Origin and Maintenance of the Long-Lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008)

2838 MONTHLY WEATHER REVIEW VOLUME 142 Origin and Maintenance of the Long-Lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008) BUO-FU CHEN Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan RUSSELL L. ELSBERRY Department of Meteorology, Naval Postgraduate School, Monterey, California CHENG-SHANG LEE Department of Atmospheric Sciences, National Taiwan University, and Taiwan Typhoon and Flood Research Institute, National Applied Research Laboratories, Taipei, Taiwan (Manuscript received 27 January 2014, in final form 21 March 2014) ABSTRACT Outer mesoscale convective systems (OMCSs) are long-lasting, heavy rainfall events separate from the inner-core rainfall that have previously been shown to occur in 22% of western North Pacific tropical cyclones (TCs). Environmental conditions accompanying the development of 62 OMCSs are contrasted with the conditions in TCs that do not include an OMCS. The development, kinematic structure, and maintenance mechanisms of an OMCS that occurred to the southwest of Typhoon Fengshen (2008) are studied with Weather Research and Forecasting Model simulations. Quick Scatterometer (QuikSCAT) observations and the simulations indicate the low-level TC circulation was deflected around the Luzon terrain and caused an elongated, north–south moisture band to be displaced to the west such that the OMCS develops in the outer region of Fengshen rather than spiraling into the center. Strong northeasterly vertical wind shear contributed to frictional convergence in the boundary layer, and then the large moisture flux convergence in this moisture band led to the downstream development of the OMCS when the band interacted with the monsoon flow. As the OMCS developed in the region of low-level monsoon westerlies and midlevel northerlies associated with the outer circulation of Fengshen, the characteristic structure of a rear-fed inflow with a leading stratiform rain area in the cross-line direction (toward the south) was established. A cold pool (Du ,23 K) associated with the large stratiform precipitation region led to continuous formation of new cells at the leading edge of the cold pool, which contributed to the long duration of the OMCS. 1. Introduction August 2009 due to Typhoon Morakot has been related to the interaction between the tropical cyclone (TC) While the heavy rainfall associated with the eyewall circulation and the southwest monsoon (Chien and Kuo region of a tropical cyclone is a primary focus for flood 2011; Lee et al. 2011). Lee et al. (2011) examined several forecasting, long-lasting heavy rainfall may also occur in factors involved in the Morakot disaster, such as the outer regions. For example, Typhoon Morakot (2009), moist and unstable air brought by the southwest mon- which was the deadliest typhoon to impact Taiwan in soon, steep topography that provided rapid lifting, and recorded history, produced record-breaking rainfall the slow movement of Morakot. .3000 mm well to the south of the center. The record One of the great challenges in forecasting the Morakot accumulated rainfall over southern Taiwan during 6–10 rainfall was the contribution from an east–west-oriented, quasi-stationary, and long-lasting convective band over the Taiwan Strait about 300 km south of the TC center. Corresponding author address: Prof. Cheng-Shang Lee, De- partment of Atmospheric Sciences, National Taiwan University, Notably, a mesoscale convective system (MCS) developed No.1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan. within this rainband (Fig. 1a), and the subsequent inter- E-mail: [email protected] action of the rainband with the steep terrain produced DOI: 10.1175/MWR-D-14-00036.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/04/21 04:30 AM UTC AUGUST 2014 C H E N E T A L . 2839 FIG. 1. Infrared satellite images of OMCSs embedded in the outer circulations of (a) Typhoon Morakot (2009), (b) Typhoon Mindulle (2004), (c) Typhoon Kalmaegi (2008), (d) Typhoon Hagupit (2008), and (e),(f) Typhoon Fengshen (2008). Thick red 3s indicate the TC centers and the three white circles indicate radii of 150, 450, and 750 km. extremely intense rainfall of approximately 1500 mm region. Predicting rainfall due to OMCSs when they are from 1200 UTC 8 August to 0300 UTC 9 August 2009. also interacting with topography is a great challenge. Because of its long duration and the orographic en- Consequently, understanding the initiation processes, hancement, this MCS accounted for a substantial frac- kinematic structure, and the maintenance of OMCSs is tion of the total precipitation during the slow passage of important. Morakot. Numerous studies (Willoughby et al. 1982, 1984; Lee et al. (2012) defined outer MCSs (OMCSs) as Barnes et al. 1983; Powell 1990b; May and Holland 1999; convective systems that develop in a distant rainband of Wang 2002, 2009; Moon and Nolan 2010) have shown a TC, have a large cold cloud shield (area of the 208-K that the TC rainbands play an important role in the cold cloud shield must exceed 72 000 km2), and persist rainfall distribution, dynamics, size, and intensity of for more than 6 h. Based on hourly infrared channel-1 TCs. Houze (2010) categorized the rainband complex of (IR1) cloud-top temperatures and passive microwave TCs as consisting of a principal rainband, secondary (PMW) images, Lee et al. (2012) documented 109 OMCSs rainbands, and distant rainbands. A principal rainband in 22% of the TCs that occurred from 1999 to 2009 in the may develop due to the convergence between the vortex western North Pacific. In addition to Typhoon Morakot flow and the environment (Willoughby et al. 1984). (Fig. 1a), other OMCSs such as in Typhoons Mindulle Several studies (Barnes et al. 1983; Willoughby et al. (2004; Fig. 1b), Bilis (2006), and Kalmaegi (2008; Fig. 1c) 1984; Powell 1990a; Hence and Houze 2008; Didlake have hit Taiwan and produced ‘‘unexpected’’ torrential and Houze 2013a,b) based on aircraft observations have rainfall because they occurred remote from the inner-core shown that the cloud structure in the upwind portion of Unauthenticated | Downloaded 10/04/21 04:30 AM UTC 2840 MONTHLY WEATHER REVIEW VOLUME 142 the principal rainband is more convective, but the clouds in the downwind portion typically consist of decaying convective cells and tend to be dominated by stratiform precipitation. These studies also reported that an overturning circulation with inflows originating from the convex (outer) side is associated with the principal rainband, and the convective cells are distributed near the concave (inner) side of the rainband axis. Furthermore, a secondary horizontal wind maximum is often observed in the midlevels and along the principal rainband (Willoughby et al. 1984; May et al. 1994; Samsury and Zipser 1995; Hence and Houze 2008). However, Ishihara et al. (1986) and Tabata et al. (1992) have shown inflows that origi- nate from the concave side, and Li and Wang (2012) have shown that convective cells may develop on the convex side of the spiral band. Whereas the major portion of the principal rainband and secondary rainband are located in the inner-core region of the TC, the distant rainbands develop in the ‘‘outer region.’’ Cecil and Zipser (2002) suggested that the outer rainband region typically begins from about 150 to 200 km from the cyclone center and is typically bounded on the inside by a precipitation-free lane ad- jacent to an inner rainband. Cecil and Zipser (2002) defined a minimum outer rainband radius of 100 km and a maximum radius of 350 km. Corbosiero and Molinari (2002, 2003) defined ‘‘outer band regions’’ as being 100– 300 km from the center of hurricanes. Houze (2010, p. 324) stated that ‘‘distant rainbands are composed of buoyant convective cells aligned with confluence lines in the large-scale, low-level wind field spiraling into the TC vortex and are radially far enough from the eye of the FIG. 2. (a) Passive microwave image as observed by the SSMIS and (b) QuikSCAT oceanic surface wind observations for the OMCS storm that the vertical structure of the convection within embedded in Typhoon Fengshen (2008). Thick black lines indicate them is relatively unconstrained by the dynamics of the the contour of zero relative vorticity based on the QuikSCAT wind inner-core vortex of the cyclone.’’ observations, and red dashed lines indicate the 2758Ccoldcloud The OMCS in this study is another type of convective shield of the outer MCSs from the IR1 image at 1000 UTC 22 Jun system that occurs in the outer region of some western 2008; (a) is a modified version from a Naval Research Labora- tory (NRL) TC_PAGES website. North Pacific TCs (Lee et al. 2012). Compared with inner-core rainbands and typical distant bands described above, these OMCSs typically have larger stratiform indicated the presence of a surface wind jet under the precipitation regions than those in typical rainbands. stratiform region, which is indicated by a zero relative Moreover, the growth of the stratiform precipitation vorticity line (Fig. 2b, thick black line). Note also that region is typically accompanied by a surface wind jet the convective cells are on the cyclonic shear side of (Lee et al. 2012). Specifically, the OMCS (Figs. 1f, 2) the jet. that developed within the outer circulation of Typhoon While the Fengshen OMCS was selected in part be- Fengshen (2008) had a large stratiform precipitation cause it occurred within a region of synoptic-scale ob- region with a moderate (215–230 K) PMW 91-GHz servations around the South China Sea, observations polarization-corrected temperature (PCT) brightness were not available to analyze the mesoscale features of temperature TB and a convective precipitation region the OMCS or the mechanism(s) that maintains the (approximately the area of very low PMW TB , 215 K) convection for the extended duration of the OMCS. In with linearly arranged convective cells (Fig.

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