observed cross-band propagation speed, as tent with the intensification of the radar-observed tracked by the outer edge of the rainband, is found to 2 precipitation from weakly to highly convective nature be 8.9 (15.2) m s 1 at the early (mature) stage. The during the early to mature stage of the rainband (cf. speeding behavior during the early to mature stage can Figs. 2 and 5). be clearly seen in the isochrones of Fig. 1 and is con- Conceptually, an optimal state means that convective sistent with the intensification of the observed cold updrafts can realize its ambient CAPE without being pool. It appears that the theoretical prediction of the inhibited significantly by either the cold pool–induced or cold pool propagation provides a reasonable range of the shear-induced circulation (Rotunno et al. 1988). estimates for the observed propagation speed of the Considering the precipitation that is caused by the re- studied rainband. lease of potential energy, instead of forced lifting, in- c. Factors related to cold pool intensification tense precipitation or deep convection is usually difficult to develop in a very small CAPE environment even if An important implication from the discussions above cold pool and shear can actually reach an optimal state. is that the intensification of the low-level cold pool may This might explain the later weakening of the TCR after play an essential role in facilitating a convective trans- 0300 UTC 28 September as it propagated into the formation of the studied TCR toward squall-line-like northern/northwestern portion of the typhoon, where features. There are several potential candidates of environmental CAPE was reduced to much lower values factors/processes that may be related to the formation (cf. Figs. 8 and 9). of prominent cold pool signatures observed in this case. The potential significance of cold pool dynamics in The prerainband, boundary layer environment was the present case, as suggested above, can be further generally characterized by relatively drier conditions addressed by comparing the propagation of the rain- (RH ; 80%–85% below 0.5 km MSL), as evident from band with the theoretically predicted propagation of a surface observations and NCEP profiles (Figs. 7a and convectively generated cold pool. Recall that the 10a). Evaporation of low-level precipitation nearby the propagation characteristics of the rainband differ sig- rainband is expected to be efficient to cool the un- nificantly from those of wave disturbances frequently saturated air. In addition, given the prevalence of a documented within TCs, as elaborated in section 3. deep layer of low-ue air in the low to midtroposphere When considering the influence of ambient winds on (cf. Fig. 10b), the occurrence of convective downdrafts, the propagation speed of an atmospheric cold pool, its if any, inside the rainband may help transport the low- velocity Vcp can be approximated by the expression ue air aloft into the lower troposphere. This process (Simpson and Britter 1980) would favor the development of negative perturbations of both temperature and moisture fields that are con- 1/2 gH(u 2 u ) sistent with the characteristics of the observed cold V 5 k y2 y1 1 bV , (2) cp u c pool for the present case, as described in section 4. y2 Another potential contributor for the intensification where H is the depth of the cold air; uy2 and uy1 are the of the observed cold pool may be related to the presence ambient and cold pool virtual potential temperatures, of considerable stratiform rain in the vicinity of the respectively; Vc is the ambient cross-band wind com- TCR. As shown in Fig. 2, a wider area of stratiform ponent ahead of the cold pool (negative for a wind to- precipitation started to occur on the rear flank of ward the cold air and vice versa); and k and b are the rainband at 2200 UTC 27 September, ;3 h prior to empirical constants. The best fit of (2) to atmospheric the convective transformation into the squall-line-like observations gives constant k and b equal to 0.9 and stage. The rear-flank stratiform precipitation would 0.6, respectively (Simpson and Britter 1980). For the provide additional cooling effects through evaporation present case, H is equal to ;2 km, as described earlier. and/or melting of hydrometeors to help organize a larger

The uy2 value is approximated by the mean virtual area of enhanced cold pool (cf. Fig. 7b). As revealed by potential temperature measured over regions ahead of the radius–time Hovmöller diagram shown in Fig. 6a, the rainband (cf. Fig. 7),whichiscalculatedtobe303.6 the appearance of the rear-ﬂank stratiform precipitation (301.5) K at IG (PJY). The minimum virtual potential seems connected to the outward propagation of the temperature observed within the rainband is equal to inner-core convection. In addition, a generally negative

301.3 (297.8) K at IG (PJY). The mean magnitude of Vc (i.e., inﬂow relative to the rainband’s motion) and a prerainband Vc averaged below 2 km MSL, based on deep layer of the rearward vertical shear between 3 and 2 Fig. 10c,iscalculatedtobe2.6(7.6)ms 1 at IG (PJY). 7 km (cf. Fig. 10c), both of which were present in the

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC 3280 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76 prerainband environment before the squall-line-like stage, would also be environmental factors favorable for upper-level hydrometeors originating from the TCR to feed the rear-flank stratiform region. Although the importance of the trailing stratiform precipitation on generating strong cold pools within ordinary convective systems has been recognized (Zipser 1977; Houze 2014), its specific role in influencing the formation of cold pools within TCs has not been identified and deserves future investigation (Eastin et al. 2012).

6. Conclusions This study used radar and surface observations to track an outer tropical cyclone rainband (TCR) of Ty- phoon Jangmi (2008) over a long period of time (;10 h) from its formative to mature stage. Based on the detailed analyses of these unique observations, some important evolving aspects of surface and precipitation characteristics for the TCR were documented in this FIG. 11. Schematic diagram illustrating the evolving aspects of study, as summarized schematically in Fig. 11.The precipitation and surface characteristics of the studied TCR from TCR was initiated on the eastern side of the typhoon its formative to mature stage (;10 h). Color shading indicates at a radial distance of ;190 km as it detached from the conceptual precipitation features of the rainband documented over upwind segment of a stratiform rainband located different time periods of observations t0–t4, and gray shading indicates the spatial variation of environmental CAPE associated close to the inner-core boundary. The outer rainband, with the typhoon circulation, with darker shading denoting larger as it propagated cyclonically outward, was observed CAPE values. The dashed circle denotes the inner-core boundary to undergo a prominent convective transformation of the typhoon, and the thick arrow indicates the TC motion. A from generally stratiform precipitation during the qualitative depiction of surface temperature and wind fluctuations earlier period to highly organized, convective precipitation associated with the TCR during its early and mature stages is shown in the upper right. at its mature stage. This observed transformation was accompanied by a clear trend of surface kinematics and thermodynamics toward squall-line-like features. In con- stage for the rainband. A relatively drier boundary layer, a trast to relatively minor surface fluctuations of the rain- deep layer of low-ue air in the low to midtroposphere, as band during its early stage, cold pool signatures, wind well as the presence of stratiform precipitation on the rear shifts, and decelerations of cross-band flow Vc across the flank of the TCR, would be potential factors favorable for leading edge of the rainband became prominent during its the intensification of the observed cold pool. In addition, mature stage (inset panels in the upper right of Fig. 11). the propagation characteristics of the TCR were observed Although the weakening of the TCR during the late to be distinctly different from those of wave disturbances period of observations appears related to its ambient frequently documented within the cores of tropical cy- low CAPE values over the northwestern portion of the clones; however, they were consistent with the theoreti- typhoon, the observed intensification of the rainband cally predicted propagation of convectively generated during its early to mature stage cannot be simply cold pools, which further supports the importance of cold explained by a nearly constant or reduced CAPE envi- pool dynamics for the present case. ronment as it propagated from east to north of the ty- Results from this study provide important observa- phoon circulation (Fig. 11). Instead, the dynamical tional evidence that a squall-line-like outer TCR may interaction between the prerainband vertical shear and originate from relatively weaker convection located cold pools, with the progression toward increasingly near the inner-core boundary of typhoon circulations. optimal conditions over time, appears to provide a rea- These findings, to some degree, confirm the speculation sonable prediction for the temporal alternation of pre- of the formation of outer TCRs by recent observational cipitation intensity. In this context, together with no and modeling studies (Li et al. 2017; YU18), which links obvious changes in ambient shear, the increasing in- the outer rainband formation to the outward propaga- tensity of cold pools may play an essential role in the tion of convective activities or rainbands initiated within convective transformation during the early to mature the inner core. Considering that the appearances of

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3281 inner rainbands are a manifestation of atmospheric waves. Mon. Wea. Rev., 134, 3073–3091, https://doi.org/ waves such as vortex Rossby waves or inertia–gravity 10.1175/MWR3250.1. waves (Willoughby 1977; Montgomery and Kallenbach Cotto, A., I. Gonzalez III, and H. E. Willoughby, 2015: Synthesis of vortex Rossby waves. Part I: Episodically forced waves in the 1997; Chow et al. 2002; Corbosiero et al. 2006) and that inner waveguide. J. Atmos. Sci., 72, 3940–3957, https://doi.org/ outer TCRs commonly exhibit a convective nature of 10.1175/JAS-D-15-0004.1. squall lines (YU18), a dynamic transformation from Didlake, A. C., and R. A. Houze Jr., 2009: Convective-scale wave-dominant convection to highly organized, squall- downdrafts in the principal rainband of Hurricane Katrina line-like precipitation is expected to occur frequently (2005). Mon. Wea. Rev., 137, 3269–3293, https://doi.org/ 10.1175/2009MWR2827.1. in the vicinity of the inner-core boundary. Neverthe- Diercks, J. W., and R. A. Anthes, 1976: Diagnostic studies of less, more case studies of outer TCRs by tracking their spiral rainbands in a nonlinear hurricane model. J. Atmos. histories of evolving and structural details will still Sci., 33, 959–975, https://doi.org/10.1175/1520-0469(1976)033,0959: be required to clarify the speciﬁc prevalence of the DSOSRI.2.0.CO;2. transformation and to identify environmental factors Eastin, M. D., T. L. Gardner, M. C. Link, and K. C. Smith, 2012: Surface cold pools in the outer rainbands of Tropical Storm or processes that are key to facilitating the develop- Hanna (2008) near landfall. Mon. Wea. Rev., 140, 471–491, ment of these scenarios. https://doi.org/10.1175/MWR-D-11-00099.1. Frank, W. M., 1977: The structure and energetics of the tropical Acknowledgments. The Doppler radar data and sur- cyclone. Part I: Storm structure. Mon. Wea. Rev., 105, , face observations used in this study were provided by 1119–1135, https://doi.org/10.1175/1520-0493(1977)105 1119: TSAEOT.2.0.CO;2. the Taiwan Central Weather Bureau (CWB) and Japan Gall, R., J. Tuttle, and P. Hildebrand, 1998: Small-scale spiral Meteorological Agency (JMA). The authors thank bands observed in Hurricanes Andrew, Hugo, and Erin. Mon. Dr. Hugh Willoughby and two anonymous reviewers Wea. Rev., 126, 1749–1766, https://doi.org/10.1175/1520-0493(1998) for providing constructive comments that improved the 126,1749:SSSBOI.2.0.CO;2. manuscript. This study was supported by the Ministry Gonzalez, I., III, A. Cotto, and H. E. Willoughby, 2015: Synthesis of Science and Technology of Taiwan under Research of vortex Rossby waves. Part II: Vortex motion and waves in the outer waveguide. J. Atmos. Sci., 72, 3958–3974, https:// Grant MOST106-2111-M-002-002-MY3. doi.org/10.1175/JAS-D-15-0005.1. Guinn, T. A., and W. H. Schubert, 1993: Hurricane spiral bands. J. Atmos. Sci., 50, 3380–3403, https://doi.org/10.1175/1520- REFERENCES 0469(1993)050,3380:HSB.2.0.CO;2. Akter, N., and K. Tsuboki, 2012: Numerical simulation of Cyclone Hamuro, M., and Coauthors, 1969: Precipitation bands of Sidr using a cloud-resolving model: Characteristics and for- Typhoon Vera in 1959 (part I). J. Meteor. Soc. Japan, 47, mation process of an outer rainband. Mon. Wea. Rev., 140, 298–309, https://doi.org/10.2151/jmsj1965.47.4_298. 789–810, https://doi.org/10.1175/2011MWR3643.1. Hence, D. A., and R. A. Houze Jr., 2008: Kinematic structure of Anthes, R. A., Ed., 1982: Tropical Cyclones: Their Evolution, convective-scale elements in the rainbands of Hurricanes Structure and Effects. Meteor. Monogr., No. 41, Amer. Meteor. Katrina and Rita (2005). J. Geophys. Res., 113, D15108, Soc., 208 pp. https://doi.org/10.1029/2007JD009429. Barnes, G. M., E. J. Zipser, D. P. Jorgensen, and F. D. Marks, 1983: Houze, R. A., Jr., 2010: Clouds in tropical cyclones. Mon. Wea. Mesoscale and convective structure of a hurricane rainband. Rev., 138, 293–344, https://doi.org/10.1175/2009MWR2989.1. J. Atmos. Sci., 40, 2125–2137, https://doi.org/10.1175/1520- ——, 2014: Cloud Dynamics. 2nd ed. Academic Press, 432 pp. 0469(1983)040,2125:MACSOA.2.0.CO;2. ——, S. A. Rutledge, M. I. Biggerstaff, and B. F. Smull, 1989: In- Bogner, P. B., G. M. Barnes, and J. L. Franklin, 2000: Conditional terpretation of Doppler weather radar display of midlatitude instability and shear for six hurricanes over the Atlantic mesoscale convective systems. Bull. Amer. Meteor. Soc., 70, Ocean. Wea. Forecasting, 15, 192–207, https://doi.org/10.1175/ 608–619, https://doi.org/10.1175/1520-0477(1989)070,0608: 1520-0434(2000)015,0192:CIASFS.2.0.CO;2. IODWRD.2.0.CO;2. Chen, B.-F., R. L. Elsberry, and C.-S. Lee, 2014: Origin and ——, and Coauthors, 2006: The Hurricane Rainband and Intensity maintenance of the long-lasting, outer mesoscale convective Change Experiment: Observations and modeling of hurri- system in Typhoon Fengshen (2008). Mon. Wea. Rev., 142, canes Katrina, Ophelia, and Rita. Bull. Amer. Meteor. Soc., 87, 2838–2859, https://doi.org/10.1175/MWR-D-14-00036.1. 1503–1521, https://doi.org/10.1175/BAMS-87-11-1503. Chen, Y., and M. K. Yau, 2001: Spiral bands in a simulated hurri- Jorgensen, D. P., 1984: Mesoscale and convective-scale charac- cane. Part I: Vortex Rossby wave veriﬁcation. J. Atmos. Sci., teristics of mature hurricanes. Part I: General observations by 58, 2128–2145, https://doi.org/10.1175/1520-0469(2001)058,2128: research aircraft. J. Atmos. Sci., 41, 1268–1285, https://doi.org/ SBIASH.2.0.CO;2. 10.1175/1520-0469(1984)041,1268:MACSCO.2.0.CO;2. Chow, K. C., K. L. Chan, and A. K. H. Lau, 2002: Generation of Kurihara, Y., 1976: On the development of spiral bands in a tropical moving spiral bands in tropical cyclones. J. Atmos. Sci., 59, cyclone. J. Atmos. Sci., 33, 940–958, https://doi.org/10.1175/ 2930–2950, https://doi.org/10.1175/1520-0469(2002)059,2930: 1520-0469(1976)033,0940:OTDOSB.2.0.CO;2. GOMSBI.2.0.CO;2. Li, Q., and Y. Wang, 2012: A comparison of inner and outer Corbosiero, K. L., J. Molinari, A. R. Aiyyer, and M. L. Black, 2006: spiral rainbands in a numerically simulated tropical cyclone. The structure and evolution of Hurricane Elena (1985). Part Mon. Wea. Rev., 140, 2782–2805, https://doi.org/10.1175/ II: Convective asymmetries and evidence for vortex Rossby MWR-D-11-00237.1.

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC 3282 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 76

——, ——, and Y. Duan, 2017: A numerical study of outer rain- Rozoff, C. M., W. H. Schubert, B. D. McNoldy, and J. P. Kossin, band formation in a sheared tropical cyclone. J. Atmos. Sci., 2006: Rapid filamentation zones in intense tropical cyclones. 74, 203–227, https://doi.org/10.1175/JAS-D-16-0123.1. J. Atmos. Sci., 63, 325–340, https://doi.org/10.1175/JAS3595.1. Ligda, M. G. H., 1955: Hurricane squall lines. Bull. Amer. Meteor. Saha, S., and Coauthors, 2010: The NCEP Climate Forecast System Soc., 36, 340–342, https://doi.org/10.1175/1520-0477-36.7.340. Reanalysis. Bull. Amer. Meteor. Soc., 91, 1015–1057, https:// Lin, Y., Y. Li, Q. Li, M. Chen, F. Xu, Y. Wang, and B. Huang, 2018: doi.org/10.1175/2010BAMS3001.1. A long-lasting vortex Rossby wave–induced rainband of Ty- Samsury, C. E., and E. J. Zipser, 1995: Secondary wind maxima in phoon Longwang (2005). Bull. Amer. Meteor. Soc., 99, 1127– hurricanes: Airflow and relationship to rainbands. Mon. Wea. 1134, https://doi.org/10.1175/BAMS-D-17-0122.1. Rev., 123, 3502–3517, https://doi.org/10.1175/1520-0493(1995) Macdonald, N. J., 1968: The evidence for the existence of Rossby- 123,3502:SWMIHA.2.0.CO;2. like waves in the hurricane vortex. Tellus, 20A, 138–150, Sawada, M., and T. Iwasaki, 2010: Impacts of evaporation from https://doi.org/10.1111/j.2153-3490.1968.tb00358.x. raindrops on tropical cyclones. Part II: Features of rainbands Marks, F. D., Jr., 2003: State of the science: Radar view of tropical and asymmetric structure. J. Atmos. Sci., 67, 71–81, https:// cyclones. Radar and Atmospheric Science: A Collection of doi.org/10.1175/2009JAS3040.1. Essays in Honor of David Atlas, Meteor. Monogr., No. 52, Amer. Senn, H. V., and H. W. Hiser, 1959: On the origin of hurricane Meteor. Soc., 33–73, https://doi.org/10.1175/0065-9401(2003) spiral rain bands. J. Meteor., 16, 419–426, https://doi.org/ 030,0033:SOTSRV.2.0.CO;2. 10.1175/1520-0469(1959)016,0419:OTOOHS.2.0.CO;2. ——, and R. A. Houze Jr., 1987: Inner core structure of Hurricane Shapiro, L. J., and H. E. Willoughby, 1982: The response of bal- Alicia from airborne Doppler radar observations. J. Atmos. Sci., anced hurricanes to local sources of heat and momentum. 44, 1296–1317, https://doi.org/10.1175/1520-0469(1987)044,1296: J. Atmos. Sci., 39, 378–394, https://doi.org/10.1175/1520- ICSOHA.2.0.CO;2. 0469(1982)039,0378:TROBHT.2.0.CO;2. May, P. T., and G. J. Holland, 1999: The role of potential vorticity Simpson, J. E., and R. E. Britter, 1980: A laboratory model of an generation in tropical cyclone rainbands. J. Atmos. Sci., 56, atmospheric mesofront. Quart. J. Roy. Meteor. Soc., 106, 485– 1224–1228, https://doi.org/10.1175/1520-0469(1999)056,1224: 500, https://doi.org/10.1002/qj.49710644907. TROPVG.2.0.CO;2. Skwira, G. D., J. L. Schroeder, and R. E. Peterson, 2005: Surface Molinari, J., D. M. Romps, D. Vollaro, and L. Nguyen, 2012: CAPE observations of landfalling hurricane rainbands. Mon. Wea. in tropical cyclones. J. Atmos. Sci., 69, 2452–2463, https:// Rev., 133, 454–465, https://doi.org/10.1175/MWR-2866.1. doi.org/10.1175/JAS-D-11-0254.1. Tang, X., W.-C. Lee, and M. Bell, 2014: A squall-line-like principal Montgomery, M. T., and R. J. Kallenbach, 1997: A theory for rainband in Typhoon Hagupit (2008) observed by airborne vortex Rossby waves and its application to spiral bands and Doppler radar. J. Atmos. Sci., 71, 2733–2746, https://doi.org/ intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc., 10.1175/JAS-D-13-0307.1. 123, 435–465, https://doi.org/10.1002/qj.49712353810. ——, ——, and ——, 2018: Subrainband structure and dynamic Moon, Y., and D. S. Nolan, 2015: Spiral rainbands in a numerical characteristics in the principal rainband of Typhoon Hagupit simulation of Hurricane Bill (2009). Part II: Propagation of (2008). Mon. Wea. Rev., 146, 157–173, https://doi.org/10.1175/ inner rainbands. J. Atmos. Sci., 72, 191–215, https://doi.org/ MWR-D-17-0178.1. 10.1175/JAS-D-14-0056.1. Tompkins, A. M., 2001: Organization of tropical convection in low Nolan, D. S., and J. A. Zhang, 2017: Spiral gravity waves radiating vertical wind shears: The role of cold pools. J. Atmos. Sci., 58, from tropical cyclones. Geophys. Res. Lett., 44, 3924–3931, 1650–1672, https://doi.org/10.1175/1520-0469(2001)058,1650: https://doi.org/10.1002/2017GL073572. OOTCIL.2.0.CO;2. Parsons, D. B., 1992: An explanation of intense frontal updrafts Ushijima, T., 1958: Outer rain bands of typhoons. J. Meteor. Soc. and narrow cold-frontal rainbands. J. Atmos. Sci., 49, Japan, 36, 1–10, https://doi.org/10.2151/jmsj1923.36.1_1. 1810–1825, https://doi.org/10.1175/1520-0469(1992)049,1810: Wakimoto, R. M., 1982: The life cycle of thunderstorm gust fronts as AEFIFU.2.0.CO;2. viewed with Doppler radar and rawinsonde data. Mon. Wea. Powell, M. D., 1990a: Boundary layer structure and dynamics in outer Rev., 110, 1060–1082, https://doi.org/10.1175/1520-0493(1982) hurricane rainbands. Part I: Mesoscale rainfall and kinematic 110,1060:TLCOTG.2.0.CO;2. structure. Mon. Wea. Rev., 118, 891–917, https://doi.org/10.1175/ Wang, Y., 2002: Vortex Rossby waves in a numerically simulated 1520-0493(1990)118,0891:BLSADI.2.0.CO;2. tropical cyclone. Part I: Overall structure, potential vorticity, and ——, 1990b: Boundary layer structure and dynamics in outer hur- kinetic energy budgets. J. Atmos. Sci., 59, 1213–1238, https:// ricane rainbands. Part II: Downdraft modification and mixed doi.org/10.1175/1520-0469(2002)059,1213:VRWIAN.2.0.CO;2. layer recovery. Mon. Wea. Rev., 118, 918–938, https://doi.org/ ——, 2009: How do outer spiral rainbands affect tropical cyclone 10.1175/1520-0493(1990)118,0918:BLSADI.2.0.CO;2. structure and intensity? J. Atmos. Sci., 66, 1250–1273, https:// Riemer, M., M. T. Montgomery, and M. E. Nicholls, 2010: A new doi.org/10.1175/2008JAS2737.1. paradigm for intensity modification of tropical cyclones: Willoughby, H. E., 1977: Inertia-buoyancy waves in hurricanes. Thermodynamic impact of vertical wind shear on the inflow J. Atmos. Sci., 34, 1028–1039, https://doi.org/10.1175/1520- layer. Atmos. Chem. Phys., 10, 3163–3188, https://doi.org/10.5194/ 0469(1977)034,1028:IBWIH.2.0.CO;2. acp-10-3163-2010. ——, 1978: A possible mechanism for the formation of hurricane Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A theory for rainbands. J. Atmos. Sci., 35, 838–848, https://doi.org/10.1175/ strong, long-lived squall lines. J. Atmos. Sci., 45, 463–485, https:// 1520-0469(1978)035,0838:APMFTF.2.0.CO;2. doi.org/10.1175/1520-0469(1988)045,0463:ATFSLL.2.0.CO;2. ——, 1988: The dynamics of the tropical hurricane core. Aust. Roux, F., J. Testud, M. Payen, and B. Pinty, 1984: West African squall- Meteor. Mag., 36, 183–191. line thermodynamic structure retrieved from dual-Doppler radar ——, 1990: Temporal changes of the primary circulation in tropical observations. J. Atmos. Sci., 41, 3104–3121, https://doi.org/10.1175/ cyclones. J. Atmos. Sci., 47, 242–264, https://doi.org/10.1175/ 1520-0469(1984)041,3104:WASLTS.2.0.CO;2. 1520-0469(1990)047,0242:TCOTPC.2.0.CO;2.

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC OCTOBER 2019 Y U E T A L . 3283

——, J. A. Clos, and M. G. Shoreibah, 1982: Concentric eye walls, 2000–08. J. Atmos. Sci., 68, 1568–1585, https://doi.org/10.1175/ secondary wind maxima, and the evolution of the hurricane 2011JAS3725.1. vortex. J. Atmos. Sci., 39, 395–411, https://doi.org/10.1175/ ——, and C.-L. Tsai, 2013: Structural and surface features of arc- 1520-0469(1982)039,0395:CEWSWM.2.0.CO;2. shaped radar echoes along an outer tropical cyclone rainband. ——, F. D. Marks Jr., and R. J. Feinberg, 1984: Stationary and J. Atmos. Sci., 70, 56–72, https://doi.org/10.1175/JAS-D-12- moving convective bands in hurricanes. J. Atmos. Sci., 41, 090.1. 3189–3211, https://doi.org/10.1175/1520-0469(1984)041,3189: ——, and ——, 2017: Structural changes of an outer tropical cy- SAMCBI.2.0.CO;2. clone rainband encountering the topography of northern Wu, C.-C., K. K. W. Cheung, and Y.-Y. Lo, 2009: Numerical study Taiwan. Quart. J. Roy. Meteor. Soc., 143, 1107–1122, https:// of the rainfall event due to the interaction of Typhoon Babs doi.org/10.1002/qj.2994. (1998) and the northeasterly monsoon. Mon. Wea. Rev., 137, ——, C.-Y. Lin, L.-W. Cheng, J.-S. Luo, C.-C. Wu, and Y. Chen, 2049–2064, https://doi.org/10.1175/2009MWR2757.1. 2018: The degree of prevalence of similarity between outer Yu, C.-K., and C.-L. Tsai, 2010: Surface pressure features of tropical cyclone rainbands and squall lines. Sci. Rep., 8, 8247, landfalling typhoon rainbands and their possible causes. https://doi.org/10.1038/s41598-018-26553-8. J. Atmos. Sci., 67, 2893–2911, https://doi.org/10.1175/ Zipser, E. J., 1977: Mesoscale and convective-scale downdrafts as 2010JAS3312.1. distinct components of squall-line structure. Mon. Wea. Rev., ——, and Y. Chen, 2011: Surface ﬂuctuations associated with 105, 1568–1589, https://doi.org/10.1175/1520-0493(1977)105,1568: tropical cyclone rainbands observed near Taiwan during MACDAD.2.0.CO;2.

Unauthenticated | Downloaded 09/26/21 12:31 PM UTC