mesoscale vortex generation and merging …...358 mesoscale vortex generation and merging process...

ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 27, NO. 2, 2010, 356–370

Mesoscale Vortex Generation and Merging Process:

A Case Study Associated with a Post-Landfall

Tropical Depression

YU Zifeng∗1,2 (喻自凤), LIANG Xudong1,2 (梁旭东),YU Hui1,2 (余晖), and Johnny C. L. CHAN3

1Shanghai Typhoon Institute, China Meteorological Administration, Shanghai 200030

2Laboratory of Typhoon Forecast Technique, China Meteorological Administration, Shanghai 200030

3Laboratory for Atmospheric Research, Department of Physics & Materials Science,

City University of Hong Kong, Hong Kong

(Recived 17 June 2008; revised 20 April 2009)

ABSTRACT

An observational analysis of satellite blackbody temperature (TBB) data and radar images suggeststhat the mesoscale vortex generation and merging process appeared to be essential for a tropical-depression-related heavy rain event in Shanghai, China. A numerical simulation reproduced the observed mesoscalevortex generation and merging process and the corresponding rain pattern, and then the model outputswere used to study the related dynamics through diagnosing the potential vorticity (PV) equation. Thetropical depression (TD) was found to weaken first at lower levels and then at upper levels due to negativehorizontal PV advection and diabatic heating effects. The meso-vortices developed gradually, also from thelower to the upper levels, as a result of positive horizontal PV advection and diabatic heating effects in thedownshear left quadrant of the TD. One of these newly-generated vortices, V1, replaced the TD ultimately,while the other two, V2 and V3, merged due to the horizontal PV advection process. Together with theredevelopment of V1, the merging of V2 and V3 triggered the very heavy rain in Shanghai.

Key words: mesoscale vortex, tropical depression, heavy rain, potential vorticity

Citation: Yu, Z. F., X. D. Liang, H. Yu, and J. C. L. Chan, 2010: Mesoscale vortex generation and mergingprocess: A case study associated with a post-landfall tropical depression. Adv. Atmos. Sci., 27(2), 356–370,doi: 10.1007/s00376-009-8091-x.

1. Introduction

Since the late 1980s, the availability of mesoscaleinformation, particularly from satellites and radar,and the increased application of probability and scaleinteraction concepts have led to the recognition of theimportance of mesoscale systems in tropical cyclones(TCs). Holland (1995) hypothesized the importanceof mesoscale vortex interaction in TC genesis. Ritchieand Holland (1997) discussed scale interactions dur-ing the formation of Typhoon Irving and pointed outthat mesoscale interactions with low-level, large-scalecirculations and with other mid-level, mesoscale fea-

tures resulted in the development of vorticity in themid levels. Simpson et al. (1997) also showed that thedevelopment of, and the apparently stochastic interac-tions between, mesoscale vortices and associated con-vective systems were an integral component of the gen-esis of TC Oliver. Montgomery et al. (2002) showedthat the radial mixing of angular momentum can beaccomplished by asymmetric eddies in the eyewall ofreal TCs, which would be essential for maintaining theTC structure. Wang (2002) argued that strong pertur-bation from an outer spiral rainband can amplify thevortex Rossby waves in the eyewall, causing a large dis-tortion of the eyewall and partial eyewall breakdown

∗Corresponding author: YU Zifeng, [email protected]

NO. 2 YU ET AL. 357

accompanied by a weakening of the TC. TC eyewallbreakdown/recovery is then accompanied by a weak-ening/intensifying cycle of the TC. Such a partial eye-wall cycle is an asymmetric process. In addition, Hen-dricks et al. (2004) found that the “vortical” hot tower(VHT) was essential in the formation of TC Diana(1987), and a VHT paradigm in tropical cyclogenesiswas given and discussed by Montgomery et al. (2006).Reasor et al. (2005) supported a stochastic view of TCgenesis in which multiple lower-to-middle-troposphericmesoscale cyclonic circulations were involved in build-ing the surface cyclonic circulation of Dolly. Multiscalevortex formation and interaction may be a common as-pect of many tropical cyclogenesis events (Sippel et al.,2006). Luo and Liu (2007) investigated the interactionbetween a tropical storm and its adjacent mesoscaleconvective vortices (MCVs) using a barotropic primi-tive equation model. Their results showed that the in-teraction may result in vortex self-organization. Theintensity of the initial tropical storm controlled thecomplexity of the self-organization process and deter-mined whether the vorticity of the MCVs was eitherfully or partially absorbed. Therefore, mesoscale vor-tex formation and interaction can be very importantfor TC genesis over oceans and for structure variation.

Much observational research has shown a close con-nection between heavy rain in China and the interac-tion among multi-vortices (e.g. Lu, 1986; Sun andAn, 2001; Chen et al., 2003, 2004), especially the vor-tex merging phenomenon, which has been studied formany years (Ritchie and Holland, 1997; Enagonio andMontgomery, 2001; Ritchie, 2003). Some of this re-search has been on landfall TCs. For example, thewell known heavy rain event in August 1975 caused bya post-landfall typhoon in inland China was found tobe linked with a vortex merging process (Chen, 1977).The most intensive three-hour rain amount was 566mm when the vortices merged together. Lin and Buhe(2003) simulated a heavy rain event associated with alandfall tropical depression (TD) which occurred on 5August 2001 in Shanghai and found that the strongconvection was related to a merging of two mesoscaleconvective cloud clusters.

Structural changes of a TC at landfall may result inspatial redistributions of the strongest winds and heav-iest rainfall. Unfortunately, a lack of data precludesa detailed description and analysis of the mesoscalestructure of TCs at landfall (Powell, 1987, 1990; Pow-ell et al., 1996). The dynamics of mesoscale vortexgeneration and merging processes related to TC heavyrainfall remains unclear. In this paper, the same caseas that used by Lin and Buhe (2003) will be studiedbased on a simulation with the fifth generation PennState University–NCAR Mesoscale Model (MM5) ver-

sion 3.6. This heavy rain event was associated witha post-landfall TD moving into the neighboring areasof Shanghai and was not predicted by numerical mod-els or subjective forecasting at the time (Yang et al.,2004). Urban and suburban areas of Shanghai wereall affected, with 24-hour rainfall coming close to 300mm. Some short notes and studies (Shao and Huang,2002; Yang et al., 2003, 2004; Yu et al., 2005) havebeen published which show the existence of a favorableweather situation, with multiple mesoscale vortices ina post-landfall TD background; good moisture and in-stability conditions for the case based on some datasources. Fu et al. (2004) produced a much more com-prehensive observational analysis and pointed out thatthe continuous development of a mesoscale convectivesystem in Shanghai was the reason for this heavy rain-fall event. However, no further analysis has been madeon the mechanism of the system redevelopment andevolvement. Because it was related with a new vortexformation and replacement of the original TD center,it is also a good case to study post-landfall TC evolu-tion.

An observational investigation of the mesoscaleprocesses based on Geostationary MeteorologicalSatellite-5 (GMS-5) infrared brightness temperature(TBB) data and radar images is presented in section2. The model and its configuration are described insection 3, and comparisons between observations andsimulations of the mesoscale vortices associated withthe heavy rain are made in section 4. The dynamicsassociated with the development of the mesoscale vor-tices and the vortex merging processes are describedin sections 5 and 6, respectively. A summary and con-clusions are presented in section 7.

2. Observational analysis

Seen from the conventional observations of windvectors and geopotential height at 500 hPa, the post-landfall TD was located in the adjoined area of Zhe-jiang, Jiangsu and Anhui Provinces at 0800 LocalStandard Time (LST, Beijing Time) 5 August 2001(Fig. 1a). Twelve hours later, the depression movednortheastward to the west of Shanghai (Fig. 1b).

Figure 2 shows the cloud evolution process duringthis rain process, analyzed from the GMS-5 TBB data.It is shown that the cloud system associated with theTD was quite asymmetric at the initial time 1300 LST5 August 2001. In the next three hours, the cloud sys-tem developed to its south and extended to a muchlarger area with a minimum TBB of −55◦C, implyinga development of the southern part of the TD system.But the west side of the cloud system shrank from1600–1700 LST 5 August 2001, accompanied by an

358 MESOSCALE VORTEX GENERATION AND MERGING PROCESS AND A POST-LANDFALL TD VOL. 27

Fig. 1. Conventional observations of 500 hPa wind vec-tors and geopotential height (in 10 gpm) at (a) 0800 LST5 August 2001 and (b) 2000 LST 5 August 2001. Circlesdenote the geopotential height contours of 5880 gpm forthe TD. The three provinces, Anhui, Jiangsu, and Zhe-jiang, as well as Shanghai municipality, are marked byAH, JS, ZJ and SH, respectively.

eastward development of the system. Then, from 1700LST 5 August 2001, several mesoscale cloud massesappeared and developed in the south and east, andone of them moved northeastward and across Shang-hai very slowly. The hourly rainfall data from gaugestations in the areas of Shanghai, Zhejiang and Jiangsu(shown as SH, ZJ and JS in Fig. 3a) were also analyzedto assess the rain process (Fig. 3b). There was qual-ity control in the rain gauge data used, and the gaugestations were densely distributed in the Shanghai area,while very sparse in other areas such as Zhejiang andJiangsu (Fig. 3a). Therefore, the rain gauge data canbe used to reflect well the whole rain process, especiallyin Shanghai. At 2000 LST 5 August 2001, it can beseen that the northeastward-moving cloud masses had

resulted in the beginning of the heavy rainfall processof Shanghai at 2000 LST 5 August 2001 (Fig. 3b).

Shown by the cloud maps, the two mesoscale cloudmasses near Shanghai merged after 2000 LST 5 August2001, while another mesoscale cloud mass was newlydeveloping to their west. Radar reflectivity observa-tion from a Gemtronic 360AC Doppler radar locatedin Hongqiao airport, Shanghai (1.0◦ elevation angle),also caught the vortex merging process over the region(Fig. 4). Strong reflectivity (>35 dBZ) appeared firstin the west of Shanghai at about 1816 LST 5 August2001 and moved eastward at 1916 LST 5 August 2001.A relatively weak reflectivity zone in the northern areaof Zhejiang Province was also observed at 1816 LST 5August 2001, which moved northward into HangzhouBay and developed in the subsequent one and a halfhours. The two strong reflectivity areas were con-nected with each other and merged gradually over theShanghai area until 2100 LST 5 August 2001. Then,at 2200 LST 5 August 2001, the newly developed cloudmass to the west “ingested” the eastern merged cloudmasses and expanded quickly in area (Fig. 2). Be-cause of a radar observational failure from 2200–2300LST 5 August 2001, the reflectivity was not contin-uously observed for the later rain process. However,from the cloud maps between 2000 LST 5 August 2001and 0200 LST 6 August 2001, the cloud mass kept in-tensifying and growing, with its lowest TBB being lessthan －70◦C (Fig. 2). And, during that period, thecenter position of the cloud mass was almost stationaryover Shanghai, and accompanied by the most intenserainfall process, with a maximum hourly rain amountof 82.7 mm at 0100 LST 6 August 2001 (Fig. 3b).The maximum six-hour accumulated rainfall reached180 mm, a once-in-fifty-years value.

At 0300 LST 6 August 2001, and accompanied byan eastward movement of cloud masses, the area withTBB below −70◦C began to shrink and the observedrain rate in Shanghai began to weaken, suggesting anoverall weakening of the system. Later, the systemwas largely gone from Shanghai and the heavy rainfallprocess related to it had ceased.

These results suggest that this heavy rain processmight be related to the generation and merging ofmeso-vortices associated with the post-landfall TD.

3. Model description

This study utilizes the Penn State University-NCAR Mesoscale Model (MM5) version 3.6. The com-putational domains consist of a 27 km grid with a meshsize of 201×201 (D01), two-way nested (Dudhia et al.,2005) with a 9 km grid with a mesh size of 121×121(D02). The domain center is at 31.0◦N, 121.0◦E. All

NO. 2 YU ET AL. 359

Fig. 2. Blackbody brightness temperature (TBB, units: ◦C) of the Geostationary Meteoro-logical Satellite-5 (GMS-5) from 1300 LST 5 August 2001 to 0800 LST 6 August 2001 nearShanghai. The typhoon mark in the top figures denotes the center position of the TD. AH,JS, ZJ and SH are as in Fig. 1.

grids have 23 levels in the vertical. The simulationsfor D01 and D02 are initialized at 0800 LST 5 Au-gust 2001, and integrated for 24 hours. The modelphysics include the Grell convective scheme for cumu-lus parameterization and the Hong and Pan Medium-Range Forecast (MRF) scheme for planetary boundarylayer parameterization (Grell et al., 1994; Hong andPan, 1996). No cumulus parameterization is used inD02. Global Final analyses of the US National Centers

for Environmental Prediction with 1◦ × 1◦ horizontalresolution are used as the model first guess field andboundary conditions.

4. Overview of the simulation

Whether the simulation can reflect the observedmesoscale vortex generation and merging process isexamined in this section by comparing the simulated


a

b

Fig. 3. (a) Distribution of rain gauge stations used in the study; (b) hourly gauge rainfall from1700 LST 5 August 2001 to 0400 LST 6 August 2001 (units: mm). JS, ZJ and SH are as in Fig. 1.

850 hPa vorticity field (Fig. 6), radar reflectivity (Fig.7) and rainfall (Figs. 5b and 8) with the observations.Since the average distance between automatic gaugestations in Shanghai was around 10 km but the gaugedata in other areas were very sparse, the observed rain-fall in Shanghai can be reasonably compared with therain rate outputs of D02 at a 9 km horizontal resolu-tion.

Seen from the simulated 850 hPa vorticity field att = 5 hours (1300 LST 5 August 2001), an area withhigh vorticity (named V1 in Fig. 6) existed to thesoutheast of the post-landfall TD and at the adjointregion of Jiangsu, Zhejiang and Anhui Provinces. The

location of the TD is defined by the circulation cen-ter and shown by a typhoon mark in the figures. Theobserved development of the southern part of the TDsystem was reproduced well, as shown by the gener-ation and development of several mesoscale vorticitycenters to the south of V1 in the following hours (cf.Fig. 6 and Fig. 2). During that period, V1 weakenedgradually, corresponding to the observed weakening ofthe western part of the system. Among these newly-generated vorticity centers, one is named as V2 and be-lieved to be the representation of the mesoscale cloudmass causing the rain in Shanghai before 2000 LST5 August 2001. It can be seen that V2 also moved

NO. 2 YU ET AL. 361

Fig. 4. Radar reflectivity (dBZ) of Gemtronic 360AC Doppler radar (1.0◦ elevation angle)from 1800–2100 LST 5 August 2001. Areas of radar reflectivity greater than 15 dBZ areshaded. The location of radar, Hongqiao, is indicated by the black dot. “Bay” marked inthe first figure represents Hangzhou Bay.

(a) (b)

Fig. 5. Total rainfall from 2000 LST 5 August 2001 to 0200 LST 6 August 2001 (units: mm)from (a) the observation, and (b) the simulation.

across Shanghai and merged with another mesoscalesystem, V3, at 2100 LST 5 August 2001 in the east ofShanghai, quite similar to the TBB features.

Figure 7 shows the simulated radar reflectivity at850 hPa for 1800–2100 LST 5 August 2001, and it canbe seen that at 1800 LST 5 August 2001 there weretwo high reflectivity areas as the observation, but theircenters were in the southwest of Shanghai and in Zhe-jiang, respectively (Fig. 4). While the southern onewas very close to the observed, the simulated northernone was more southern than the observed, with theirlocation distance apart being around 50 km. At 1900LST 5 August 2001, the two high reflectivity areasapproached much closer to each other, as in the ob-servation, but the simulated southern one seemed tomove northward too slowly. Then, at 2000 LST 5 Au-gust 2001, they began to merge over Hangzhou Bayin the simulation, and, at 2100 LST 5 August 2001,the merged one moved to Shanghai at last, which wasthe same as in the observation. Therefore, this vor-tex merging process was reproduced well by the sim-ulation, as caught by the observed radar reflectivity(see Fig. 4), and also consistent with the cloud map

analysis (Fig. 2), though there were small differencesfor the vortices’ locations. Furthermore, correspond-ing with the vortex merging process, the simulatedhourly rainfall began at 2000 LST 5 August 2001 inthe Shanghai area, as in the observed rainfall (cf. Fig.3b and Fig. 8).

From 2200 LST 5 August 2001 to 0200 LST 6 Au-gust 2001, V1 redeveloped to the west of Shanghai,representing well the observed redevelopment of thewestern part of the system in the simulation, but V1was located a little more westward than the observedand the simulated merged system over Shanghai weak-ened much more quickly than observed (Fig. 6). Thus,the simulated hourly rain center was further west to-wards Shanghai, and then the simulated hourly rain-fall in Shanghai from 2300 LST 5 August 2001 to 0200LST 6 August 2001 was too weak compared with theobserved rain (cf. Fig. 8 and Fig. 3b). Therefore, al-though the simulated six-hour rainfall had two centers(Fig. 5b), as in the observation (Fig. 5a), with a westcenter having a maximum rainfall of 130 mm and aneast one of 87 mm, they were lower than the observedat 140 mm and 180 mm. This relatively low simu-


Fig. 6. Simulated 850 hPa vorticity from 1300 LST 5 August 2001 to 0400 LST 6 August 2001(units: 1×10−5 s−1). The typhoon mark in the top figures denotes the center position of the TD.V1, V2 and V3 are the three mesoscale systems important for the heavy rain process in Shanghai.AH, JS, ZJ and SH are as in Fig. 1.

lated rainfall center amount in the east was mainlydue to the faster weakening of the mesoscale systemsover Shanghai in the simulation compared to the ob-servation.

Later, after 0200 LST 6 August 2001, there weretwo more vorticity clusters moving over Shanghai inthe simulation, and this was seemingly different fromthe observed continuous decaying of a unique cloudcluster (Fig. 2), which may have been caused by thedrifted model boundary conditions from the model ini-tial conditions. However, since this study is mainly fo-cused on the mesoscale vortex generation and mergingprocess, and the simulation before 0200 LST 6 Au-gust 2001 appears reliable enough based on the above

analyses, this does not adversely influence the analysisresults of the paper.

So, generally speaking, there were discrepancies be-tween the simulation and the observation. For exam-ple, the length of time the simulated merged mesoscalesystem lasted and intensified over Shanghai was aboutfour hours shorter than in the observation, and thuspredicted weaker rainfall for the central Shanghai areathan was observed. However, the most importantthing is that the simulation can represent well the ob-served evolution of mesoscale systems, including themesosale vortex generation and merging process on theTD background, and corresponding rain intensifica-tion and rain distribution patterns in Shanghai. Since

NO. 2 YU ET AL. 363

Fig. 7. Simulated 850 hPa radar reflectivity of the D02 run at 1800, 1900, 2000 and 2100LST 5 August 2001, respectively.

Fig. 8. Simulated hourly rainfall from 1700 LST 5 August 2001 to 0400 LST 6 August 2001 (units: mm).

this paper is mainly focused on the mesoscale vortexgeneration and merging process before 0200 LST 6 Au-gust 2001, which was reproduced well in the simulatedresults, then the simulated output before 0200 LST 6August 2001 is enough to carry out further analysesin follow-up work.

5. Generation process of the mesoscale sys-tems

The remnant cyclonic circulation of the post-landfall TD remained obvious in the mid troposphere

at 1000 LST 5 August 2001 (Fig. 9), with the max-imum PV (>1.5 PVU, PVU= 10−6 K m2 kg−1 s−1)located to the south and southeast of the circulationcenter. At this time, the magnitude of the 200–850hPa vertical environmental wind shear within a ra-dius of 180 km from the TD center was 3 m s−1, andpointing south (179◦) (Fig. 9). The PV maximumwas therefore located in the downshear left area, whichis consistent with the results of previous studies (e.g.Willoughby et al., 1984; Marks et al., 1992; Franklin etal., 1993; Gamache et al., 1997; Corbosiero and Moli-nari, 2002; Black et al., 2002). In the following sev-


Fig. 9. Simulated PV (greater than 1.5 PVU are shaded) and streamlines at 600 hPa.The circle represents a radius of 180 km from the TD center. The arrow is shown forthe environmental vertical wind shear direction at 1000 LST 5 August 2001.

eral hours, the direction of vertical wind shear changedonly a little around 180◦ (not shown). By 1600 LST 5August 2001 the TD weakened, with the closed cy-clonic circulation disappearing and V1 expanding sig-nificantly in scale with obvious cyclonic circulationsaround it (Fig. 9). Although the TD experienced ashort-term redevelopment process later at 1800 LST5 August 2001, it was finally replaced by V1 at 2000LST 5 August 2001.

The western and eastern side of the TD was de-fined as its left and right side when facing to the north.Both V2 and V3 were developed in the southeasternpart of the TD (see Fig. 6). Examination of water va-por conditions for the western and eastern parts of theTD at 1000 LST 5 August 2001 showed that the rela-tive humidity in the eastern part of the TD was muchhigher than that in the western part from 750 hPa tohigher levels (Fig. 10a). The divergence of the east-ern part was negative in the lower levels and positivein the higher levels (Fig. 10b), while it was opposite

in the western part, accompanied by air rising in theeast and sinking in the west (Fig. 10c). These differ-ences indicate that the eastern side of the TD was mostlikely of tropical mode, characterized by the enhancednorthward intrusion of warm, moist air which createda local environment favorable for a tropical mode ofredevelopment (McTaggart-Cowan et al., 2003).

In order to understand the dynamics in the vortexgeneration and replacement process, a PV budget iscarried out. The PV tendency equation can be writ-ten as:

∂P

∂t= −∇3 · (VhP − gQζa + g∇3θ×F + wkP ) , (1)

where P is PV (Hoskins et al., 1985) and defined as:

P = −gζa · ∇3θ = −g(fk +∇3 × V ) · ∇3θ , (2)

where k is the unit vector in the vertical direction, fthe Coriolis parameter, g the gravitational acceler-

NO. 2 YU ET AL. 365

Rh

Div

W

(a)

(b)

(c)

Pre

ssu

reh

Pa

()

Pre

ssu

reh

Pa

()

Pre

ssu

reh

Pa

()

Fig. 10. Comparison for the west side and east side ofthe TD at t= 2 h (valid 1000 LST 5 August 2001): (a)relative humidity (Rh, units: %); (b) divergence (Div,units: 1×10−5 m s−1); and (c) vertical velocity (w, units:1×10−3 m s−1).

ation, Q the diabatic heating rate (in K s−1), F is fric-tion, w is vertical velocity, θ is potential temperature,t is time, ζa is the 3D absolute vorticity vector, and

Vh = ui + vj ,

∇3 ( ) = i∂ ( )∂x

+ j∂ ( )∂y

+ k∂ ( )∂z

,

where u and v are the zonal and meridional winds,respectively, and i and j are the unit vectors in thezonal and meridional directions, respectively.

The left side of Eq. (1) represents the PV tendency(PVT) and the terms on the right side are divergenceof PV flux due to horizontal advection (VPV), diver-

gence of PV flux due to vertical advection (WPV), di-vergence of PV flux due to diabatic heating (QPV),and divergence of PV flux due to friction (FPV), re-spectively.

TD, V1, V2 and V3 were analyzed individually toinvestigate how different PV terms contributed to theirevolution. The stream line centers were used to definethe TD and V1 centers. The centers of V2 and V3were defined by considering both the maximum reflec-tivity and vorticity centers at each level. Each termin the PV equation was calculated in the Lagrangianframework and averaged within a radius of 180 km, 50km, 30 km, and 30 km respectively for the TD, V1,V2 and V3 (radii of 180 km and 50 km can include themain cyclonic circulations of the TD and V1 at 1000LST 5 August 2001. 30 km was the mean radius of V2and V3 positive vorticity of 20×10−5 s−1 at 850 hPa.).

It can be seen that the PVT term for the TD in thelow levels was negative from 1000–1200 LST 5 August2001, contributed to by both the VPV and QPV terms(Figs. 11a, b, c). From 1500 LST 5 August 2001 thePVT turned positive in the low and middle levels, cor-responding to the short-term re-intensification of theTD later on (Fig. 9). At 1800 LST 5 August 2001 thenegative PVT center moved to the middle and upperlevels from the low levels, with negative VPV being themain contributor. The QPV term also contributed tothe TD later weakening. After 1900 LST 5 August2001 the TD center was replaced by V1. The WPVand FPV terms had no obvious effects to PV tendencyof the TD (not shown).

For V1, it had a positive PVT center in the lowerand middle levels at 1200 LST 5 August 2001 (Figs.11d, e, f), with a positive VPV center in the mid-dle levels and a positive QPV center in the lower lev-els. Afterward, some small areas of negative PVT ap-peared at 1500, 1600 and 1700 LST 5 August 2001,corresponding to the weakening process of V1 duringthat period. Notably negative VPV was appearing inthe middle and upper levels. Since 1800 LST 5 Au-gust 2001, PVT of V1 was positive vertically but withits maximum in the middle and high levels. PositiveVPV and QPV had very important effects on the de-velopment of V1 at around 1900 LST 5 August 2001when V1 replaced the TD center. Thus, V1 developedgradually from the low levels to the upper levels, withthe contribution of both VPV and QPV. For V2 andV3, their development was very similar to that of V1(not shown).

The weakening process of the TD and developmentof the mesoscale systems V1, V2 and V3 can be il-lustrated by a schematic diagram (Figs. 12a, b, c),with TD weakening gradually from the low levels tothe upper levels resulting from negative horizontal PV


PVT

Pre

ssu

reh

Pa

()

TD

5AUG2001

5AUG2001

5AUG2001

(a) (b) (c)

VPV QPV

PVT

Pre

ssu

reh

Pa

()

V1

5AUG2001

6AUG2001

VPV QPV

5AUG2001

6AUG2001

5AUG2001

6AUG2001

(d) (e) (f) Fig. 11. PVT, VPV and QPV term variations with time for TD and V1. (a), (b), (c) are PVT,VPV and QPV terms for TD respectively, and (d), (e), (f) are the same but for V1. Each termis calculated by averaging each term in a radius of 180 km and 50 km around the centers of TDand V1 (units: 1×10−6 PVU s−1). Dashed lines show the negative value areas, and black areas areundefined areas of TD and V1.

advection and diabatic heating effects, and the meso-vortices of V1, V2 and V3 developing also from the lowlevels to the upper levels on a favorable background asa result of positive PV horizontal advection and dia-batic heating effects.

6. Merging of V2 and V3 and the redevelop-ment of V1

As seen in Fig. 13a, V2 and V3 were very closeto each other at 1920 LST 5 August 2001. PositiveVPV was, for V2 and V3, at 850 hPa. Their vorticitywas greater than 40×10−5 s−1 and 100×10−5 s−1, re-spectively. Approximately one hour later, V2 and V3began to connect with each other (Fig. 13b). How-ever, the VPV of V3 turned negative and its vortic-ity decreased to 90×10−5 s−1, while V2 was develop-ing. It seems that V3 was ingested by V2. At t=2040LST 5 August 2001, the vorticity of V2 increased to100×10−5 s−1, but V3 merged gradually with V2 (Fig.13c).

To study the evolution of V2 and V3, the PV termswere analyzed from 1900–2100 LST 5 August 2001 in10-min intervals (Fig. 14). For V2, the PVT was pos-itive in the low levels but negative in the high levelsduring almost the whole merging process (Fig. 14a).

VPV was the main contributor to the PVT term ofV2 (Fig. 14b). Its distribution pattern was very simi-lar to PVT, while QPV was always positive in the lowand middle levels (Fig. 14c). For V3, the PVT waspositive at first (1910–1940 LST 5 August 2001) in thelow levels, but turned negative from around 2010 LST5 August 2001 (Fig. 14d), when V3 began to connectwith V2 (Fig. 13b). VPV was also the main contrib-utor to the PVT term of V3 (Fig. 14e). Thus, it canbe concluded that V3 was ingested by V2 because ofthe negative low-level horizontal PV advection for V3(positive for V2). This is also shown in the schematicdiagram (Fig. 12d).

After V2 and V3 merged at 2100 LST 5 August2001, V1 intensified substantially with its maximumPVT center at about 400 hPa (Fig. 11d). Positive PVhorizontal advection and the QPV term contributed tothe redevelopment of V1 (Figs. 11e, f). Besides, thebroader environmental conditions had been changedby now. As analyzed in Fig. 10a, the relative humid-ity for the TD cyclonic system was very asymmetricat 1000 LST 5 August 2001, and even at 1900 LST5 August 2001 the moisture distribution around thecyclone system was still very asymmetric (Fig. 15a).However, it began to become more symmetric so thata circle of high moisture over the V1 appeared at 2200

NO. 2 YU ET AL. 367

TDV1

+VPV

-VPV-QPV

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V1TD

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E

N

300

600

1000

(hpa)

Environmental flow Cyclonic flow

V1

V1 V2+V3+VPV+QPV

V1 V2+V3

Z

E

N

300

600

1000

(hpa)

+QPV+VPV V1 V2+V3

(e) (f)

+VPV

+VPV

Fig. 12. A 3D schematic diagram to show how the TD, V1, V2 and V3 evolved.Six stages are described, as above.

LST 5 August 2001 (Fig. 15b). Up until 0000 LST 6August 2001 V1 had developed into a circular shapedsystem, while the vortex over Shanghai moved north-ward without connections with any southern vortex(Fig. 6). The moisture circle around V1 began tobreak down at 0000 LST 6 August 2001 (figure notshown). QPV and VPV in the middle levels turnednegative, which would cause the PV of V1 to decrease(Fig. 11d). The vortices separated from V1, indicat-ing that the V1 reintensification process may not lastmuch longer. The V1 redevelopment and weakeningprocesses are also shown schematically in Figs. 12e

and 12f. After 0200 LST 6 August 2001 the systembegan to weaken, which caused the heavy rain processto decrease.

7. Summary and conclusions

In this paper, the mesoscale vortex generation andmerging process was found to be essential for a TD-related heavy rain event by analyzing the observationalsatellite TBB data and radar images. A numericalsimulation was conducted to reproduce the heavy rainprocess. The simulation represented reasonably the


a b c

Fig. 13. Diagnosis of the horizontal PV advection term of the PV equation (VPV, greater than 0 are withsolid contours; less than 0 are with dashed contours; units: 1×10−6 PVU s−1), and the vorticity (shaded,units: 1×10−5 s−1) at 850 hPa: (a), (b) and (c) are at t=11.4 h, 12.2 h, and 12.6 h (valid 1920, 2010, 2040LST 5 August 2001), respectively.

(a)

(d) (e) (f)

Pre

ssu

reh

Pa

()

V2

V3

Pre

ssu

reh

Pa

()

PVT

(b)

VPV

(c)

QPV

VPV QPVPVT Fig. 14. PVT, VPV and QPV term variations with a time interval of 10 min for V2 and V3 from 1900–2100 LST 5 August 2001 (time shown by x-axis). (a), (b) and (c) are the PVT, VPV and QPV terms forV2, respectively, and (d), (e) and (f) are the same but for V3. Each term is calculated by averaging eachterm in a diameter of 30 km around the centers of V2 and V3. Dashed lines show the negative value areas.Units: 1×10−6 PVU s−1.

observed generation and merging process of mesoscalevortices and corresponding rainfall in Shanghai.

With the model output, a 3D description of thenewly generated meso-vortices and their merging pro-cess is given by a schematic diagram (Fig. 12), withrelated dynamics analyzed by diagnosing the PV equa-tion. Six stages are described schematically, which in-clude the beginning stage of meso-vortices, the devel-oping stage of vortices V1, V2 and V3, the short-termstage of TD reintensification and V1 weakening, thestage of replacement of the TD by V1, the stage of V2and V3 merging and V1 reintensification, and the stageof V1 weakening. It is shown that the TD was weak-

ened from the lower levels to the upper levels, withnegative horizontal PV advection and diabatic heat-ing effects. The meso-vortices were developed gradu-ally, also from the lower levels to the upper levels un-der the effects of the positive horizontal PV advectionand diabatic heating in the downshear left quadrant ofthe TD. The newly-generated vortex, V1, replaced theTD. The merging process of V2 and V3 was controlledby the horizontal PV advection process and accom-panied by the redevelopment of the V1 system, whichbrought very heavy rain to Shanghai. Since the synop-tic pattern changed at last and V1 was separated fromthe other vortices, then it may not maintain its mois-

NO. 2 YU ET AL. 369

Fig. 15. Simulated 500 hPa wind vectors and relative humidity (greater than 70%is shaded): (a)t= 11 h; (b) t = 14 h (valid 1900 and 2200 LST 5 August 2001,respectively).

ture supply. In addition, mainly under the negativehorizontal PV advection and diabatic heating effects,the short intensification of V1 ceased and began toweaken.

Though the case in this study was a post-landfallTD and its redevelopment ceased ultimately, there wasdownshear-left local environment, increasingly sym-metric moisture distribution, and multiple vorticesformation and merging during the cyclone’s redevel-opment stage. In fact, these features are also in-cluded in TC genesis and development (Reasor et al.,2005; Molinary et al., 2006). Simpson et al. (1997)concluded that the development of, and interactionbetween, mesoscale vortices and associated convec-tive systems was an integral component of the gen-esis of TC Oliver. The downshear reformation of TCGabrielle (2001) prior to landfall was investigated byMolinary et al. (2006), who verified the downshear ef-fects on TC formation. Therefore, even though thecase studied in this paper is a post-landfall TD, andthe re-intensification of the vortex is short-lived, theinteraction between the post-landfall TD and othernewly-generated vortices is important to understand,which may aid in predictions of these events, especiallyof landfall storms with re-intensification, suddenly-increasing rain intensity and other disasters.

Acknowledgements. The authors wish to thank Ms.

CHEN Peiyan of the Shanghai Typhoon Institute for her

help handling the GMS-5 TBB data. This work is sup-

ported by the State 973 Program (2009CB421505). It is

also additionally supported by the National Natural Sci-

ence Foundation of China (Grant Nos. 40405012, 40830958

and 40705024), the Ministry of Science and Technology of

China (Grant No. 2005DIB3J104), and Shanghai Meteo-

rological Bureau (Grant Nos. 2009ST11, MS200821).

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