Journal of the Meteorological Society of Japan, Vol. 84, No. 4, pp. 783--803, 2006

783

Convective Cloud Systems over the Tibetan Plateau and Their Impact
on Meso-Scale Disturbances in the Meiyu/Baiu Frontal Zone—A Case Study in 1998—
Tetsuzo YASUNARI and Takeshi MIWA1
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan(Manuscript received 3 June 2005, in final form 12 May 2006)

Abstract
The relationship between convective activity over the Tibetan Plateau (TP) and meso-scale cloud systems in the Baiu/Meiyu frontal zone, are examined for the 1998 summer monsoon season by using
GEWEX Asian Monsoon Experiment (GAME) reanalysis data and Geostationary Meteorological Satellite (GMS)-IR Tbb data.
Diurnal cycle of the plateau-scale heat low associated with convective clouds is dominated, forming aplateau-scale convergence line. Moisture transport from south of TP is essential for developing convective clouds, and the convergence line. Occasionally, the convergence line extends to the eastern edge of
the TP, which triggers a meso-scale cyclonic vortex over the head of the Yangtze River basin through theplateau edge cyclogenesis (PEC). This vortex induces a strong low level jet with moisture inflow to theeast of it, which facilitates further development of the vortex to a meso-a scale cloud system embeddedin the Baiu/Meiyu front (BMF) over China. The occurrence of PEC is likely to be modulated by an interaction between mid-latitude westerly waves, with a quasi-biweekly time scale, passing over the TP, anda shorter-period oscillation of the monsoon trough over the Indian sub-continent, with around a 4–7 day
period.

1.

Introduction

The Baiu/Meiyu front (BMF) is one of themost remarkable subtropical fronts in theworld, and produces a large amount of precipitation over East Asia covering China throughJapan during early summer. Its strong (orweak) activity sometimes causes floods (ordroughts) in this region. Therefore, it is important to understand genesis and evolution processes of the rainfall systems, embedded in
BMF.
Corresponding author: Tetsuzo Yasunari, Hydrospheric Atmospheric Research Center, Nagoya
University, Furo-cho, Chikusa-ku, Nagoya 4648601, Japan.
E-mail: yasunari@hyarc.nagoya-u.ac.jp
1 Present affiliation: Japan Meteorological Agency( 2006, Meteorological Society of Japan

Many previous studies for the several decades related to the BMF activities have been
made from various viewpoints summarized asfollows;(1) BMF corresponds to a quasi-stationary
cloud zone located along the northern periphery of the warm and humid maritime
tropical air mass, and is characterized by
the large meridional moisture gradients(e.g., Ninomiya and Muraki 1986; Ninomiya
and Murakami 1987).
(2) BMF is associated with a strong southwesterly at 700–900 hPa called the low level jet(LLJ), which transports a large amount ofmoisture to the BMF, and generates not
only moisture gradients but also a thick,moist, neutral layer (e.g., Ninomiya andAkiyama 1974; Akiyama 1975).

784

Journal of the Meteorological Society of Japan(3) The structure of the BMF changes from region to region, as well as in the seasonal
march, e.g., the BMF over China has little
temperature gradient but large moist gradients, whereas over Japan, the temperature gradient is more dominate (Kato 1985).
(4) the BMF embeds many medium to mesoscale disturbances, which move eastward
along the front as developing and cause
heavy rains. The disturbances tend to develop near Japan, where the temperature
gradient is large (e.g., Iwasaki and Takeda
1993; Ninomiya 2000; Shibagaki et al. 2000).
In understanding genesis and development of
the BMF rainfall systems over China, we cannot ignore the orographic effects of the Tibetan
Plateau (TP). Kato (1985) suggested that the
rapid heating over the plateau in early summer
results in weakening and disappearance of theupper westerly jet, flowing to the south of theplateau. This heating causes separation of
the upper frontal zone from the lower frontal
zone over East Asia and promotes the changeof frontal characteristics. Through the numerical experiments, Sun and Lorenzo (1984)
showed that the orography of the plateau
played a key role to maintain the southwesterly
LLJ in the southeastern side of the plateau. Ninomiya and Muraki (1986) suggested that the
TP partially contributes to generate a moisturegradient in the BMF through restraining thesouthern humid air mass from moving northby an orographic effect. Murakami and Huang(1984) studied cyclogenesis and developing processes of disturbances over the area from the
TP to East Asia during early summer in 1979.
They indicated that some of the vortices over
the TP, which are frequently generated in theboundary layer over the plateau, move eastward as developing, and cause rainfall overSouth China.
During the Baiu/Meiyu season in 1998,
severely-heavy rain and flood events occurredin the Yangtze River basin in late June throughJuly. Many studies indicated that heavy rains
and floods in China during the Baiu/Meiyu season were closely associated with the meso-scaledisturbances generated over the Sichuan basin,
which is called the southwest (SW) vortex (Taoand Ding 1981; Ding 1992). The disturbances
are thought to be generated, and develop by la-Vol. 84, No. 4

tent heating, through convective activities originating over the TP, and moving eastward toward the Plain area of China (Wang 1987; Kuo
et al. 1986; Wang et al. 1993; Cheng et al. 1998).
Some studies focused on the heavy floods over
Yangtze River basin in 1998, pointing out thatthe convective activity over the TP might havesome relationship to the heavy rain over the
Yangtze River basin. For instance, Li and Chen
(2001) showed a negative correlation between
the convective activity over the BMF and thatover the TP for rainy season in 1998. However,few studies have been devoted to understanding the full evolution processes, including therole of diurnal cycle of convective activity over
the TP, development of the meso-scale systems,
and their consequences in rain systems in the
BMF.
Fortunately, the period from April to Octoberin 1998 corresponded to the intensive observing
period (IOP) of GAME (GEWEX Asia Monsoon
Experiments). One of the objectives of GAMEIOP was to obtain better data for full understanding of energy and water cycle processes
in the Asian monsoon system, including the diurnal cycle, water vapor transport, meso-scaledisturbances and roles of land-atmosphere interactions. Enhanced radiosonde observations
based on four times (00, 06, 12, 18Z) per day
were made at more than 100 sites in the Asianmonsoon region, including the TP, the Yangtze
River basin, Thailand and so on. Based on theseobservations, the objectively analyzed GAME
reanalysis data was produced with better quality and high time-space resolution. As part of
GAME-IOP, coordinated intensive observations
were made over the TP, under the cooperation
between GAME-Tibet, and the second Chinese
Tibetan Plateau scientific experiment (TIPEX).
Some of the meteorological results during the
GAME-IOP over the TP were already reported
elsewhere (e.g., Uyeda and Yamada 2001; Xu
et al. 2002; Wang et al. 2003; Fujinami et al.
2005; Yamada and Uyeda 2006).
The purpose of this study are to scrutinize
the relationship between convective activity
over the TP, and that over the BMF during the
Baiu/Meiyu season in 1998, and solve how convective activity over the TP influenced theheavy floods over the Yangtze River Basin byusing GAME reanalysis data, and GMS-IR Tbb
data.

August 2006

T. YASUNARI and T. MIWA

785

Fig. 1. Study area.

2.

Data and method of analysis

We used GAME reanalysis data Version 1.1,
to analyze the atmospheric field over the study
area, with spatial resolution of 0.5 and temporal resolution of 6 hours for May to September
in 1998, in the Asia area. The data assimilation
was made by the Japan Meteorological Agency(JMA) global operational model, using the enhanced radiosonde observation data during the
GAME-IOP, as well as global operational observation data. The details of this data are described in Yatagai et al. (2000), and Ueda et al.
(2003). We also used the Geostationary Meteorological Satellite (GMS)-IR Tbb (Infrared
black-body equivalent temperature) data, to analyze the convective activity over Asian monsoon region from June to July in 1998, in particular East Asia from the TP to Japan. The
study area including the orography of the TP isshown in Fig. 1. Tbb represents cloud top temperature calculated from the intensity of the infrared radiation from earth, when an area iscovered by clouds. Lower Tbb values correspondto higher cloud top height, and means intensive
convective activity. GMS-IR Tbb data, with 1.0horizontal resolution and with 3 hourly time
resolution, are used in this study.
3.

The convective activity over East Asia
during the Baiu period in 1998

To understand the relation between the convective activity over the TP, and that over the
BMF zone, the longitude-time section of Tbb
along 30 N from June to July in 1998 is produced, as shown in Fig. 2. We find two major
regions with active convection as indicatedwith low Tbb values in Fig. 2. One is over the
TP to the west of 105 E, the other region isover the plain area in China through the East
China Sea. The convective activity over the TPshows a prominent diurnal change, in particular since June 13, which corresponds to theonset of rainy season in the eastern part of the
TP (Li and Chen 2001). The convectively active
region with diurnal cycle first appeared overthe eastern part of the TP at the beginning ofJune, expanded westward after June 13, and786

Journal of the Meteorological Society of Japan

Vol. 84, No. 4

Fig. 2. Longitude-time section of Tbb along 30 N during June to July in 1998. The area lower than
260 K on Tbb is shaded to show the cloud activity. In this latitude, the area of 80–100 E corresponds to TP.

reached to the western part (80 E) by the endof the month. The westward expansion of active
convection was closely related with the westward movement of summer monsoon onset over
the TP, as shown by Li and Chen (2001). Another characteristic of the convective activity
over the TP, was its quasi-periodic variationsof about two weeks as is remarkably seen in
the amplitude modulation of diurnal change ofTbb with maxima in 1–10, 16–20, and the endof July. This quasi periodic variation of convective activity was known as a sub-monthly-scale
intraseasonal variation, notably seen in the
Asia monsoon area (e.g., Nitta 1983; Fujinami
and Yasunari 2001, 2004). On the other hand,
the major region of convective activity over theplain area in China, and the East China Sea,
moved westward toward the TP in the latter
half of June. Afterward, it disappeared in thefirst half of July, but clearly appeared again
July 20. This area of active convection corresponds to the mean position of the BMF. The
convective activity over the BMF also showed
weak diurnal change, though it is not so clear
in comparison with that over the TP. This diurnal change is, though, likely to have a close relation with the generation of meso-a-scale cloudclusters, affected by the diurnal change of instability, closely associated with solar radiation(Kato et al. 1995).
These two regions of active convection over
the TP and the MBF are separated clearly, one

August 2006

T. YASUNARI and T. MIWA

to the east, and the other to the west of around105 E, where the eastern edge of the TP is located. However, the area of active convection
over the TP occasionally moves eastward to
the East China Sea through the plains in China
for June 20–23, 26–28 and July 18–21. Interestingly, the convective cloud systems over the
TP seem to propagate into the BMF zone,
when the convective activities with diurnal
cycles over the TP are enhanced. In this paper,
we examine two notable cases, June 20–23 andJuly 18–21, to diagnose the atmospheric conditions related to development of the cloud systems in the MBF. The modulation mechanismof convective activities over the TP will also be
discussed in section 7.
4.

The atmospheric condition over theTP

Large diurnal cycle is one of the most remarkable features of the atmosphere over the
TP (Yanai and Li 1994; Kuwagata et al. 2001;
Fujinami et al. 2005). Before we examine thedevelopment of convective systems over the TP
and BFZ, we need to understand the mean diurnal cycle over the TP, during the summer
monsoon period. First, the diurnal changes of
convective activity, temperature and moisture
flux are examined for June to August in 1998.
Two typical cases are also investigated as casestudies.
4.1

The diurnal change over the TP during
summer in 1998
Figure 3 shows mean Tbb, temperature andmoisture transport fields at 500 hPa at eachtime (00, 06, 12, 18Z) of day averaged for theperiod from June through August. These Universal Time (UT) of the day roughly correspond
to 6, 12, 18 and 24 local time (90 E) over the TP(90 E). The Tbb field shows the convective
activity of a diurnal cycle is most active overthe southern part of the TP around 30 N, witha maximum in the evening (12Z) and minimum
around noon of local time (06Z). As shown in
Fig. 2, the diurnal cycle of Tbb over the TP
was intensified since the middle of June. In
contrast, that to the south of the TP, centeredin the Assam region (around 27 N, 90 E)
showed nearly in the opposite phase to thatover the TP, with a maximum in the morning,
and a minimum in the evening. This feature of

Fig. 3. The diurnal variation of Tbb,
500 hPa temperature and 500 hPa
moisture flux around TP averaged from
June to August in 1998. The shading
area shows Tbb lower than 260 K. Contour interval for temperature is 2 K.
Unit of moisture flux is g g 1 m s 1 . Topographic contour for 3,000 m is also
shown (thick solid line).

787

788

Journal of the Meteorological Society of Japan

convection, and rainfall in the Assam region,
was also noticed by the previous studies (Prasad 1974; Murakami 1983).
The temperature field at 500 hPa also showeda prominent diurnal cycle over the central TP.
The lowest temperature appeared at 00Z, witha minimum value of about 272 K, and the highest at 12Z with a maximum value of about
276 K. The temperature outside of the TP isnearly stable through the day. The large amplitude of the diurnal cycle over the TP is mostly
due to sensible heating from the elevated surface, and latent heating through convection
over the TP.
In the moisture transport field, a strong
southwesterly inflow was remarkably seen from
Assam/South China to the TP through the day,but the maximum tends to appear in the evening (12Z). A remarkable feature, is the appearance of two convergence zones over thesouthern (30 N), and northern (34 N) part of
the TP at around 12Z. The southern one corresponds to the active convective zone, indicatedby low Tbb. These two convergence zones are
generated along the two mountain ranges (i.e.,
the Kunlung mountain range, and the great &trans-Himalaya mountain range) in the TP orography, which suggests that the convergencezones are associated with the mountain andvalley wind circulation system. A moisture divergence area is also noted along the northern
periphery of the TP, which may suggest a moisture source from the northern foothills (or theoasis zone) of the Kunglung mountain range.
In addition, the two convergence zones seem tomerge in the central eastern part of the TP(32 N/95 E) at 12Z. At 18Z, the two convergence zones are reformed to a plateau-scale cyclonic circulation over the TP. These processes
are described in more detail in Fujinami et al.
(2005). The diurnal change of the moisture flux
field is similar to that of the wind field, shownby Yanai and Li (1994).
4.2

Convective activity and moisture transport
over the TP for 20–23 June and 18–21
July
In reference to Fig. 2, we have chosen two remarkable cases, when convective clouds systemgenerated over the TP moved eastward to the
China plain; one case is June 20–23, and theother July 18–21. Figures 4 and 5 show Tbb,

Vol. 84, No. 4

and 500 hPa moisture flux at 00z, 12z for June
20–23 and July 18–21, respectively. In June
20, the convective activity over the TP was
active along the southern edge at 12Z, and two
moisture convergence zones were generated
over the TP at 12z, as shown in Fig. 3. On June
21, the moisture inflow from south of the TP
was very active, and a cyclonic circulation wasformed over the north TP (35 N/90 E). Thismoisture flux field was totally different fromthe average diurnal change shown in Fig. 3.
At 12Z, two convergence zones developed andmerged together, with more westward expansion than the mean case (Fig. 3). In addition,
the eastern edge of the convergence zone
reached to the eastern edge of the TP, with alength of 2000 km. The strong convergencezone was located along 33 N, which was about
3 degree to the north of the mean position inlatitude. The area of strong convection expanded over most parts of the eastern TP. The
active moisture transport area spread over thesouthern side of the convergence zone, andreached the eastern edge of the TP. At 00z on
June 22, the convergence line stretching east
to west disappeared, but a new convergence extending south to north, and a cyclonic circulation developed over the central TP. The moisture inflow from the south spread over thesoutheastern part of the cyclonic circulation.
The active moisture transport area also spreadeastward, and its edge reached around 110 E.
The convective activity became active to theeast of the new convergence zone. At 12z, thetwo convergence zones appeared to be similar
to those indicated in Fig. 3. The active moisturetransport spread eastward, and the convective
area moved eastward, respectively. On June
23, the area of active convection, and strong
moisture flux, showed an almost similar pattern to the mean diurnal change. It should be
noted, however, that a cyclonic circulation was
generated more southeastward over the TP(28 N/108 E).
Figure 5 shows the moisture flux and Tbb
distribution during July 18 to 21. On July 18,
the moisture inflow from the south was active
over the eastern TP, and the convective activity
was active over the central TP, even at 00z. At
12z, two convergence zones were formed in thediurnal change, and the convective activity wasstronger over most parts of the TP. On July 19,

August 2006

T. YASUNARI and T. MIWA

789

Fig. 4. Tbb and 500 hPa moisture flux over TP at 00Z and 12Z from 20 to 23 June. Shaded areashows Tbb less than 260 K same as Fig. 3. Unit of moisture flux is g g 1 m s 1 . Topographic contourfor 3,000 m is shown (thick solid line).

the convergence zone extended eastward to theeastern edge of the TP, and the convective cloudsystem also moved eastward without decay
at 00Z. At 12Z, two convergence zones wereformed again, with the eastern edge extending
more eastward. The convective area spreadover the entire TP. On July 20, the moist strong
westerly area, convergence zone and active convective area expanded to the east of the TP. At
12Z, a very long convergence zone, with meandering was formed, with a trough over central
TP, and a ridge over the southeast TP, as790

Journal of the Meteorological Society of Japan

Vol. 84, No. 4

Fig. 5. Same as Fig. 4 but from 18 to 21 July.

shown in the moisture flux fields. On July 21,
the trough developed and moved further eastward. Wang et al. (2003) also examined thiscase of July, suggesting the importance of moisture convergence in the boundary layer for theformation of meso-scale systems.

5.

Cyclogenesis at the eastern edge of TP

In section 4, we have shown the two cases
when convective cloud systems generated over
the TP moved eastward to the China plain. In
these cases, the convergence zone with convec-

August 2006

T. YASUNARI and T. MIWA

tive clouds were formed over the TP, andreached to the eastern edge of the TP, where ameso-scale system, with cyclonic circulation ina moisture flux field was generated.
This section describes in more detail how thegeneration of the meso-scale convective systemoccurred. Figure 6 shows the longitude verticalsection of relative vorticity, and meridional
wind along 30 N for June 21–22, when the diurnal convective system moved from the TP, tothe plain area of China. Since the convective
system over the TP tends to move or expand
eastward toward the eastern edge of the TP atlate evening to mid-night to form relative vorticity tubes of 200–300 km scale over the plateau edge, we plot the diagrams for 18Z and
6Z, rather than 12Z and 0Z, to show moreclearly the extreme phases of evolution. A vertically extended, positive vorticity was found in
the lower to middle troposphere around 125 E,
which corresponds to a meso (b to a scale) cyclonic disturbance in the MBF zone. Additionally, a positive vorticity tube in the lower troposphere was generated, and developed over theeastern edge of the plateau, around 100 E on
June 21. At the same time, a LLJ, with a strong
southerly core with more than 9m/s, appeared
at 600 hPa around 105 E. On June 22, the vorticity tube moved eastward and reached around105 E, and developed into another tall vorticity
tube, extended from the surface to 300 hPa. In
association with this vortex, the strong southerly also expanded in the lower troposphere.
At 18z on June 22, the northerly wind area appeared in the mid troposphere to the west of
the strong southerly. These features indicatethat a cyclonic disturbance was generated tothe east of the TP.
Figure 7 shows the longitude vertical section
as Fig. 6, except for July 19–20 along 32 N. On
July 19, we could see a positive vorticity tube,
extended from 600 hPa to 400 hPa at the eastern edge of the TP at 06Z. The vorticity tube
moved eastward and left from the TP, developing from the surface to 300 hPa through 18Z on
July 20. Nearly simultaneously, a strong southerly core was generated at 600 hPa at the eastern side of the TP around 108 E, as is seen at
18Z on July 19 through 18Z on July 20. The
southerly core developed from the surface to
300 hPa, coupled with the positive vorticity
tube. The northerly wind was also found be-

791

tween the TP and the southerly core, whichreached from the surface to 400 hPa, implying
development of the cyclonic disturbance in the
middle atmosphere. These characteristics ofthe cyclogenesis found near the eastern edge of
the TP, were very similar to those found for thecase on June 21–22.
6.

Cyclone development over East Asia

In section 4 and 5, we described that the cyclogenesis occurred over the eastern foot of the
TP (and the Sichuan basin), when the convergence zone with convective cloud systems over
the TP developed, and expanded eastward
across the eastern edge of the TP. In this section, we examine the processes; how these cyclonic disturbances formed over the Sichuanbasin, and further developed as meso-a scale
systems, in the BMF zone.
Figure 8 shows the daily mean Tbb, geopotential height and vorticity at 850 hPa, andmoisture flux integrated from surface to
300 hPa, during June 20–27. In this period,
the BMF in East Asia was located from south
China northeastward to the Japan Islands,
and the moist southwest monsoon flow wasstrong along the southern periphery of the
BMF. The subtropical anticyclone (Pacific high)
was also well established, supplying anothermoisture source from the Philippine Sea area
toward the BMF. To the north of the BMF, ageneral decrease of height at 850 hPa towards
the north, was recognized in a relatively dry
air mass. It should be noted that a cyclonic circulation or disturbance with large-scale convection, was located over the Ganges Plainnear the foothill of the Himalayas, which was
responsible for enhancing a moist southerly
flow toward the TP. This enhanced moist flow
may have formed a thick moist layer at thenear surface of the TP (Wang et al. 2003), andenhanced moist convective instability there.
The eastward movement of the convective cloudsystem over the TP, and cyclogenesis at theeastern edge of the TP occurred on June 21–22.
Under this synoptic situation, a meso-a-scale
circulation system was formed at the easternedge of the TP on June 21–22, as described in
section 4 and 5. This meso-system enhanced
moist southwesterly flow to its southeast flank,and dry northern air to its northwest flank (asseen in Fig. 8(h)) on June 23. This enabled the792

Journal of the Meteorological Society of Japan

Vol. 84, No. 4

Fig. 6. Vertical cross section of vorticity (left) and meridional (right) wind along 30 N at 06Z and 18Z
during 21 to 22 June. Shaded area of vorticity and meridional wind show positive value and southerly more than 6 m/s, respectively.

August 2006

T. YASUNARI and T. MIWA

Fig. 7. Same as Fig. 5, expect that the latitude is 32 N and the period is from 19 to 20 July.

793

794

Journal of the Meteorological Society of Japan

Vol. 84, No. 4

Fig. 8. Tbb, 850 hPa geopotential height (left), moisture transport and 850 hPa vorticity distribution(right) over Asia monsoon region during 20 to 27 June. Shading shows Tbb with lower than 260 K.
Unit of moisture flux is g kg 1 m s 1 . Topographic contour for 1,500 m is shown (thick solid line).

August 2006

T. YASUNARI and T. MIWA

Fig. 8. (cont.)

795

796

Journal of the Meteorological Society of Japansystem to have abruptly developed to a medium or near-synoptic scale system with morethan 1000 km horizontal scale, presumably
through intensified moist convective instability.
On June 24–25, this system further developed,by merging with the other system, which had
already been located over the Yangtze-river basin (at 125 E as shown in Fig. 6). These features on June 24–25 strongly suggest that theoverall role of the cyclonic circulation genesis,
at the eastern edge of the TP, is to activate the
BMF itself by enhancing the moisture gradientacross the BMF. On June 26–27 the developed
system over the lower reach of the Yangtzeriver basin, persisted as seen in Tbb and moisture flux fields, but a major cyclonic center (asseen in the vorticity field) moved eastward to
Korea, and the Japan island area.
Figure 9 shows the same diagrams as Fig. 8,
but for the case of July 19 to 27. A characteristic feature in this period was that there was noapparent BMF at the beginning stage (Jul 19)
though the monsoon southwesterly was flowing
into central China. Another distinct difference
was the existence of a synoptic-scale cyclonic
circulation system in northern China, and the
moist flow was converging both from the southwest and the northwest, to the Yangtze river
basin in central China. A meso-a scale system
was located over the East China Sea, associated with the synoptic system to the north.
On July 19 to 20, a cyclonic vortex at theeastern edge of the TP appeared. Over Bangladesh, northern India and near the foothills of
the Himalayas, a convective system was located, which seems to enhance moisture inflow
to the TP, as was similar to the case in June.
Simultaneously, another meso-a-scale vortex
was formed to the east, presumably by the enhanced moist southwesterly flow, associatedwith the vortex genesis at the edge of the TP.
On July 21 through 23, these vortices further
developed to form a zonaly-elongated BMF,
from the foot of the TP, to the East China Sea.
It should be noted that the synoptic system tothe north already passed through to further
east, and no more apparent northwesterly intrusion was seen to the BMF zone in thisstage. In other words, the BMF convergencewith meso-scale systems were newly formed tothe south of the convergence, as seen on July
19 to 20. On July 23–24, a new synoptic-scaleVol. 84, No. 4

disturbance was generated near the Yangtze
River mouth, and further developed over the
East China Sea on July 25–26.
7.

Discussion

We have diagnosed the atmospheric circulation changes, for the two typical cases when
the cloud systems originally generated over the
TP developed at the eastern edge of the TP, andfurther developed to the meso-scale convective
systems in the BMF zone in east Asia. Throughthe analyses, we have found some common
characteristics as follows.
The convective activity with moisture flux
convergence over the TP, showed clear diurnal
cycles to form the convergence zone over central part of the plateau, basically caused bystrong atmospheric diabatic heating (Li andYanai 1996 etc.). Occasionally, the convective
activity is enhanced and as a result, the convergence zone over the central plateau, developed
to reach the eastern edge of the TP. When theconvergence zone over the TP extends further
eastward, to reach the eastern edge of TP, a
meso-(b to a)scale vortex tube is generated
with a vertical extension from the surface up
to 300 hPa, as has clearly been shown in Fig. 4
and Fig. 5. We tentatively call this phenomenon the Plateau Edge Cyclogenesis (PEC).
Though we need further study on the dynamics
of the PEC, we can speculate that the verticalstretching of the cyclonic vortex, originallyformed over the TP associated with the convergence zone, is a fundamental mechanism for
the PEC, due to down-slope movement of thevortex at the edge of the Plateau. In addition,
the intensified vorticity associated with thisstretching of the vortex column is associatedwith the enhanced moist southern flow, to theeast of the vortex, which in turn strengthens
the cyclonic vortex, through enhanced latentheating to develop a meso-scale convective system. Thus, the PEC is an essential mechanism
for the meso-scale cyclogenesis over the Sichuan basin area. These meso-scale systems,
triggered by PEC, could be included to thosenamed as Southwest (SW) vortices by Chinese
meteorologists, which play an important rolein causing heavy rainfall over the BMF zone
in China. Some previous studies also suggested the importance of dynamical, as well as
thermo-dynamical, effect of the eastern edge ofAugust 2006

T. YASUNARI and T. MIWA

Fig. 9. Same as Fig. 8, but from 18 to 25 July.

797

798

Journal of the Meteorological Society of Japan

Fig. 9. (cont.)

Vol. 84, No. 4

August 2006

T. YASUNARI and T. MIWA

799

Fig. 10. Time series of the area averaged 500 hPa geopotential height over TP (30–35 N/80–100 E)
and meridional moisture transport averaged 90–100 E along 30 N. The area with 90–100 E along30 N corresponds to the southeastern part of TP. Three major events of PEC are shown with solidlines and arrows in the above. Unit of geopotential height (left axis) is geopotential meter. Unit ofmoisture flux (right axis) is g g 1 m s 1 .

the TP for the SW vortices (Wang and Orlanski
1987; Wang et al. 1993). In the concept of PEC,
however, we have noted the enhancement ofplateau-scale convective systems, and/or convergence zone over the TP, appearing in strong
diurnal cycle and could interact dynamicallywith the Plateau edge slope orography, toabruptly form leeside meso-scale cyclogenesis.
Enhanced moisture supply to the east of theTP, accompanied by this cyclogenesis is also an
important process for further development toconvective meso system in the BMF zone.
What triggers PEC to form meso-scale systems to the east of the TP? We have noticed
that the PEC in cases of June–July 1998, occurred associated with enhanced southerly
moisture inflow from the Indian plain. Then,

what caused strong pulses of moisture inflow
over the TP. To examine this, the time seriesof the meridional moisture transport, averaged
90–100 E along 30 N and 500 hPa geopotential
height, averaged over the TP (30–35 N/80–100 E), are produced as shown in Fig. 10. Three
major events of PEC are seen during the period(middle of June, early July and middle of July),
though we described only two cases: middle ofJune and middle of July. In the case of early
July, the meso-scale system developed throughPEC moved further northward to Northern
China (Shinoda et al. 2005), that could not appear in the longitude-time section in Fig. 2.
From this figure, we can find that when theheight over the TP is low, the southerly moisture inflow is active. The two cases analyzed

800

Journal of the Meteorological Society of Japan

circumstantially here, corresponded to the low
geopotential height and active moisture inflow
from the south. Furthermore, we can noticevariations of two time-scales; one 4–6 days period, and another quasi-biweekly period, in thetime series of geopotential height. The quasibiweekly variation can be seen in the longitude
time section of Tbb (Fig. 2). On the other hand,the 4–6 days variation can also be seen in
the time series of moisture inflow (Fig. 10). So,
the formation of the strong convergence zone
over the TP, is thought to be associated closelywith the 4–6 days variation in mid-latitude
westerly, or monsoon low, formed over a monsoon trough along the Ganges River (Murakami1976). The invasion of westerly disturbanceinto the TP, may be responsible for triggering
the moisture inflow, the formation of enhanced,
elongated convergence zones, with strong convective activity (Fujinami and Yasunari 2001,
2004).
To clarify the relationship among PEC, theintraseasonal westerly trough movement, andmonsoon low with 4–6 days variation, we use aband-pass filter to pick up 4–6 days variation,and biweekly (10–20 days) variation on geopotential height. The predominance of the biweekly variation in convective activity, andgeopotential field over the TP in the 1998 summer, is already reported by Fujinami and Yasunari (2004). Figure 11 shows the time series of(a) filtered geopotential height over the TP(30–35 N/80–100 E), and (b) Indian subcontinent (20–25 N/70–90 E), with 4–6 days filteredmoisture inflow (30 N/90–100 E). From thisfigure, we notice that the major three PEC
events (i.e., June 21–23, July 6–8, July 19–21),
as shown with solid lines and arrows, occurred
during the negative phases of 10–20 days, filtered geopotential height over the TP, overlapped with the negative phases of 4–7 days,
filtered height fluctuation. These negative
phases in 4–7 day filtered height, are well consistent with the positive phases of the 4–7 day
filtered moisture transport over the TP. Thatis, the PEC is suggested to be closely associatedwith the interaction between the two different
time-scale variations. Furthermore, negative
phases of the 4–7 days geopotential heightvariation over the TP, and those over the monsoon trough region of the Indian subcontinent(Fig. 11[b]), appear nearly in the same timing.

Vol. 84, No. 4

This may imply that the enhancement of southerly moisture inflow over the TP occurs whenthe mid-latitude westerly trough and the monsoon trough is synchronized under the largerscale, intraseasonal variation of waves over the
TP.
In this study, we have scrutinized the twocases in 1998, when the PEC triggered thedevelopment of meso-scale systems; one embedded in the BMF in June, and the otherreforming the BMF in July. These cases, in addition to another case in the beginning of July,corresponded to the abnormally heavy rainfall,
and flood events in the plain area in China. It
is interesting to note that, in the two cases in
middle June and middle July, another cyclogenesis occurred over the lower reach of the
Yangtze river basin, through the East China
Sea, which seemingly enhanced the heavy rainfall, and flood condition there.
8.

Conclusion

In this study, we tried to solve the relation
among convective activity over the TP, cyclogenesis at the edge of the TP, called the Plateau Edge Cyclogenesis (PEC), and the development of the meso-scale cloud systems overthe BMF zone. These meso-scale systems caused
the abnormally heavy rainfall and floods havingin the Yangtze River basin, during the Baiu
season in 1998.
The heavy floods occurred over the basin in
the middle to late June and July. During these
periods, we found two significant events, when
the cloud systems generated over the TP moved
eastward, and developed into meso-scale cloudsystems over the BMF, and intensified the convective activity over south China.
The development of cloud systems and convergence zones over the TP, occurred by the enhanced southerly moisture inflow. Though theconvergence zone is formed over the TP, every
day in the summer monsoon period, it occasionally extended to the eastern edge of the TP, due
to increase of moisture inflow. The cyclonic vortex, with convective cloud systems, were generated along the convergence zone, and expanded
to the eastern edge of the TP.
When the cloud systems reached over theeastern edge of the TP, the PEC occurred byvertical stretching of the vortex, and latentheating due to convective activity. In generat-August 2006

T. YASUNARI and T. MIWA

801

Fig. 11. Time series of 4–7 days (dot line) and 10–20 days (solid line) filtered area average geopotential height over (a) TP and (b) Indian subcontinent. The areas for averaging 500 hPa geopotential
height and for 700 hPa geopotential height are 30–35 N/80–100 E and 20–25 N/70–90 E, respectively. Dashed lines show 4–7 day filtered meridional moisture transport averaged for 90–100 E
along 30 N. Three major events of PEC are shown with solid lines and arrows in the above. Unitof geopotential height (left axis) is geopotential meter. Unit of moisture flux (right axis) is
g g 1m s 1.

ing a disturbance, a strong southerly wind core
is formed at the eastern edge of the TP, whichis responsible to strong convection. At the sametime, an area of northerly wind is formed along

the eastern slope of the TP. These major PEC
events are likely to be related to the movementof large-scale westerly troughs onto the TP
with about a biweekly period, and its interac-

802

Journal of the Meteorological Society of Japantion with shorter-period monsoon activity with
4–7 day period.
From these analyses, it has been found thatthe convective activity over the TP is closely
associated with the meso-scale systems, withheavy rainfall over the BMF in China, throughPEC over the eastern edge of the TP, throughthe Sichuan basin. However, we also noticeother cases of cyclogenesis over the Sichuan basin, and heavy rain cases in the Yangtze Riverbasin, not directly related to PEC. These additional factors also need to be examined, to fully
understand the dynamics of genesis and development of meso-scale cloud/precipitation systems, in the BMF zone. The detailed processes,
and possible mechanisms, of these major cyclogeneses will be reported elsewhere.
Acknowledgements
We thank Dr. H. Fujinami of the HyARC,
Nagoya University for assisting us to edit therevised manuscript as well as his valuable suggestions and comments on the paper. We are
greatly indebted to Dr. Y.-M. Kodama, editor
in charge of this paper in JMSJ, for his constructive comments and suggestions. We also
thank Prof. F. Kimura and Dr. H. Ueda of the
University of Tsukuba for their valuable comments. Thanks are also extended to Dr. N. Yamazaki, Meteorological Research Institute of
JMA, for useful comments in using the GAME
reanalysis data.
References
Akiyama, T., 1975: Southerly transversal moisture
flux into the extremely intense rainfall zone in
the Baiu season. J. Meteor. Soc. Japan, 53,
304–316.
Chang, C.-P., S.C. Hou, H.C. Kuo, and G.T.J. Chen,
1998: The Development of an Intense East
Asian Summer Monsoon Disturbance with
Strong Vertical Coupling. Mon. Wea. Rev., 126,
2692–2712.
Ding, Y., 1992: Summer Monsoon Rainfalls in China.
J. Meteor. Soc. Japan, 70, 373–396.
Fujinami, H. and T. Yasunari, 2001: The seasonaland intraseasonal variability of diurnal cloud
activity over the Tibetan Plateau. J. Meteor.
Soc. Japan, 79, 1207–1227.
——— and ———, 2004: Submonthly Variability ofConvection and Circulation over and aroundthe Tibetan Plateau during the Boreal Summer. J. Meteor. Soc. Japan, 82, 1545–1564.

Vol. 84, No. 4———, S. Nomura, and T. Yasunari, 2005: Characteristics of Diurnal Variations in Convection
and Precipitation over the Southern Tibetan
Plateau during Summer. SOLA, 1, 49–52.
Iwasaki, H. and T. Takeda, 1993: Structure and behavior of meso-scale cloud clusters travelingover the Baiu-frontal zone. J. Meteor. Soc.
Japan, 71, 733–747.
Kato, K., 1985: On the Abrupt Change in the Structure of the Baiu Front Over the China Continent in Late May of 1979. J. Meteor. Soc.
Japan, 63, 20–36.
———, 1989: Seasonal Transition of the lower-level
circulation systems around the Baiu front in
China in 1979 and its relation to the northernsummer monsoon. J. Meteor. Soc. Japan, 67,
249–265.
———, J. Matsumoto, and H. Iwasaki, 1995: Diurnal
Variation of Cb-Clusters over China and Its
Relation to Large-scale Conditions in the Summer of 1979. J. Meteor. Soc. Japan, 73, 1219–
1234.
Kuo, Y.-H., L. Cheng, and R.A. Anthes, 1986: Mesoscale Analyses of the Sichuan Flood Catastrophe, 11–15 July 1981. Mon. Wea. Rev., 114,
1984–2003.
Kuwagata, T., A. Numaguti, and N. Endo, 2001: Diurnal variation of water vapor over the central
Tibetan Plateau during summer. J. Meteor.
Soc. Japan, 79, 401–418.
Lorenzo, D. and S.-J. Chen, 1984: Genesis of vortex
and shear line over the Qinghai-Tibetan
Plateau—Numerical experiment. Paper presented at the Symposium on the Tibetan Plateau and Mountain Meteorology, Beijing, 19–
24 March 1984.
Li, W. and L. Chen, 2001: Characteristics of seasonalvariation of rainfall over the Tibetan Plateau
during summer 1998 and its impact on east
Asian weather. Acta. Meteor. Sinica, 15, 293–
309.
Li, C. and M. Yanai, 1996: The Onset and Interannual Variability of the Asian Summer Monsoon
in Relation to Land-Sea Thermal Contrast. J.
Climate, 9, 358–375.
Murakami, M., 1976: Analysis of summer monsoon
fluctuations over India. J. Meteor. Soc. Japan,
54, 15–31.
———, 1983: Analysis of the deep convective activity
over the western Pacific and Southeast Asia.
Part T: Diurnal variation. J. Meteor. Soc. Japan, 61, 60–76.
Murakami, T. and W.-G. Huang, 1984: Orographic
Effects of the Tibetan Plateau on the Rainfall
Variations Over Central China During the 1979
Summer. J. Meteor. Soc. Japan, 62, 895–909.

August 2006

T. YASUNARI and T. MIWA

Ninomiya, K. and T. Akiyama, 1974: Band structureof meso-scale echo clusters associated with lowlevel jet stream. J. Meteor. Soc. Japan, 52,
300–313.
——— and H. Muraki, 1986: Large-Scale Circulations over East Asia during Baiu Period of
1979. J. Meteor. Soc. Japan, 64, 409–429.
——— and T. Murakami, 1987: The early summer
rainy season (Baiu) over Japan. Monsoon Meteorology, C.-P. Chang and T.N. Krishnamurti,
Eds., Oxford Univ. Press, 93–121.
———, 2000: Large- and meso-a-scale characteristicsof Meiyu-Baiu front associated with intense
rainfalls in 1–10 July 1991. J. Meteor. Soc.
Japan, 78, 141–157.
Nitta, T., 1983: Observational study of heat sources
over the eastern Tibetan Plateau during thesummer monsoon. J. Meteor. Soc. Japan, 61,
590–605.
Prasad, B., 1974: Diurnal Variation of rainfall in
Brahmaputra Valley. Indian Journal of Meteorology and Geophysics, 25(2), 245–250.
Shibagaki, Y., M.D. Yamanaka, S. Shimizu, H.
Uyeda, A. Watanabe, Y. Maekawa, and S. Fukao, 2000: Meso-b to g-scale wind circulations
associated with precipitation cloud near Baiu
front observed by the MU and meteorologicalradars. J. Meteor. Soc. Japan, 78, 69–91.
Shinoda, T., H. Uyeda, and K. Yoshimura, 2005:
Structure of moist layer and sources of waterover the southern region far from the Meiyu/
Baiu front. J. Meteor. Soc. Japan, 83, 137–152.
Sun, S.-Q. and D. Lorenzo, 1984: Some numerical experiments of the Tibetan Plateau influence on
low-level jet in East Asia in summer. Paper
presented at the Symposium on the Tibetan
Plateau and Mountain Meteorology, Beijing,
19–24 March 1984.
Tao, S.-Y. and Y.-H. Ding, 1981: Observational Evidence of the Influence of the Qinghai-Xizang(Tibet) Plateau on the Occurrence of the Heavy
Rain and Severe Convective Storms in China.
Bull. Amer. Meteor. Soc., 62, 23–30.
Ueda, H., H. Kamahori, and N. Yamazaki, 2003: Sea-

803

sonal Contrasting Features of Heat and Moisture Budgets between the Eastern and Western Tibetan Plateau during the GAME IOP. J.
Climate, 16, 2309–2324.
Uyeda, H., H. Yamada, J. Horikomi, R. Shirooka, S.
Shimizu, L. Liping, K. Ueno, H. Fujii, andT. Koike, 2001: Characteristics of convective
clouds observed by a Doppler radar at Naquon Tibetan Plateau during the GAME-Tibet
IOP. J. Meteor. Soc. Japan., 79, 463–474.
Wang, B., 1987: The Development Mechanism for Tibetan Plateau Warm Vortices. J. Atmos. Sci.,
44, 2978–2994.
——— and I. Orlanski, 1987: Study of a Heavy Rain
Vortex Formed over the Eastern Flank of the
Tibetan Plateau. Mon. Wea. Rev., 115, 1370–
1393.
Wang, W., Y.-H. Kuo, and T.T. Warner, 1993: A Diabatically Driven Mesoscale Vortex in the Lee of
the Tibetan Plateau. Mon. Wea. Rev., 121,
2542–2561.
Wang, J., Y. Yang, X. Xu, and G. Zhang, 2003: A
monitoring study of the 1998 rainstorm along
the Yangtze river of China by using TIPEX
data. Advances in Atmospheric Sciences, 20,
425–436.
Yamada, H. and H. Uyeda, 2006: Transition of therainfall characteristics related to the moistening of the land surface over the central Tibetan
Plateau during the summer of 1998. Mon. Wea.
Rev. (accepted).
Yanai, M. and C. Li, 1994: Mechanism of Heating
and the Boundary Layer over the Tibetan Plateau. Mon. Wea. Rev., 122, 305–323.
Yatagai, A. et al., 2000: About GAME reanalysis
data. Journal of the Japan Society of Hydrology and Water Resources, 13, 486–495. (in Japanese)
Xu, X., S. Tao, J. Wang, L. Chen, L. Zhou, and X.
Wang, 2002: The relationship between water
vapor transport features of Tibetan Plateaumonsoon ‘‘large triangle’’ affecting region anddrought-flood abnormality of China. Acta Meteorological Sinica, 60, 257–266. (in Chinese)