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Origin and Maintenance of the Long-Lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008) [Monthly Weather Review](Monthly Weather Review Via Acquire Media NewsEdge) 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 (??<-3K) 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. (ProQuest: ... denotes formulae omitted.) 1. Introduction While the heavy rainfall associated with the eyewall region of a tropical cyclone is a primary focus for flood forecasting, long-lasting heavy rainfall may also occur in outer regions. For example, Typhoon Morakot (2009), which was the deadliest typhoon to impact Taiwan in recorded history, produced record-breaking rainfall .3000 mm well to the south of the center. The record accumulated rainfall over southern Taiwan during 6-10 August 2009 due to Typhoon Morakot has been related to the interaction between the tropical cyclone (TC) circulation and the southwest monsoon (Chien and Kuo 2011; Lee et al. 2011). Lee et al. (2011) examined several factors involved in the Morakot disaster, such as the moist and unstable air brought by the southwest mon- soon, steep topography that provided rapid lifting, and the slow movement of Morakot. One of the great challenges in forecasting the Morakot 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. Notably, a mesoscale convective system (MCS) developed within this rainband (Fig. 1a), and the subsequent inter- action of the rainband with the steep terrain produced extremely intense rainfall of approximately 1500 mm from 1200 UTC 8 August to 0300 UTC 9 August 2009. Because of its long duration and the orographic en- hancement, this MCS accounted for a substantial frac- tion of the total precipitation during the slow passage of Morakot. Lee et al. (2012) defined outer MCSs (OMCSs) as convective systems that develop in a distant rainband of a TC, have a large cold cloud shield (area of the 208-K cold cloud shield must exceed 72 000 km2), and persist for more than 6 h. Based on hourly infrared channel-1 (IR1) cloud-top temperatures and passive microwave (PMW) images, Lee et al. (2012) documented 109 OMCSs in 22% of the TCs that occurred from 1999 to 2009 in the western North Pacific. In addition to Typhoon Morakot (Fig. 1a), other OMCSs such as in Typhoons Mindulle (2004; Fig. 1b), Bilis (2006), and Kalmaegi (2008; Fig. 1c) have hit Taiwan and produced ''unexpected'' torrential rainfall because they occurred remote from the inner-core region. Predicting rainfall due to OMCSs when they are also interacting with topography is a great challenge. Consequently, understanding the initiation processes, kinematic structure, and the maintenance of OMCSs is important. Numerous studies (Willoughby et al. 1982, 1984; Barnes et al. 1983; Powell 1990b; May and Holland 1999; Wang 2002, 2009; Moon and Nolan 2010) have shown that the TC rainbands play an important role in the rainfall distribution, dynamics, size, and intensity of TCs. Houze (2010) categorized the rainband complex of TCs as consisting of a principal rainband, secondary rainbands, and distant rainbands. A principal rainband may develop due to the convergence between the vortex flow and the environment (Willoughby et al. 1984). Several studies (Barnes et al. 1983; Willoughby et al. 1984; Powell 1990a; Hence and Houze 2008; Didlake and Houze 2013a,b) based on aircraft observations have shown that the cloud structure in the upwind portion of 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 cir- culation 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 sec- ondary 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 storm that the vertical structure of the convection within them is relatively unconstrained by the dynamics of the inner-core vortex of the cyclone.'' The OMCS in this study is another type of convective system that occurs in the outer region of some western North Pacific TCs (Lee et al. 2012). Compared with inner-core rainbands and typical distant bands described above, these OMCSs typically have larger stratiform precipitation regions than those in typical rainbands. Moreover, the growth of the stratiform precipitation region is typically accompanied by a surface wind jet (Lee et al. 2012). Specifically, the OMCS (Figs. 1f, 2) that developed within the outer circulation of Typhoon Fengshen (2008) had a large stratiform precipitation region with a moderate (215-230 K) PMW 91-GHz polarization-corrected temperature (PCT) brightness temperature TB and a convective precipitation region (approximately the area of very low PMW TB , 215 K) with linearly arranged convective cells (Fig. 2a). Fur- thermore, the Quick Scatterometer (QuikSCAT) satellite indicated the presence of a surface wind jet under the stratiform region, which is indicated by a zero relative vorticity line (Fig. 2b, thick black line). Note also that the convective cells are on the cyclonic shear side of the jet. While the Fengshen OMCS was selected in part be- cause it occurred within a region of synoptic-scale ob- servations around the South China Sea, observations were not available to analyze the mesoscale features of the OMCS or the mechanism(s) that maintains the convection for the extended duration of the OMCS. In this study, Weather Research and Forecasting Model (WRF) simulations of the OMCS embedded in Typhoon Fengshen (2008) (Figs. 1f, 2) are analyzed. These simu- lations provide the three-dimensional wind field asso- ciated with convective cells, the multicellular cycle of the convective system, and the development of the OMCS. Descriptions of the case, WRF, and the verifi- cations are provided in sections 2 and 3, respectively. The initiation of the OCMS is described in section 4, and the kinematic structure and the multicellular cycle of the OMCS are in section 5. Discussion and conclusions are provided in section 6. 2. Overview of Typhoon Fengshen (2008) and the synoptic environment The track of Fengshen and the accumulated rainfall estimated by the Tropical Rainfall Measuring Mission (TRMM) from 16 to 23 June 2008 are shown in Fig. 3. Tropical Storm (TS) Fengshen had rapidly intensified into a typhoon (TY) before making its first landfall on Samar Island in the Philippines. On 0000 UTC 21 June, TY Fengshen turned to the northwest and passed Metro Manila at approximately 0000 UTC 22 June with winds of 45.8 m s21. After TY Fengshen left Luzon Island, it moved northward and later made its second landfall at Shenzhen, Guangdong Province, at approximately 2200 UTC 24 June. The intense rainfall near the TC center and the rainfall enhancement due to the topogra- phy were expected, but Fengshen also produced two OMCSs (Figs. 1e,f). It is the northern OMCS (Fig. 3,black dotted circle) that is the focus of this study. According to the methodology of identifying OMCSs in Lee et al. (2012), this second OMCS (Figs. 1f, 2) developed at ap- proximately 0600 UTC 22 June and terminated at ap- proximately 1500 UTC when Fengshen was just moving off Luzon. The OMCS in Fengshen was a ''south type'' that typically has a monsoon flow to the south (Lee et al. 2012). Some environmental factors at the closest 6-hourly analysis to the south-type OMCS initiation time associated with the 62 TCs during 1999-2009 are compared with a control sample of 1192 times that the TCs had no OMCS and with those environmental fac- tors existing prior to the OMCS in Fengshen (Fig. 4). These environmental factors are composited relative to the TC centers west of 1458E and south of 268N from June to September and are extracted from the final operational global (FNL) analyses of the National Centers for Environmental Prediction (NCEP). Compared to the control sample with no OMCS (Fig. 4a), a larger size and more intense TC outer circulation, plus the presence of stronger westerly winds to the southwest of the center, are indicated for the sample of TCs with an OMCS to the south (Fig. 4d). The asym- metry of the environmental flow is even more evident for the Fengshen case (Fig. 4g) with the strongest winds in west-southwesterlies that originated in the Southern Hemisphere. The average wind speed of about 11 m s21 in the black boxes in Figs. 4d and 4g indicates the im- portance of strong environmental winds to the south and wrapping around the east side of the TCs with a south- type OMCS. Notice also a band of maximum northerly winds (Fig. 4g) that is displaced well to the west of the center, which is a factor in the development of the OMCS in this Fengshen case. Vertical wind shear (VWS) is hypothesized to have a role of the azimuthal distribution of the convection associated with Fengshen. Corbosiero and Molinari (2002, 2003) indicated lightning strike maxima are con- centrated in the downshear right quadrant in the outer rainband region. Lee et al. (2012) also indicated that the OMCSs were concentrated in the downshear right quadrant. Compared to the control sample (Fig. 4b)orthe sample of the south-type OMCSs (Fig. 4e), the 850-200-mb (1 mb 5 1 hPa) deep-layer mean VWS calculated in a radial ring between 2 8 and 8 8 latitude of Fengshen (Fig. 4h) was also toward the southwest, but had a very large magnitude (14.3 m s21). Notice also the area of strong easterly 850-500-mb local VWS over the South China Sea to the west of Fengshen (Fig. 4h). Two other environmental factors related to tropical MCSs are the lower-tropospheric moisture and the sta- bility, which is represented in Figs. 4c, 4f, and 4i by the 850-mb minus 700-mb equivalent potential temperature uE. The sample of south-type OMCSs (Fig. 4f) has a more concentrated moisture band associated with the westerly winds of the East Asian summer monsoon (recall Fig. 4d) than in the control (Fig. 4c) and has an extensive potentially unstable region to the north- west of the center. Given small horizontal temperature gradients in the region, this very large DuE between 850 and 700 mb in Fig. 4i implies the midlevel air is much drier than the low-level air in the outer regions of Fengshen. It is hypothesized that the interaction of the outer circulation of Fengshen with the topography of Luzon may account for the band of maximum northerly winds to the west of the center (Fig. 4g). This hypothesis and the roles of the asymmetric distribution of moisture and the dynamical role of the VWS on the formation of the OMCS will be examined in section 4 using a high- resolution simulation. It is anticipated that this combi- nation of topographic interaction, asymmetric moisture distribution, and VWS as in the Fengshen case will be relevant to other cases of south-type OMCSs that have caused heavy rainfall over Taiwan. 3. Model description and verifications of the simulations a. Model description A 60-h simulation with the Advanced Research WRF (Skamarock et al. 2005), version 3.4.1, was initialized at 0000 UTC 21 June, with the innermost domain in- tegrated from 0000 UTC 22 June to 0000 UTC 23 June to simulate the mesoscale features of the OMCS in Fengshen. WRF is a fully compressible, Eulerian, and nonhydrostatic model. WRF uses terrain-following, hydrostatic pressure vertical coordinates with 35 levels from the surface to 20 hPa. In this study, four nested domains that are fixed geographically are employed with two-way interaction between the inner grids (Fig. 5a). These four domains have horizontal gridpoint spacings of 36, 12, 4, and 1.33 km and grid dimensions of 229 3 142, 391 3 316, 646 3 631, and 400 3 400, respectively. The initial and lateral boundary conditions for the model are from the NCEP FNL analyses, which are available on the surface and at 26 vertical levels from 1000 to 10 mb, and these analyses are also used as the forcing in the WRF four-dimensional data assimilation grid nudging technique in the first and second domains throughout the model simulation. The convection-allowing physics on all grids utilizes the new Thompson et al. (2008) microphysics scheme, which includes ice, snow, and graupel processes. The Grell and Devenyi (2002) ensemble convective param- eterization scheme is used in the outer two domains. The Yonsei University (YSU) planetary boundary layer (PBL) scheme (Noh et al. 2003) is used to calculate the vertical fluxes of sensible heat, moisture, and momen- tum at the lower boundary. The Dudhia shortwave ra- diation scheme (Dudhia 1989) and the Rapid Radiative Transfer Model (RRTM) longwave radiation scheme (Mlawer et al. 1997) are used. To ensure a more reasonable TC structure and max- imize agreement with the observed track (Fig. 5b), an artificial vortex to represent Fengshen in the initial conditions is created with the WRF tropical cyclone bogus scheme. After some experimentation, a bogus vortex with an intensity of 30 m s21 at a radius of 200 km was adopted. A modified Rankine wind profile is spec- ified beyond the radius of maximum winds that is similar between the surface and 600 mb and linearly decreases to near zero at 100 mb. b. Verifications and preliminary analyses of the OMCS The simulated track and the Joint Typhoon Warning Center (JTWC) best track for Fengshen agree well (Fig. 5b), especially from 0000 UTC 22 June to 0000 UTC 23 June that includes the period of the OMCS de- velopment. The track errors during the entire simulation period are less than 120 km and are less than 55 km during 22 June. In addition, the simulated intensities on 22 June 2008 agree with those in the JTWC best track file. Whereas the minimum sea level pressures according to the JTWC were 978 hPa at 0600 and 1800 UTC, the simulated pressures are 982 and 981 hPa, respectively. An estimate of the rainfall from 0300 to 1800 UTC 22 June (Fig. 6a) was obtained from the Climate Prediction Center morphing technique (CMORPH) dataset (Joyce et al. 2004). These precipitation analyses at very high spatial and temporal resolution have been exclusively derived from low orbiter satellite microwave observa- tions with features that are translated via spatial prop- agation information based on geostationary infrared satellite data. Note the rainfall maximum of approxi- mately 300 mm located in the outer region southwest of the TC center. Although the simulated region of maxi- mum rainfall is shifted to the south by approximately 100 km, the rainfall amount agrees well with the CMORPH observations (Fig. 6b). A key convective feature in the visible imagery (Figs. 7d-f) and in the simulation (Figs. 7a-c) before the OMCS developed is a nearly east-to-west band of moderate convection plus a nearly north-to-south band of shallow convection. The initiation of this north-to- south band of shallow convection and its subsequent interaction with the east-to-west band is considered to be the crucial contribution to the formation of the long- lasting OMCS in Fengshen (Figs. 7c,f). In the simulation, the moisture band originates from an area of high water vapor mixing ratio between 168- 188N and 1188-1208E(Fig. 7g) at 2300 UTC 21 June and contributes to the formation of the north-to-south band of shallow convection at 0500 UTC 22 June (Fig. 7b, red ellipse, and Fig. 7h, letter A). At the downstream end of moisture band A, the surface convective available poten- tial energy (CAPE) is approximately 3600-4000 Jkg21, and the level of free convection is less than 400 m (Fig. 7i), which are conditions favorable for an outburst of deep convection. Note also that a north-south-oriented region of dry air at a larger radius is also wrapped around the outer TC circulation (Fig. 7i). The simulation of the extensive east-west region of radar reflectivity.40dBZ 3h later at1100UTC (Fig.8a) is taken to represent the OMCS development. Note the north-south band of shallow convection that is con- tinuing to feed into the OMCS. The first clue as to the source of this north-south convection is from the QuikSCAT oceanic surface winds (Fig. 8d). Note that the surface winds northeast of Luzon at 300 km from the center of Fengshen are deflected around the topogra- phy. Some distance to the northwest of Luzon, these easterly surface winds turn southward and accelerate. However, the northerly surface winds are still at such a large radius at 16 8 N to the west of Fengshen that they continue southward into the OMCS rather than being able to reach the center. The simulated surface winds (Fig. 8b) have a similar westward flow past the northern tip of Luzon and then turn southward and accelerate, but similarly pass well to the west of the low pressure center and continue toward the simulated OMCS. The north-south-oriented band of shallow convection south of 178NinFig. 8a may be associated with a region of confluence in the surface wind directions in Fig. 8b. As indicated in section 2, the environmental VWS is hypothesized to be the dynamical origin of the north-south band of shallow convection. Deep-layer mean (28-88 radius) VWS from 850 to 500 mb and from 850 to 200 mb in the Fengshen case are indicated in Fig. 8c. While a reversal from low-level northerlies in the band of shallow convection to upper-tropospheric easterlies is expected, even the 0-3-km local VWS is strong, and the 850-500-mb mean VWS is easterly at 6.9m s21 (Fig. 8c, blue arrow). Whereas the OMCS develops in the down- shear left side of the 850-200-mb mean VWS (Fig. 8c,red arrow), the shallow convective band is on the downshear right side. This aspect of the formation of the OMCS in Fengshen will be discussed further in section 4c. To summarize, the model simulates the TC track and intensity rather well, especially on the day the OMCS occurs. Although the simulated TC is larger and the region of maximum rainfall is shifted to the south, the mesoscale convective features of the OMCS are generally well simulated by the model. Therefore, this WRF sim- ulation will be used in the following sections to further examine various mechanisms that are considered to have led to the development, the kinematic structure, and the maintenance of the long-lasting OMCS in Fengshen (2008). 4. Mechanisms leading to the OMCS development a. Effect of Luzon terrain As described in section 3b and first indicated by the QuikSCAT wind vectors in Fig. 8d, the low-level circu- lation of Fengshen is deflected around the northern end of Luzon and flows westward some distance before turning southward. Although the OMCS is too far south in the simulation (Fig. 8), the surface wind vectors (Fig. 8b) are deflected around the northern tip of Luzon. Backward tracers at low levels from the simulated OMCS also confirm the role of the Luzon terrain in deflecting the TC circulation (not shown). Thus, the associated moisture band passes well to the west of the center of Fengshen rather than being drawn toward the inner core. This role of Luzon terrain in the development of the OMCS is further investigated with a model sensitivity test with a 30% reduction (TER30) and an increase to 150% (TER150) of the terrain height. No special data assimilation was used as the 6-hourly NCEP fields were used in the nudging technique for both altered terrains. In the TER150 sensitivity test, the TC moved offshore within 12 h and the accumulated rainfall was primarily offshore with a maximum .400 mm in the OMCS (Fig. 9b). The deflection of low-level flow around the enhanced Luzon topography, and the westward dis- placement and broader moisture band (Fig. 9d,green dashed line) are much more pronounced than for mois- ture band A in Fig. 7h. Another effect of the terrain may have been leeside subsidence given the indications of dry air downstream of where the airflow passes over Luzon (note red contours indicating mixing ratios less than 15 g kg21 in Fig. 9d as well as in Figs. 7g and 7h. Note also the maximum rainfall in the OMCS in Fig. 9b is associ- ated with a concentrated region of ice, snow, and graupel that is downshear from the center (Fig. 9f). By contrast, the maximum rainfall in the TER30 sensitivity test with the reduced terrain is associated with the outer TC circulation (Fig. 9a). Note that the surface streamlines are minimally affected by the reduced terrain (Fig. 9c), and the TC circulation is more symmetric about the center (in a flat terrain test the circulation is sym- metric; not shown). Although there is a relative maximum in the water vapor mixing ratio to the west of Luzon, it is associated with a rainband that spirals around the inner core rather than leading to an OMCS. The combination of ice, snow, and graupel in Fig. 9e has isolated maxima along that rainband that are more representative of con- vective cells rather than the OMCS in Fig. 9f. To diagnose these altered terrain effects on the TC structure and the flow characteristics between TER30 and TER150, the Okubo-Weiss quantity Q after Okubo (1970) and Weiss (1991) is calculated: ... where S1 = [(?u/?x)2(?y/?y)], S2 5[(?y/?x)1(?u/?y)], and z 5 [(?y/?x) 2 (?u/?y)] are the strain rates S1 and S2 due to stretching deformation and shearing deformation, and z is the vertical relative vorticity. In regions where vorticity dominates strain (Q , 0), trajectories of two neighboring particles do not separate exponentially in time (Schubert et al. 1999) and coherent structures such as mesovortices can survive. In the region where strain dominates vorticity (Q . 0), the vorticity gradient is intensified to form long, thin vorticity streamers. In such regions, fluid elements are stretched and the exponen- tial divergence of nearby particles leads to the so-called chaotic stirring in two-dimensional turbulence (Okubo 1970; Weiss 1991; Elhmaidi et al. 1993; Kevlahan and Farge 1997). In the region where the TC inner-core circulation is interacting with the Luzon terrain, the Q , 0 region in TER30 (Fig. 10a) is much larger than that of TER150 (Fig. 10b). Thus, the TC simulated in TER30 maintains a stronger, more axisymmetric, inner-core structure and thus may be less influenced by the environmental VWS. In the TER30 (Fig. 10a), the regions with large Q are smaller and are closer to the inner-core region of the TC. Note that the vorticity-dominated region in TER150 (Fig. 10b) extends farther to the west of northern Luzon such that the strain-dominated region that is as- sociated with the north-south moisture band is pushed westward to the outer region of the TC. Both this elon- gated moisture band and the east-west band where the OMCS forms have larger Q values and have larger con- vergence in the boundary layer (Fig. 10b). Therefore, leeside subsidence may have also contributed additional moisture convergence to the elongated moisture band. These sensitivity tests confirm the important role of the Luzon terrain in deflecting the low-level flow in such a way that the north-south-elongated moisture band passes well to the west of the TC center. Rather than feeding moist air into the inner core of the TC, this elongated moisture band extends to the intersection with the monsoon airstream to the southwest and con- tributes to the development of the OMCS. b. Role of the elongated moisture band After the deflection of the low-level, moist monsoonal air around Luzon and its turning southward well to the west of Luzon, a key question is how the moisture be- comes concentrated into an elongated north-south band of shallow convection. The moisture budget equation is written in the flux form: ... (1) where q is specific humidity, u (y) is the eastward (north- ward) horizontal velocity, w is the vertical velocity, and E ( C ) is evaporation (condensation). From left to right, the terms in Eq. (1) are the moisture tendency (TEND), the horizontal moisture flux convergence (HMFC), the vertical moisture flux convergence (VMFC), and the sources and sinks (S). The HMFC may be rewritten as the terms of horizontal advection (HADV) and horizontal divergence (HDIV): ... (2) These moisture budget terms are calculated from a 5-min model output. A temporal and vertical-spatial average is defined as ... with T2 - T1 = 4h, Z1 = 0.1 km, and Z2 5 1 km. The combination of HMFC and VMFC averaged between 0.1 and 1 km and from 0300 to 0700 UTC in the control simulation is plotted in Fig. 11a. The positive total moisture flux convergence (MFC; Fig. 11a) is large near both the strong moisture band A (Fig. 7h) and the region of the decaying convective band (Figs. 7a,b). The negative total MFC is mainly in the region of the de- caying convective band from 1168 to 1188E and from 138 to 148N. Near moisture band A, positive (negative) HADV is found to the west (east) side of the moisture bands, and the HDIV is collocated with the band. Whereas the horizontal advection term is clearly related to the westward displacement of the moisture band, the moisture convergence represented by HDIV is the key factor that concentrates the moisture into an area that is already moist. The time series of terms in the moisture budget from 0.1 to 1 km in the upstream region of the OMCS (box in Fig. 11a between 14.78 and 16.78N and 116.58 and 118.58E) indicates the variations of the TEND term are dominated by the HADV term on a short temporal scale, but the HDIV 1 VMFC combination contributes to the positive TEND before the moisture maximum occurs at approximately 0715 UTC (Fig. 11b). After the OMCS develops, both the HADV and the S have minimum values, so the low-level moisture begins to decrease due to the vertical flux to the convection of the OMCS. A second region of high surface moisture flux is as- sociated with the interaction between the TC circulation and the monsoonal westerlies in the southern to eastern quadrants of the TC (Fig. 11c). Therefore, it is proposed that the southwesterly monsoon flow has an important role in supplying moisture to the TC as well as via its role of convergence with the north-south band in providing favorable thermal and moisture conditions for the OMCS development. c. Role of vertical wind shear The environmental VWS in the outer circulation of the TC may be the dynamical origin of the shallow convection in the elongated moisture band based on idealized numerical experiments of Riemer et al. (2010), who showed that the wavenumber-1, asymmetric, up- ward, vertical motion is on the downshear right side in the outer region of the TC. Riemer et al. (2010) illus- trated the resulting convective asymmetry was due to the tilt of the outer vortex by showing that the wavenumber- 1 vorticity asymmetry above the boundary layer ex- tended out to a 150-200-km radius, which produces frictional convergence in the boundary layer and thus forces upward motion based on Ekman pumping theory. Whereas Riemer et al. (2010) imposed a simple wind profile with easterly VWS, the VWS direction in the Fengshen case rotated anticlockwise and increased with height. In the Fengshen simulation at 0200 UTC 22 June, a large azimuthal wavenumber-1 vorticity asymmetry at 1.5-km height is located to the downshear right side of 850-200-mb VWS (Figs. 12a,b). This vorticity asymme- try is also collocated with an elongated region of con- vergence from 158 to 188N and near 1188E. Thus, the interaction of the TC and VWS may have had an im- portant role in concentrating the moisture and thus in- creasing the CAPE at the downstream end of the band. Note also that another band structure of convergence with higher rain rate is associated with the decaying east-west convective band (Fig. 12b). The shallow convection develops from 0200 UTC 22 June as the moisture is concentrated in the elongated band region (see streamlines in Figs. 12c,d). The 0-6-km local VWS greater than 15 m s21 is associated with the shallow convection in the region from 158 to 188N and from 1168 to 1188E at 0200 UTC and then moves downstream and intensifies at 0500 UTC (Fig. 12d). Note also the large (.21 m s21) 0-6-km VWS that is associated with the decaying east-west convective band between 138 and 148N. Since the 0-6-km VWS vector within this elongated band of shallow convection has a direction shift of about 90 8 to the low-level winds, these conditions are favorable for developing a large stratiform precipitation region on the convex side. The VWS may also have had an important role in the OMCS devel- opment via separating the region of upward motion from the region of precipitation and thereby preventing new convective cells from being suppressed by the rainfall. 5. Kinematic structure and the maintenance mechanisms To analyze possible maintenance mechanisms of the OMCS, the kinematic structure and multicellular cycle of it will be examined in this section. a. Analysis of the simulated OMCS The mesoscale convective features of the OMCS are well simulated by the model (Fig. 13). The atmospheric column ice, snow, and graupel hydrometer mixing ratio (Figs. 13a,b) is distributed similarly to the PMW 91-GHz PCT TB observed by the Special Sensor Microwave Imager/Sounder (SSMIS) (Figs. 13c,d). The convective cells in both the simulation and the PMW TB are located to the north or in the central area of the stratiform precipitation region. Recall from Fig. 8a that the simulated OMCS in the mature stage at 1100 UTC 22 June 2008 consists of a convective line (maximum radar reflectivity . 50 dB Z ) and a stratiform precipitation region (35- to 45-dBZ region) to the south of the convective line. This OMCS is approximately 275 km southwest of the TC center and is collocated with the 850-hPa positive vorticity band that is independent to the region of large positive vorticity associated with the TC inner-core circulation. Recall also from Fig. 8b that a surface jet is collocated with the stratiform precipitation region of the simulated OMCS, but is slightly shifted cyclonically inward to- ward the TC center. Furthermore, the OMCS has de- veloped in the confluence region between the northerly wind relative and the monsoonal westerly wind (Fig. 8b), which is therefore in an environment of large VWS (see section 4c). Previous studies (McCaul and Weisman 1996; Houze 2010; Akter and Tsuboki 2012) that have addressed distant rainbands have generally assumed that cold pools in the TC are relatively weak due to small evap- orative cooling rates associated with the stratiform precipitation. In this simulation, the large stratiform precipitation region that developed in association with the OMCS has a cold pool with surface potential temperatures 2-3 K colder than in the upstream region (Fig. 14a). The vertical velocity at 1.5-km height smoothed to 25 km (Fig. 14b) has an area of upward motion on the northern edge of the OMCS that is as- sociated with the convective precipitation region and an area of downward motion under the stratiform precipitation region that is specifically associated with the maximum cold pool. The time evolution of the OMCS is described on a western region between 117.78 and 118.08 E and in an eastern region between 118.28 and 118.58E as the west- ern and central portions of the elongated north-south moisture band converges with monsoonal westerly flow (Fig. 15). In the western portion of the OMCS, a series of convective cells (defined by column mixing ratio of ice, snow, and graupel .25 3 1024 kg kg21) form and propagate southeastward (Fig. 15a) as the surface winds within the north-south band exceed about 16 m s21 (Fig. 15c). Local enhancement of these northwesterly winds is simulated in conjunction with the convective cells and localized maxima in the monsoonal flow occur where the enhanced northwesterly winds converge with the monsoonal flow. As described in section 4b, the central core of the north-south band has a high moisture con- tent (.18 g kg21, Fig. 15e). Because this band is more moist than the monsoonal flow to the south, horizontal moisture convergence occurs and contributes to the development of the convective cells at the intersection of the north-south band with the monsoonal flow. The key difference in the eastern region (Fig. 15, right column) is that the central core of the north-south moisture band with more northerly wind components (Fig. 15d, top) is interacting with the monsoonal flow to produce a continuous, and more stationary, line of convective cells that then form the long-lasting OMCS (Fig. 15b, along the thick, black dashed line). Except for the region of weaker northwesterly winds with some- what drier air (Fig. 15f, dashed triangle), these more northerly winds again are more moist than the mon- soonal air and lead to horizontal moisture convergence along the convective line at the northern edge of the OMCS. This convective line is also the boundary be- tween warmer air in the north-south band and the cold pool air under the stratiform cloud region that has po- tential temperatures of 299-300 K (Fig. 15h). It is also under the stratiform cloud region that the monsoonal westerly winds have a low-level jet with wind speeds of 22 m s21 (Fig. 15d). A momentum budget (not shown) confirms the conclusion of Trier et al. (1998) that such a convective line oriented perpendicular to the low-level VWS tends to increase the VWS by vertically trans- ferring momentum against the gradient. b. Kinematic structure of the OMCS The kinematic structure of the OMCS is displayed by taking a 2-h (0815-1015 UTC) average of the zonal-mean fields in the eastern portion of the OMCS (Fig. 16). Because the orientation of the convective line along the northern edge of the OMCS is roughly east to west, the zonal wind u and meridional wind y will be considered as the along-line and cross-line velocities in Fig. 16. Note the convergence between the stronger (weaker) north- erly (southerly) winds on the north (south) side of the convective line, which is consistent with the reflectivity (Fig. 16a) and vertical velocity (Fig. 16b). That is, the convective precipitation region (14.28-14.78N) has large upward vertical velocity, and the stratiform pre- cipitation region (13.58-14.28N) has downward vertical velocity below the freezing level. While northerly flow in the (Northern Hemisphere) tropics is typically cooler and drier than a warm, moist monsoonal flow, the airstream at the downstream end of the elongated moisture band is warmer (Fig. 15h) and more moist (Fig. 15f) than the monsoonal flow. In the cold pool region between 13.38 and 14.58N, the surface air temperatures are more than 2 K colder than that of the upstream (northerly flow) environment (Fig. 16b). Several studies (e.g., Barnes et al. 1983; Yamasaki 1983; Powell 1990a; Eastin et al. 2012) have documented the existence of cold pools in rainbands in which the surface air was more than 2 K colder than the surrounding en- vironment. However, the area of cold pools identified in those studies did not extend over as large an area as the OMCS in this study. Note the gradient of u at the northern edge of the cold pool is large, which is favor- able for triggering new convective cells. Viewed relative to this southward-moving OMCS, the low-level rear inflow from the north-south band is sup- plying warm, moist air with a very high uE (Fig. 16e)thatis then lifted in the convective region and forms the leading stratiform precipitation region to the south of the convec- tive region. Although the maximum northward cross-line velocities occur near the boundary of the convective region and the stratiform region (Fig. 16d), some of the outflow from the convective is back toward the north (Fig. 16b), which leads to the backward (northward) overhang in the reflectivity aloft (Fig. 16a). Thus, the kinematic structure of this OMCS is similar to the rear-fed, leading stratiform archetype of Pettet and Johnson (2003),whofoundin case studies that a considerable fraction of the leading stratiform systems in the central United States are sus- tained by the inflow of high uE air from behind the system. The multicellular time evolution of the convective line is displayed in Fig. 17 by taking 10-min averages of the zonal-mean fields in the eastern portion of the OMCS. Although the convective structure between 0930 and 0940 UTC (Fig. 17a) is basically similar to the averaged kinematic structure (Fig. 16a) with a mature cell (cell A) and the stratiform region south of 148N, the initial stage of cell B is also indicated above the leading edge of the cold pool. After 5 min (Fig. 17b), cell A begins to dissi- pate while cell B continues develop. As cell A dissipates, cell B is in its mature stage between 0940 and 0950 UTC (Fig. 17c). After another 10 min (Fig. 17d), a new cell (C) begins at the leading edge of the cold pool. During this period (0930-1000 UTC), the downward vertical velocity within the stratiform precipitation region of the OMCS becomes more organized and stronger. The magnitude of the cold pool is also enhanced with minimum Du ,23K near 148N. It is also suggested that the dry air that extends around the western side of the OMCS at low levels to midlevels (Figs. 7i, 16e) may enhance the evaporation cooling under the stratiform precipitation region and thus may have contributed to the formation of such a strong mesoscale cold pool in the OMCS. 6. Summary and discussion Since OMCSs are a special type of long-lasting rain- band that occurs in the outer region of some western North Pacific TCs, forecasting the heavy rainfall associated with the OMCSs is a critical issue for TC rainfall forecasting and disaster warning operations. This study extends the OMCS climatological analysis of Lee et al. (2012) and then uses a WRF model simulation to examine the de- velopment, kinematic structure, and the maintenance mechanisms of a ''south-type'' OMCS that occurred in- the outer circulation of Typhoon Fengshen (2008). The Fengshen OMCS had some similar conditions as a com- posite of 62 south-type OMCSs documented by Lee et al. (2012): (i) presence of extended monsoonal flow to the southwest of the TC center that wraps around the southern and eastern quadrants; (ii) a narrow moisture band that extends north to south well to the west of TC center (rather than spiraling into the center); and (iii) the OMCS forms where the southern end of the north-south moisture band interacts with the monsoonal flow. The track and minimum sea level pressure of Fengshen were well simulated, and the simulated rainfall distri- bution during the OMCS period (0300-1500 UTC) agreed well with the CMORPH rainfall, except that the location was shifted to the south by approximately 100 km. In addition, the mesoscale convective features including the shallow convection before the OMCS initiation, the convective and stratiform structure of the OMCS, and the surface jet under the stratiform region are well simulated. Thus, the simulation is the basis for an analysis of the development, kinematic structure, and maintenance of the Fengshen OMCS that is summarized in the conceptual model in Fig. 18. The role of Luzon topography was investigated with a model sensitivity test with 30% (TER30) and 150% (TER150) terrain height and demonstrated that the deflection of the TC low-level circulation by the terrain caused the development of an elongated moisture band well to the west of the center such that the OMCS de- velops in the outer region of the TC (Fig. 18b, black streamlines). The Okubo-Weiss calculation indicated that the strain-dominated region was associated with the elongated moisture band to the west of northern Luzon and that leeside subsidence may have also con- tributed additional moisture convergence to the elon- gated moisture band. In the reduced 30% terrain height simulation, the moisture band/rainband formation spi- raled into the TC inner core (Fig. 18b, dashed stream- lines). A similar deflection of the TC low-level flows by Taiwan terrain may have contributed to the formation of other OMCSs that have developed near Taiwan [e.g., TY Kalmaegi (2008) in Fig. 1c]. A moisture budget indicates moisture associated with the TC-monsoon interaction is transported cyclonically around the TC and becomes concentrated in the elon- gated north-south band to the west of the TC center due to large moisture flux convergence. Thus, very large convective available potential energy (;3600-4000 J kg21) and a low level of free convection (,400 m) was simu- lated at the downstream end of the north-south mois- ture band where it interacted with the monsoonal flow and produced the OMCS (Fig. 18b, star). One of the environmental characteristics of OMCSs is the northeasterly vertical wind shear. In the Fengshen case, the deep-layer VWS was particularly large (14 m s21), and even the lower-tropospheric VWS was large. A large azimuthal wavenumber-1 vorticity asymmetry at 1.5-km height (Fig. 18b, red dashed line) is simulated on the downshear right side of 850-200-mb VWS and is consid- ered to have produced frictional convergence in the boundary layer that then led to shallow convection along the moisture band. Therefore, both the large moisture flux convergence and the frictional convergence in the elon- gated north-south band due to the VWS are considered to have contributed to the moisture source for the OMCS. The kinematic structure of the convection of the OMCS is composed of a continuous line of convective cells and a large stratiform precipitation region to the south of the convective line as shown in the schematic in Fig. 18c. Thus, this OMCS has a structure in the cross- line direction similar to the rear-fed inflow and leading stratiform type of MCS (Pettet and Johnson 2003). Furthermore, a strong cold pool (Du ,23 K) is associ- ated with the large stratiform precipitation region. New cells are continually formed when the air with high uE values from the north converges with the leading (northern) edge of the cold pool. In this manner, the continuous formation of new cells may contribute to the longer duration of this OMCS than exists for the TC distant rainbands described in the introduction. In par- ticular, the large stratiform region of this OMCS is unique compared to distant rainbands and the upwind portion of primary rainbands that are generally lacking in stratiform precipitation (Houze 2010; Akter and Tsuboki 2012; Eastin et al. 2012), and the stratiform region of primary rainbands typically develops in its downwind position (Hence and Houze 2008; Didlake and Houze 2013b; Li and Wang 2012). The development of the large stratiform precipitation region to the south of the convective line leads to this OMCS having a large spatial scale. Yuter and Houze (1998) and Houze (2004) suggested that such large stratiform regions are sustained by the environmental flow. Future studies will focus on the sensitivity of the environmental conditions that lead to the development and maintenance of the stratiform precipitation regions of OMCSs. Lee et al. (2011) found OMCSs occurred in 22% of all TCs in the western North Pacific. Among the 62 south- type OMCSs that are associated with the summer mon- soon, 6 of them were terrain-influenced cases similar to the Fengshen case with the TC passing to the south of the terrain (Fig. 18d). Six other terrain-influenced south-type OMCS cases were associated with a TC passing to the north of the terrain, which are therefore similar to the TY Morakot (2009) case in Fig. 1a. Another 13 terrain- influenced south-type OMCS cases were associated with a TC approaching the coast of southeast China. Whereas the total number of these terrain-influenced OMCS is 26, they only account for 42% of 62 south-type OMCSs. Further study is required of the other south-type OMCS cases that were not terrain-influenced to gain a better understanding and improve the capability of the numerical models and thus improve forecasts of the OMCSs in TCs, which will lead to more accurate and timely warnings of heavy rainfall events associated with this type of rainband systems. Acknowledgments. Buo-Fu Chen is supported by the National Taiwan University, and Professor Cheng-Shang Lee is supported by the National Taiwan University and the Taiwan Typhoon Flood Research Institute of the National Applied Research Laboratories. This re- search is supported by the National Science Council of the Republic of China (Taiwan) under Grants NSC 98-2625-M-002-002andNSC99-2625-M-002-013-MY3. Professor Russell L. Elsberry is supported by the Ma- rine Meteorology section, Office of Naval Research. 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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) Corresponding author address: Prof. Cheng-Shang Lee, De- partment of Atmospheric Sciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan. E-mail: [email protected] (c) 2014 American Meteorological Society |