[August 24, 2011] A Subtropical Oceanic Mesoscale Convective Vortex Observed during SoWMEX/TiMREX [Monthly Weather Review] Tweet (Monthly Weather Review Via Acquire Media NewsEdge) ABSTRACT This study examines a subtropical oceanic mesoscale convective vortex (MCV) that occurred from 1800 UTC 4 June to 1200 UTC 6 June 2008 during intensive observing period (IOP) 6 of the Southwest Monsoon Experiment (SoWMEX) and the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). A dissipating mesoscale convective system reorganized within a nearly barotropic vorticity strip, which formed as a southwesterly low-level jet developed to the south of subsiding easterly flow over the southern Taiwan Strait. A cyclonic circulation was revealed on the northern edge of the mesoscale rainband with a horizontal scale of 200 km. An inner subvortex, on a scale of 25-30 km with maximum shear vorticity of 3 × 10-3 s-1, was embedded in the stronger convection. The vortex-relative southerly flow helped create local potential instability favorable for downshear convection enhancement. Strong low-level convergence suggests that stretching occurred within the MCV. Higher ?e air, associated with significant potential and conditional instability, and high reflectivity signatures near the vortex center suggest that deep moist convection was responsible for the vortex stretching. Dry rear inflow penetrated into theMCVand suppressed convection in the upshear direction. A mesolow was also roughly observed within the larger vortex. The presence of intense vertical wind shear in the higher troposphere limited the vortex vertical extent to about 6 km. 1. Introduction a. Background The joint Taiwan-United States multiagency Southwest Monsoon Experiment (SoWMEX) and Terraininfluenced Monsoon Rainfall Experiment (TiMREX) observed several heavy rainfall events over southern Taiwan and the South China Sea during the period from 15 May to 30 June 2008 (Jou et al. 2010). During this time, the mei-yu front is often present near Taiwan and is typically characterized by a horizontal shear zone from the low troposphere up to 700 hPa with a moderate-weak temperature gradient over Taiwan and southern China (Chen and Chang 1980). Southwesterly monsoon flow, located along the south side of the mei-yu front, typically transports high equivalent potential temperature ?e northeastward, creating an environment with strong horizontal gradients of moisture and wind shear (Chen and Yu 1988; Jou and Deng 1992). Mei-yu fronts are often accompanied by cloud bands elongated along the front and a series of mesoscale disturbances growing along the horizontal shear zone (Fang 1985; Ninomiya et al. 1988a,b; Chen 1992; Ding 1992; Li et al. 1993; Kuo andHorng 1994;Du and Cho 1996; Li et al. 1997; Chang et al. 1998, 2000; Chen 2004; Yamasaki 2005; Chen et al. 2008). In this sense, the mei-yu front is sometimes better characterized as a narrow strip of cyclonic vorticity and humidity contrast than a true atmospheric front. b. Overview of the 5-6 June MCV One of the objectives of SoWMEX/TiMREX was to study the dynamic, kinematic, and thermodynamic characteristics of the mesoscale disturbances associated with the southwest monsoon flow upstream of Taiwan. Mesoscale disturbances typically move eastward or eastnortheastward along the mei-yu front and can produce large amounts of precipitation in Taiwan (Chen and Yu 1988; Ray et al. 1991; Lin et al. 1992; Jou and Deng 1998; Chen et al. 2001; Johnson and Bresch 1991). During intensive observing period (IOP) 6 of SoWMEX/TiMREX (1800 UTC 4 June-1200 UTC 6 June), a quasi-stationary mei-yu front with a weak horizontal temperature contrast stretched across southern China and the middle of Taiwan. A series of mesoscale convective systems (MCSs) occurred south of the front over southern Taiwan. One of these MCSs exhibited a comma-shaped convective system and a clear spiral cloud pattern associated with cyclonic circulation on its northwest side, which was captured by visible satellite images (Fig. 1). The cyclonic circulation is referred to as a mesoscale convective vortex (MCV). The convection system associated with the MCV propagated directly toward southern Taiwan and brought flooding rainfall to the coastal area, almost 200 mm in 2 h. c. Review of mesocyclones associated with mei-yu fronts Within mei-yu fronts, numerous MCSs, accompanied by mesoscale cyclones in the lower troposphere, have been investigated in the vicinity of the Yangtze River Valley (;30°N) (Akiyama 1984a,b; Zhang et al. 2003; Chen et al. 2008). These include mesoscale disturbances that originate along the eastern flank of the Tibetan Plateau called the southwest vortex (Ye 1981; Tao and Ding 1981; Wu and Chen 1985; Kuo et al. 1986, 1988; Wang 1987; Wang and Orlanski 1987; Wang et al. 1993; Chen et al. 1997; Chang et al. 2000) and the meso-betascale vortex over the downstream portion of the Yangtze River (Geng et al. 2004). The significant increase in relative vorticity associated with the development of the southwest vortex has largely been attributed to diabatic heating effects due to convection, with about a 20% contribution from preexisting ambient vorticity near the mature stage of the MCS (Wang et al. 1993). Chen et al. (2008) suggested that cyclogenesis was a result of conditional instability of the second kind (CISK) in which the frontal potential vorticity (PV) centers and cumulus convection reinforce each other through a positive feedback process. A climatological study by Lee et al. (2006) showed that 11 tropical or subtropical cyclones have formed within the mei-yu front during the previous 30 yr. The incipient low-level disturbances originated over land and the low-level circulations strengthened while moving over the open ocean along the stationary mei-yu front. Increased sea surface temperature (SST), strengthened northeasterlies (Lee et al. 2006), and the monsoon air from the boundary layer ahead of the MCS feed deep convection with outflow from the MCS occurring in mesoscale upper levels (Zhang et al. 2003) and are considered to be instrumental formesoscale oceanic cyclogenesis in the subtropical region. Owing to relatively few direct observations, the detailed structure of MCVs associated with the mei-yu wind shear zone over the subtropical ocean has not been as well documented as MCVs over continents. The southwest vortex is frequently observed between 700 and 850 hPa, with a horizontal scale of several hundred kilometers. In cases of more intense development, the coherent cyclonic circulation core extends from the surface layer to 300 hPa with no vertical tilt (Ding et al. 2001). A leeside subtropical MCV, persisting for 24 h near the southeastern coast of Taiwan on 16 June 1987, was observed by airborne Doppler radar (Yu et al. 1999). The vortex had an average relative vorticity of 7 × 10-4 s-1 at 2.5 km MSL, with a diameter less than 70 km and a vertical scale that extended to 7 km. The vortex was intimately coupled to convective precipitation, rather than stratiform precipitation, within a developing MCS. A numerical study by Zhang et al. (2003) showed that a subtropical oceanic MCV, accompanied by a mesolow, was well developed below 500 hPa in the trailing stratiform region behind the leading convective line. d. Objective and outline of paper To investigate the associated subsynoptic features and structure of the MCV over the subtropical ocean, three dropsonde flight missions were conducted during IOP 6 of SoWMEX/TiMREX along with 3-hourly launches from a dense array of soundings over and near Taiwan. The unique aspect of this study is the analysis of these enhanced observations to determine the structure of the subtropical oceanic MCV. The data sources used in the study are stated in section 2. The characteristics of the environment and the kinematic and thermodynamic structure of the subtropical oceanic MCV are described in section 3. The context of the findings for the present case is discussed in section 4 along with a comparison to midlatitude continental MCVs. Section 5 summarizes the results with a schematic diagram. 2. Data and analysis methods The principal data used in this study are atmospheric sounding data, radar radial velocity and composite reflectivity, satellite images, and global analyses. The National Centers for Environmental Prediction (NCEP) final operational global analysis data (FNL) from the period of 3-6 June 2008 were used to describe the synoptic environment. The data are available every 6 h with horizontal grid spacing of 1.08 3 1.08 and 26 mandatory levels from 1000 to 10 hPa. Fields utilized in the present analysis are sea level pressure, geopotential height, temperature, sea surface temperature, relative humidity, and u and y winds. Half-hourly visible images during daytime and IR images during nighttime from the Multi-functional Transport Satellite (MTSAT) of theJapan Meteorological Agency (JMA) were used. The water vapor channel data were also used to diagnose the humidity distribution in the midtroposphere. A multiradar reflectivity composite (Chang et al. 2009) and the radial velocity data from the Cigu radar are used for tracking the motion, precipitation, and kinematic structure of the vortex. The Cigu radar is a Gematronik 1500S Doppler radar, with specifications similar to theWeather SurveillanceRadar-1988 Doppler (WSR-88D). Located at 23.147°N, 120.086°E (Fig. 2), this radar is capable of detecting the radial velocity within a 230-km range. The radial velocity data are used in depicting the MCV dipole feature. The quality control (QC) process involved ground clutter removal and wind field unfolding. Quality-controlled sounding data from thirteen 3-hourly rawinsonde sites (including ship soundings) and dropsondes deployed from the Astra SPX jet during IOP 6 of SoWMEX/TiMREX were the principal data sources in the present case (Fig. 2). The sounding data were subjected to fourmajorQC steps and are hence known as level-IV data (Ciesielski et al. 2010). First, an internal consistency check was used to set flagged or bad data to a missing value; it included gross limit checks on all parameters and rate-of-change checks on temperature, pressure, and ascent rate (Loehrer et al. 1996). The raw data were processed via the Atmospheric Sounding Processing Environment (ASPEN), which analyzed the data, performed smoothing, and removed suspect data points. Then, in step two, time series and scatterplots of the data were created to check the range of values of pressure, temperature, and relative humidity. The soundings were also visually evaluated on the skew T-logp diagrams for outliers or other obvious problems. An essential QC step is humidity bias correction, including using intercomparison datasets to develop a humidity correction for the Vaisala RS80 system and a daytime dry bias correction for the Vaisala RS92 system. Finally, the ''buddy check'' was used in which rawinsondes adjacent in time and in close proximity to each other were visually inspected to ensure continuity of features. Dropsondes possessed fewer questionable or bad data (both were less than 5%) than rawinsonde sites, but they also had a higher percentage of missing data: 26% of height data were missing, with 19% wind data missing on mission 8 and 10% of RH values missing on mission 9. To calculate the convective available potential energy (CAPE) and convective inhibition (CIN) from parcel theory, we used a mixed parcel-either an average over the lowest 50 hPa or below the lifting condensation level (LCL), whichever was lower. To accommodate the fact that the average flight level was about 160 hPa (;14 km), the first available data level of dropsondes was only slightly above 200 hPa; therefore, CAPE was computed from all positive buoyancy at or below 200 hPa for both rawinsondes and dropsondes. This level was typically well below the parcel equilibrium level (;150 hPa during SoWMEX/TiMREX). The speed and direction of the oceanic vortex motion were estimated from radar animations to be about 5.7 m s-1 from 251° toward the east-northeast. Based on the translation speed, all data from flight mission 8 (2145-2347 UTC 4 June 2008) were time-space corrected to 0000 UTC 5 June, and the maximum displacement was about 46 km. The data of flight mission 9 (0516-0629 UTC 5 June 2008) were not time-space corrected because themovement of the lower-tropospheric portion of the vortex was indeterminate. The MCV had an estimated radius of maximum wind R of about 100 km, with a maximum wind V close to 20 m s-1; the vertical shear across the depth (;6 km) of the vortex (Du) was 3 m s-1. The relevant time scales are the inertial period 2pp/V, about 9 h, the shear time scale 2R/?u, about 18 h, and the advective time scale 2R/U, about 10 h, with U 5 5.7 m s-1. All of these time scales vary between 9 and 18 h. The typical time (~±1 h) and space (~±25 km) corrections were much smaller than the characteristic time and space scales of the vortex. Both plan views and vertical cross sections using the same sounding data were constructed to examine the MCV structure. The perturbation fields are computed by subtracting the four quadrants' mean relative to the vortex center. Questionable soundings with a majority of data missing or with profiles obviously unrepresentative of mesoscale variations (e.g., soundings dropped into convective elements) were not utilized in the MCV analysis. The mesoscale analyses and derivatives were constructed by 2D interpolation on a 0.58 3 0.58 grid using all the available dropsonde and rawinsonde observations. The east-west vertical cross sections used the gridded values on the axis crossing the 850-hPa cyclonic circulation center. Adropsonde without winds in the boundary layer was also used. (The region where these missing data affected derivative computations is hatched in Fig. 15.) Because of the resolution of sounding observations (about 0.88-18), the asymmetric observations distributed with a northeast- southwest orientation, and a higher percentage of missing data, the sampling uncertainties of the mesoscale structure in the present case should be considered. 3. Subtropical oceanic mesoscale convective vortex a. Overview of mesoscale disturbances During 4-5 June 2008, a mei-yu front was situated across the middle of Taiwan, the Taiwan Strait, and southern China (Fig. 3). At 2313 UTC 3 June (Fig. 3a), three mesoscale disturbances were aligned along the mei-yu frontwith a wavelength of approximately 600 km. MCS III, located on the western portion of the mei-yu frontal cloud band, had a cyclonic circulation as evidenced in observations from the Hong Kong radar data around 0000 UTC 4 June (radar data not shown). Convection within each of these MCSs dispersed as they migrated eastward on 4 June (Fig. 3b). Animation of half-hourly IR satellite images (not shown) indicated that the cloud top ofMCS III decreased and the smallest area of low cloud-top temperature (<200 K) occurred at 1457 UTC 4 June. The convection was then reestablished, becoming the so-called MCS III9 (Fig. 3c). Later, the visible satellite images revealed a clear spiral cloud pattern associated with a cyclonic circulation on the northwest periphery of the consolidated comma-shaped cloud (Figs. 1, 3d, 11).MCS III9 and its associatedMCV changed direction to a northeastward path, and its convection reached southern Taiwan after 2300 UTC. Around 0600 UTC 5 June, MCS III9 decayed over Taiwan's terrain with the MCV becoming difficult to define (Fig. 3e). On 6 June, the remnant cyclonic disturbance and its associated convection reorganized again over the ocean northeast of Taiwan (Fig. 3f). TheMCV's track is shown in Fig. 4. The satellite images demonstrate that the cloud structure of MCS III9 developed rapidly between 1800 UTC 4 June and 0000 UTC 5 June. In this region, the ocean surface temperature was at or greater than 28°C. Farther to the northeast, ocean surface temperatures were around 27°C. b. Synoptic environment and ambient vorticity strip The antecedent synoptic conditions are shown in Fig. 5. As the MCV developed, the mei-yu front, located across Taiwan and the Taiwan Strait, was characterized by horizontal wind shear at 850-700 hPa.Amidtropospheric trough passed between 1208 and 130°E, which was well to the north of the mei-yu front. Very weak pressure and temperature gradients were present in the vicinity of Taiwan. Humidity contrasts occurred through the horizontal shear zone, with the highest mixing ratios of water vapor found within the southwesterly flow. Zonally averaged (118.58-120°E) Hovmoller diagrams of geopotential height, zonal wind, temperature, mixing ratio, and relative vorticity at 850 hPa from the NCEP FNL analyses during 3-7 June 2008 are presented in Fig. 6. A pressure trough and a strong temperature gradient passed between 308 and 35°N at the beginning of the event (Figs. 6a,c) and then the height gradient weakened on later days. Strong westerlies were evident south of the midlatitude trough on 3-4 June (Fig. 6b). In the wake of the midlatitude trough, there was warming and drying between 258 and 30°N (Figs. 6c,d). Farther south, a persistent cyclonic shear zone lay near 24°N. This mei-yu wind shear zone, which was collocated with cooler temperatures (below 18°C; Fig. 6c), strengthened late in the day on 4 June. Accompanied by an increasing lowlevel westerly flow (Fig. 6b), the moist extent widened on 5 June (Fig. 6d). A strip of positive relative vorticity characterized the wind shear zone. The vorticity strengthened on 5 June and its horizontal scale extended to about 500 km (Fig. 6e). The 1000-500-hPa thickness fields at 1200UTC 4 June and 0000 UTC 5 June (Fig. 7) show the northerly flow passing through the isobaric thickness line north of the Taiwan Strait. North of this area, the northwesterly flow led to cold advection. The northeasterly and easterly flow subsided between 238 and 25°N, and farther south, the westerly-southwesterly jet attained a local maximum of 15 m s-1, creating a channel with ?e air greater than 340 K at 700 hPa. The high ?e channel extended for more than 2000 km between 28°N to the east and 22°N on the west end. The easterly subsidence warming contributed to decreasing the temperature contrast across the mei-yu front and resulted in a relatively barotropic environment with primarily a gradient in water vapor mixing ratio (Figs. 6, 7). The developing mesoscale cyclonic disturbances were biased toward the zone of high ?e air (Fig. 7).At 0000UTC 5 June, the vortex near 125°E (associated with MCS I) weakened while the other two vortices over the Taiwan Strait merged (MCS III9) and strengthened. Because there were no direct observations over the ocean on 4 June near these features, the magnitude of the vorticity should be interpreted with caution. c. Distribution of thermodynamic parameters and precipitation As determined from the two dropsonde flights during the day on 5 June (around 0000 and 0600 UTC), the CAPE in the vicinity of the MCV center was less than 1500 J kg-1 (Fig. 8). The positive area on the skew T-logp diagrams was generally thin through a deep layer and the CAPE was relatively small, which is a typical feature of storm environments over the tropical and subtropical oceans (Lucas et al. 1994; LeMone et al. 1998). At 0000 UTC 5 June (Fig. 8a), the dropsonde with the highest CAPE (2375 J kg-1) descended through the southwesterly flow to the southwest of the MCV, in close proximity to vigorous convective cells. There were two dropsondes located on the southeastern periphery of the MCV that were characterized by modest CAPE (794 and 1291 J kg-1, respectively) and almost noCIN.A similar situation existed at 0600 UTC (Fig. 8b); a dropsonde also revealed moderate CAPE and little CIN near the southern coast of Taiwan. These locations were near the convective clouds and had relatively higher water vapor content. The soundings, which were located to the rear and on the northern periphery of theMCV center, demonstrated either small CAPE or larger CIN and corresponded to a dry environment. At 0600 UTC, the dry air extended into the MCV center. Thus, there were two different regimes: organized and deep moist convection on the east and southeast side of the vortex, and clear air or scattered clouds to the west and northwest. Figure 9 shows a dropsonde skew T-logp diagram from each of these regimes. DropsondeA(Fig. 9a) was moist through the troposphere (indeed, it was in the MCS) with a nearly saturated boundary layer. Dropsonde A possessed smaller spikes, some with weak superadiabatic layers and near-zero lapse-rate layers nearby, suggesting sampling of the convective scale environment in which strong updrafts and downdrafts were causing a mixing of air from different layers. The prevailing southwesterly monsoon flow rotated to southerly inflow on this flank of the MCV. The lowertropospheric winds were primarily southerly with some evidence of an increase with height. The wind direction, veering and backing twice with height below 500 hPa, also suggests a significant influence of convection. The wind speed increased above 500 hPa, and the vertical wind shear between 500 and 200 hPa was 12.9 m s-1. Since dropsonde A was strongly influenced by convective-scale features, it is excluded frommesoscale weather analyses. The profile fromdropsondeB (Fig. 9b) exhibited a capping inversion above 900 hPa that was responsible for greater CIN and only scattered, low clouds in this area. In addition, the boundary layer was rather dry, well-mixed, and slightly warmer than that in dropsonde A. During a portion of the observing period, a mesoscale rainband formed featuring an intense bow-shaped leading line of convection and a broad stratiform precipitation shield on the upshear flank of the vortex during 2300UTC4 June and 0100UTC5 June (Fig. 10). The northward-moving convection collided with the southwest terrain of Taiwan, which contributed to flooding rains along the coastal plain around 0300-0600 UTC 5 June. At the same time, a leeside convective precipitation area developed. d. The kinematic and thermodynamic structure of the subtropical oceanic MCV In the mesoscale analyses at 0000 UTC and 0600 UTC 5 June, a cyclonic circulation associated with the MCV was apparent at 925-700 hPa (Figs. 11-13). The gradient of virtual potential temperature ?? at 925 hPa was about 1 K (100 km)21 at 0000 UTC (Fig. 11a), while on the northeast periphery of the MCV, ?? was lower. The wind direction crossed the isentropes from warm to cold air, implying warm advection, and upgliding motion occurred within the strong southerly flow associated with the MCV circulation. This weak upgliding was nearly collocated with the main convection. In addition, an enhanced ?? gradient was presented near the coast at 0600 UTC (Fig. 11b), where the isentropic ascending motion may have contributed to flooding rain along the coastal area instead of on the terrain slope farther inland. The 850- and 700-hPa maps show a mesolow within the cyclonic circulation, slightly biased to the northern periphery at 0000 UTC 5 June (Figs. 12a, 13a). The cyclonic circulation was located offshore from the Taiwan coast at 0600 UTC, and the mesolow deepened and shifted to the eastern periphery of the MCV (Figs. 12b, 13b). The northward-propagating convective cells coincided with the southerly and southwesterly winds. According to the reflectivity, the strong southerly wind observation was either within or very close to the MCS, suggesting that simple gradient wind balance might not be expected on this flank. The temperature gradient within the lower troposphere over the analysis region was weak. Corresponding to approximate thermal wind balance, the average vertical wind shear between 500 and 925 hPa was only about 3.3 m s-1 and directed from 2818 at 0000 UTC. This value of vertical wind shear would be in thermal wind balance with a temperature gradient of roughly 1.2°C (1000 km)21. The virtual temperature gradient at 850 hPa was greater, about 2°C (1000 km)21 directed toward the southwest at 0000 UTC (Fig. 12a). Six hours later, the central area of the mesolow warmed, partly obscuring the overall temperature gradient (Fig. 12b). At 0600 UTC, the wind shear was 3 m s-1 from 3518. Mesoscale maps at 700 hPa show the locally higher ue area collocated with a southwesterly wind at 0000 UTC (Fig. 13a) and with a southerly wind at 0600UTC(Fig. 13b). Apronounced ?e contrast is present on the north side of the MCV, and there was a localized area of lower ?e behind the main convection at 0000 UTC. Weak warm advection occurred on the northwest periphery. The cyclonic circulation over the lower-tropospheric MCV center was negligible at 500 hPa (Fig. 14). Cyclonic vorticity at 500 hPa may be inferred along an arc extending from northeast to east of the MCV, then trailing back to slightly south and southwestward of the MCV. Six hours later, the cyclonic circulation at 500 hPa is also shifted to the east of the MCV, but by a smaller amount than earlier. Warm advection on the east flank was present especially between 700 and 500 hPa. The wind direction veered with height and a northerly wind was revealed at 500 hPa. A large humidity contrast was apparent to the north of the MCV. The dry air penetrated into the rear flank of the MCV in the northwesterly and westerly flow, consistent with Figs. 8 and 9b. In general, the southerly and southwesterly flow possessed higher humidity relative to northwesterly flow. An east-west vertical cross section of the meridional wind component exhibited a distinct tangential velocity couplet between 5 km MSL and the boundary layer at 0000 UTC, stretching to 8 km at 0600 UTC 5 June (Fig. 15a). An asymmetric tangential velocity couplet appeared between 3 km MSL and the boundary layer with a diameter of about 200-250 km. The maximum northerly flow occurred below 1 km, and the maximum southerly flow was between 1 and 3 km at 0000 UTC before deepening to 5 km by 0600 UTC. The main convection, in which the 25-dBZ echo extended up to 8 km MSL, was collocated with southerly flow. A weaker cyclonic wind shift above the main convection was revealed at 0000 UTC. The southerly wind at 10- 11 km MSL is likely the signature of the convective outflow. The MCV apparently became deeper between 0000 and 0600 UTC, probably owing to deep convection partly within the MCV center (at 0000 UTC). The main convection moved ahead of the MCV center at 0600 UTC, and heavy precipitation occurred over coastal areas. An eastward-tilted cyclonic circulation above the mountain level was evident at 0600 UTC. The system-relative zonal winds exhibited relatively weaker and shallower front-to-rear flow penetrating through the lower portion of the vortex at 0000 UTC (Fig. 15b). Isentropic ascent (Raymond and Jiang 1990; Trier and Davis 2007) of the easterly flow below 850 hPa appeared as the low-level ?? deviation decreased westward between 1198 and 121°E. At 0600 UTC, the low-level ?? deviation minimum in the vicinity around 120.4°E enhanced the ?? gradient farther south, leading to favorable conditions for isentropic upglide. The location of the primary convection was near this upgliding region, and a deep warm core, corresponding with the convection, was revealed in the mid to upper troposphere at 0600 UTC. The system-relative1 westerly flow above the front-to-rear flow at 0600 UTC was stronger and deeper in contrast to 0000 UTC and penetrated into the MCV center. Above 500 hPa, the westerly flow increased with height with vertical wind shear between 500 and 200 hPa of 12 m s-1 at 0000 UTC and 18 m s-1 at 0600 UTC. Downshear, the precipitation appeared to be mainly stratiform with a brightband signature between 500 and 700 hPa (Fig. 15a). The maximum mesoscale vorticity exceeded 2.5 × 1024 s-1 around 118.5°E at 0000 UTC and around 119.3°E at 0600 UTC 5 June (Fig. 15c). At 0000 UTC, the strong cyclonic vorticity extended nearly to 500 hPa with a nearly upright structure, and a second maximum was revealed to the east of the lower-tropospheric vortex. Six hours later, the cyclonic vorticity strengthened and the maximum penetrated down into the boundary layer. The vorticity structure associated with the MCV was translated eastward above 700 hPa, yet remained upright throughout the midtroposphere. Low-level convergence with a magnitude of about one-quarter of the vorticity was slightly ahead of the vortex center at 0000 UTC. Stronger midtropospheric divergence over the coastal region was also evident at 0600 UTC. There was anticyclonic vorticity appearing above 400 hPa and the minimum occurred eastward of the main convective region. High values of equivalent potential temperature in the boundary layer occurred within the region of cyclonic vorticity (Fig. 15). Enhanced ?e at 850 hPa corresponded with the strong southerly flow. Potential instability, conditional instability (Fig. 9a), and high reflectivity (Fig. 13a) within the region of low-level convergence suggest that the stretching of vorticity was due to deep moist convection within the MCV. Noticeable low RH between 850 and 500 hPa (Figs. 14, 15d) and sparse precipitation in the upshear region of theMCV(Figs. 10, 13, 15) reflected the dry air transport. At 0000 UTC, the mesolow below the 700-hPa warm perturbation (Fig. 15e) was collocated with the low-level vortex (Fig. 15c); the other warm perturbation in the high troposphere and a mesolow below it reflect the MCS structure with a warm anvil (Fig. 15b). The warm perturbation in the upper troposphere deepened (Fig. 15b) and the lower-tropospheric mesolow strengthened at 0600 UTC (Fig. 15e). The gradient of geopotential height increased between 119.58 and 121°E in associationwith the intensification of southerly flow (Fig. 15a). e. Radial velocity dipole and subvortex According to the radial velocity from the Cigu radar 0.58 plan position indicator (PPI; not shown) and 1.48 PPI (Fig. 16), a subvortex appears embedded in the MCV circulation. Two horizontal scales are evident in the dipole pattern of radial velocity. Consistent with the sounding analysis (Figs. 11a-13a, 15a), the MCV circulation observed on the northern edge of the mesoscale rainband is roughly 160-200 km across, although it is difficult to be precise because the receding velocities are mainly in clear air. Within theMCVcirculation, an inner dipole, with a horizontal scale of only 25-30 km, was first observed at 2200 UTC 4 June and became indistinct after 0230 UTC 5 June (Fig. 17). During this period, the inner subvortex moved eastward along 22.2°N, and the velocity dipole was asymmetric, similar to the tangential wind structure of the MCV. A stronger receding branch dominated at higher altitudes with stronger flow approaching the radar at lower levels. This asymmetry may be simply a superposition of the small-scale vortex onto the larger-scale southwesterly flow associated with the MCV and the environment. The maximum shear vorticity of about 3 × 10-3 s-1 occurred at 2300UTC 4 June at 4.5 km MSL (Fig. 17b). With respect to the reflectivity field (Fig. 16), the incipient mesovortex was embedded in the northern end of the stronger convective line segment (.40 dBZ) at the back edge of the mesoscale rainband. Just behind the intense convection, a weakly bow-shaped reflectivity pattern accompanied the stronger approaching flow on the southern side of the small-scale vortex at 2300 UTC (Fig. 16a). Later yet, as the small-scale vortex gradually grew in scale (Fig. 17d), the reflectivity in the convective line weakened and became less organized (Fig. 16b). We hypothesize that this small-scale vortex is analogous to line-end vortices observed in other convective systems, primarily at higher latitudes. The larger-scale cyclonic vorticity provided by the MCV and mei-yu frontal zone would strongly favor the development of the cyclonic vortex as opposed to what might otherwise be a cyclonic- anticyclonic vortex pair in the absence of background rotation. 4. Context of findings The MCV presented in this study occurred within a preexisting ambient vorticity strip (Figs. 6e, 7) and apparently resulted from deep, moist, convective precipitation (Figs. 10, 15a, 16). A subvortex, which was embedded in the MCV, exhibiting stronger shear vorticity and a shorter life cycle was also closely tied to stronger convection. For a mei-yu front, preexisting ambient vorticity, associated with significant horizontal wind shear and moderate-weak temperature gradients, is often seen; however, such a consolidated vortex occurs less frequently. One-day-average vertical cross sections normal to the mei-yu front through the Taiwan Strait in Fig. 18 show the PV evolution over 2 days. The easterly flow in the present case had subsided, thereby becoming warm and dry. The isolated PV structure around 228-23°N, which intensified and became vertically stacked, coincided with increasing barotropic wind shear below 500 hPa (also see in Figs. 6b,e). Stretching effects and latent heating due to local organized convection within the region of upright ambient vorticity contributed to the concentration and further enhancement of the vorticity (Fig. 15). In this region, the enhanced cyclonic vorticity within the mei-yu strip made the vortex formation process more efficient. Once generated, we suggest that the vortex perturbed the strip, generating an undulation on a scale of the vortex itself (Figs. 11-13, 16). Based on the spatial scale of the vortex and the short time scale over which it appeared to form, it is likely that the energy source was a diabatic energy conversion, and that the undulations of the vorticity strip were a result of this process rather than a catalyst. In contrast, a typical midlatitude MCV forms within the stratiform region or at the end of the convective line (Cotton et al. 1989; Olsson and Cotton 1997; Chen and Frank 1993), and the midtropospheric warming is usually associated with condensation heating in the stratiform region (Zhang and Fritsch 1988). The rearinflow jet in the midtroposphere, due to downdrafts from melting, evaporation, and water loading, results in strong divergence at low levels (Smull and Houze 1987). Thus, the maximum cyclonic vorticity and the maximum tangential winds tend to occur between 550 and 600 hPa (Zhang 1992; Fritsch et al. 1994; Bartels et al. 1997; Davis and Trier 2007). The differences in the present subtropical oceanic MCV are the very moist and warm boundary layer and low LCL (Fig. 9) compared to midlatitude convective environments. There was also the preexistence of large ambient vorticity (Fig. 18) and inferred relatively weak evaporation cooling (Fig. 15b) in the lower troposphere, both of which favored vorticity generation in the subtropical, oceanic vortex at a lower altitude than in midlatitude MCVs. In the present case, convection and the associated vortex may have developed more rapidly because of the presence of a warm ocean surface (Fig. 4) (Lee et al. 2006). It seems likely that the strong southwesterly flow south of the mei-yu front would have increased CAPE and reduced CIN values through enhanced surface moisture fluxes. Evidence for this is the extreme conditional instability seen in the southernmost dropsonde in Fig. 8a. This southwesterly flow, modified by the MCV circulation (Fig. 15a), transports high ?e air northward in the lower troposphere, thereby increasing potential instability ahead of the MCV center (Fig. 15d). Such an environment is favorable for convection and further heavy precipitation (Fig. 10). This is similar to the role of the enhanced nocturnal low-level jet (LLJ) associated with continental MCVs and their heavy precipitation (Cotton et al. 1989; Bartels and Maddox 1991; Olsson and Cotton 1997; Davis and Trier 2002; Trier and Davis 2002; Schumacher and Johnson 2009); such cases tend to exhibit an asymmetric precipitation structure with mesoscale rotation. In addition, the isentropic vertical motion pattern associated with the vortex-relative zonal flow region (Fig. 15b) is favorable for downshear ascent in the lower troposphere and the release of potential instability. The dry rear inflow in the midtroposphere, which was entrained into the MCV center, inhibited deep convection over its upshear region. This contributed to the asymmetry of precipitation across the vortex and to the lack of convection in its center by 0600 UTC 5 June. 5. Concluding remarks During 4-5 June 2008, a mei-yu front, characterized by strong ?e gradients, a distinct horizontal wind shift, and a weak horizontal temperature variation, was situated across the middle of Taiwan, the Taiwan Strait, and southern China. A rain belt persisted along a moderate southwesterly low-level jet around 20°N. Wavy cyclonic disturbances with active convection migrated eastward along a vorticity strip defined by a zone of strong horizontal wind shear. A decayed MCS reorganized within the vorticity strip around 1200 UTC 4 June 2008 and developed a strong MCV with a diameter of roughly 250 km, and included evidence of an embedded subvortex with a diameter of only 25-30 km. The observed magnitude of maximum shear vorticity in the subvortex reached 3 × 1023 s-1 at 4.5 km MSL. Dry, subsiding easterly flow poleward of the mei-yu front helped create a large contrast in water vapor across the shear zone with essentially no temperature contrast. Very moist air in the lower-tropospheric southwesterly flow provided a convectively unstable environment and contributed to the heavy precipitation in association with this MCV. We propose that vortex stretching associated with the deep convection concentrated the vorticity within the mei-yu vorticity strip, and hence the depth of the vortex was closely related to the depth of this shear zone. The vorticity strip thus provided a favorable ''reservoir'' of cyclonic vorticity that could be tapped by convection. Figure 19 shows a schematic of the structure and evolution of the subtropical MCV examined in this study. Stage I is based on the analyses of 0000 UTC 5 June; the MCV was intensifying and exhibited a near-upright structure with deep convection occurring near its center. The strong tangential wind and vorticity, with a cyclonic circulation diameter of 250 km, was confined below roughly 5 km MSL. The MCV induced more southerly and southeasterly flow, which brought the moisture northward to its east flank and created convective and conditional instability.Weak isentropic ascent associated with the vortex-relative easterly flow downshear of the MCV was nonetheless able to cause a focusing of the convection due to small convective inhibition, low cloud base, and low lifting condensation level in the extremely moist environment. The leading convection organized into a bow-shaped convective line, with a line-end vortex diameter of only 25-30 km evident in the Cigu Doppler radar. In contrast to the leading convection, there was only scattered precipitation on the upshear flank of the vortex and a swirl of low clouds exposed (Fig. 1). The asymmetric precipitation structure resulted from the dry system relative rear inflow, much of which came fromthe dry air north of the vorticity strip that had a history of subsidence before entering the subtropics. The low-level cyclonic circulation center was not located underneath the deep convection, which would not be optimal for the vortex to rapidly intensify. Although a tilt was discernable, the weaker vertical wind shear below 500 hPa still allowed for a relatively upright vortex. The mature phase of the vortex (Fig. 19, stage II) exhibited a deeper structure with a slight eastward tilt. The main convection was collocated with low-level convergence and midlevel divergence placed ahead of the low-level MCV center. The convective instability and low-level isentropic ascent enhanced precipitation, creating flooding over the onshore coastal region instead of on the terrain slope. Dry inflow on the upshear side of the vortex entrained into the circulation's center and inhibited further convection in the core. Splitting of the flow, induced by the central mountain range of Taiwan, resulted in convection eventually forming on the east side of Taiwan. This is the first study using the unique dataset from the East Asian region to examine a subtropical oceanic MCV in detail. We revealed the associated environmental features and the kinematic and thermodynamic structure of the MCV with an embedded strong inner subvortex. This case possessed obviously different characteristics compared to the midlatitude MCV. A critical unresolved issue is the evolution of the MCS/MCV. The overarching future objective is to examine the cyclogenesis and evolution of the MCV over the ocean via high-resolution numerical simulation. Furthermore, the orographic influence on the structure of the vortex and precipitation will also be explored in a forthcoming study. Acknowledgments. This study was supported by the National Science Council of Taiwan, under Grants NSC- 98-2745-M-002-007-MY3 and NSC-100-2915-I-002-018. The authors thank all participants involved in the planning and field phases of SoWMEX/TiMREX. Access to the satellite and radar data was provided by the Center Weather Bureau of Taiwan and the data processing was assisted by Nick C.-C. Tong and Ultimate C.-J. Jung (NTU). We also thank Paul Ciesielski (Colorado State University), Junhong Wang (NCAR), and Shao-Chin Huang (Chinese Culture University) for their support of the SoWMEX/TiMREX sounding data quality control. 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Mon. Wea. Rev., 127, 838-857. Zhang, D.-L., 1992: The formation of a cooling induced mesovortex in the trailing stratiform region of a midlatitude squall line. Mon. Wea. Rev., 120, 2764-2785. _____, and J. M. Fritsch, 1988: A numerical investigation of a convectively generated, inertially stable, extratropical warm-core mesovortex over land. Part I: Structure and evolution. Mon. Wea. Rev., 116, 2660-2687. Zhang, Q.-H., K.-H. Lau, Y.-H. Kuo, and S.-J. Chen, 2003: A numerical study of a mesoscale convective system over the Taiwan Strait. Mon. Wea. Rev., 131, 1150-1170. HSIAO-WEI LAI Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan CHRISTOPHER A. DAVIS National Center for Atmospheric Research,* Boulder, Colorado BEN JONG-DAO JOU Department of Atmospheric Sciences, National Taiwan University, and Asia-Pacific Economic Cooperation Research Center for Typhoon and Society, Taipei, Taiwan (Manuscript received 9 March 2010, in final form 5 November 2010) * The National Center for Atmospheric Research is sponsored by the National Science Foundation. Corresponding author address: Ben J.-D. Jou, Department of Atmospheric Sciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Da-an District, Taipei City 106, Taiwan. E-mail: [email protected] (c) 2011 American Meteorological Society [ Back To TMCnet.com's Homepage ]