Synoptic- and Meso-αScale Processes around the Meiyu Front in China at the
Sharp Boundary of very Wet and Dry Regions*Kuranoshin Kato and Shouichirou Ikeda(Faculty of Education, Okayama University)*Tsushima-naka 3-1-1, Okayamaa-city, 700-8530 Japan. e-mail: kuranos@cc.okayama-u.ac.jp
Abstract
The present report firstly summarizes the results of the synoptic and meso-α-scale analeses on the GAME/HUBEX,
including the case around the Japan Islands, in relation to the seasonal background of the continental-scale heat low as
pointed out by Kawamura and Murakami (1998). Then, a preliminary results of the analysis on the moisture importprocess toward the northern area of the Meiyu front in China.
Keyword:GAME/HUBEX, LAPS/CREST, Meiyu/Baiu, meso-α-low and Baiu front.
1. Introduction
The rainfall activity around the Meiyu/Baiu front issustained by the huge northward transport of moisture fromthe subtropical high region. It is noteworthy that the air
mass is extremely humid in the subtropical region just to
the south of the Meiyu/Baiu front in China and the westernJapan, influenced by the Indian monsoon airflow and theespecially warm western Pacific so-called the ﾒ warm
water pool ﾓ.
On the other hand, the extremely dry andhot region is located just to the north or northwest of the
Meiyu front in China, while the relatively cool air mass(called the "Okhotsk air mass") affects the Baiu frontal
system as a northern system.
The mean position, the intensity and the rainfall
characteristics of the Meiyu/Baiu front show considerable
seasonal transition affected both by the Asian monsoon
systems and westerly systems in the higher latitude. The
variety of the higher latitude systems results in thesignificant different features of the Meiyu/Baiu front
between the continental part of China and the Japan Islands
side. Especially, the heating from the ground over thearid and the semiarid region in China plays an essential
role in the seasonal evolution of the large-scale circulationand the rainfall characteristics around the Meiyu front(Kato, 1985, 1987).
Furthermore, the authors havepointed out that the heating from the ground around thearid region in China contributes to the maintenance of thecontinental-scale heat low in the lower layer surrounding
the Tibetan Plateau, combined with the condensation
heating on and to the south of the plateau. Such process
also seems to affect the rainfall activity around the
Meiyu/Baiu front through the temporal variation ofmoisture transport to the northern region of mean positionof the Meiyu front.
It is also noticed that the roles and the variety of theformation processes of meso- α -lows are interesting
features of the Meiyu/Baiu frontal heavy rainfalls. In
some types of the large-scale situation, the activation of thefrontal-scale rainfall results in the initiation of a meso-αscale low. An important connection of the continentalscale heat low with such evolution is also pointed out.
Thus the present lecture will discuss on the frontal activity
in the seasonal background of the heat low surrounding the
Tibetan Plateau, referring to some studies by the present

authors.
It is also interesting that the moisture content in North
China (which is located to the north of the mean Meiyu
frontal zone) increased at the onset of the Meiyu in Central
China, according to a case study for 1979 (Kato et al.,1995). While the moisture inflow into the Meiyu frontal
zone in Central China is brought mainly by the low-levelsoutherly wind of the time mean fields, the coupling withthe temporal southerly transport would be important for thefurther northward moisture transport to the northern area.
On the other hand, once the soil became somewhat wet due
to the rainfall associated with the eventual southerlytransport of moisture, re-evaporation from the groundmight partly contribute to the rainfall there. That is, a roleof the re-cycling of water vapor would not be negligible.
Furthermore, the change in the ground surface condition
such as the soil moisture content would modify the sensible
heating from the ground, as well as the latent heating.
Thus the recycling process of the water vapor there in
relation to the transient southerly transport of moisture
should be quantitatively examined.
The Huaihe River Basin (LAPS (Lower Atmosphere andPrecipitation Study) special observation area), where the
GAME/HUBEX intensive observations were also carriedout, is located just around the boundary of the very humid
and the dry regions. For example, although the Meiyu
frontal rainfall persists in this area from late June to earlyJuly as in the Changjiang River Basin, rather dry situation
is sustained there until the Meiyu onset. The present
report also introduces the preliminary results of the largescale study on the LAPS..
2 Continental-scale heat low around the Tibetan
Plateau in the mature stage of the Meiyu/Baiu season
Climatologically speaking, a continental-scale heat low
around the Tibetan Plateau is formed associated with theslowly varying seasonal transition, according to Kawamura
and Murakami (1998) (Fig. 1. They call it the L-mode).
This would mainly due to the condensation heating by themonsoon rainfall in South Asia or heating over the Tibetan
Plateau. Simultaneously, the heating from the groundaround the arid region in China would give rise of thelower atmosphere there, and nearly the highest temperature
in a year is sustained in the lower layer from early June toearly August. While the synoptic-scale cyclcones andanticyclones pass there alternatively around May, thequasi-stationary surface low covers there after June withdeepening in the evening (the low would be sustained dueto the heating from the ground). Thus the continentalscale heat low mentioned by Kawamura and Murakami(1998) would be a result of combination of these heatingprocesses.

of the heavy rainfall there during this period.
The eastward pressure gradient just to the south of the
Meiyu front in China, associated with the strong southerly
wind toward the frontal zone, was enhanced from 20 to 24
June due to the deepening of the low-pressure area around23N/100-110 E. As known by the previous studies, socalled the southwest vortex (SW vortex) propagating
eastward from the eastern foot of the Tibetan Plateau
sometimes enhances the Meiyu frontal activity (e.g.,
Murakami and Huang, 1984). The propagation of such
disturbance is also affected by the mid-latitude westerly,
although it is initiated over the plateau in association withthe diurnal variation of convective activity there.
However, the low-pressure area found for this case seems
to be the different kind of system from the SW vortex, andso on. The deepening of the low-pressure area mentioned
above corresponded to the eastward extension of thecontinental-scale heat low mentioned in Section 2.

Fig. 1 Schematic figure of the L-mode circulation in
relation to the East Asian large-scale systems (modified
from Kawamura and Murakami (1998))
Although Kawamura and Murakami (1998) alsosuggests the seasonally rapid varying mode (S-mode) also
contributes to the enhancement of low-level southerly wind
toward the Baiu/Meiyu, further northward penetration of
the southerly wind component through the mean positionof the Meiyu frontal zone in China could be realized
combined with the L-mode. In addition, the dominancy
of the L-mode in the mature Meiyu/Baiu season implies thelarge-scale eastward pressure gradient in the seasonal mean
field in East Asia, not only for the subtropical high area but
also in its northern area. In the following sections,examples of analysis results on such processes by ourgroups will be briefly introduced (Kato, et al., 2001a,
2001b,Kato and Hisayasu, 2002).
3. Large-scale Features associated with the Heavy
Rainfall in Central China in the Meiyu of 1998
The Meiyu rainfall belt sifted slightly northward to bring
the heavy rainfall around the Changjiang River Basin (Fig.
2). The Meiyu rainfall belt shifted northward to the
Huaihe River Basin (~33N) around 29 June, and further
northward with being weakened around 4 July. The
Meiyu rainfall zone shifted southward to the ChangjiangRiver Basin and was enhanced again around 20 July.
Especially in the first heavy rainfall event around theChangjiang River Basin (22-27 June), huge rainfall zone
(e.g., the zone with rainfall amount of more than 20
mm/day for the average from 110 E to 120 E) persisted
everyday without changing its latitude very much. It isnoted that the huge amount of northward moisture flux
persisted almost everyday, which contributed largely to that

Fig. 3Time-latitude section of 6-hourly rainfall
amount averaged for 112.5-120 E (over the eastern
China) based on the GAME re-analysis data (value istransformed into the unit of mm/day). The thick solidlines indicate the latitude of the ridgeline of the subtropicalhigh at 500 hPa level on the daily Weather Maps, JMA.
The Huaihe River Basin is located around the latitude
indicated as "CREST Observation Area". The mainbranch of the Changjiang River is around 30 N. The firstheavy rainfall event around the Changjiang River Basininn 1998 corresponds to Period 2 in this figure.
4. A role of the arid region in China in the initiationof meso-α-lows on the Meiyu front in China
As for the meso-α-scale systems on the Meiyu front in
China, it is known that the Tibetan Plateau plays animportant role in initiating the meso- α -scale lows
originated around the eastern foot of the Plateau and move
eastward on the Meiyu front (so-called the southwest
vortices (SW vortices)). However, we often find anothersituations in which the rainfall is firstly enhanced in the
Meiyu frontal scale to organize the convective clouds intoa meso-α-scale cloud cluster. In such cases, meso-αscale low is also initiated after the organization of theclouds. Results of the case study for ~29 June 1998 and~2 July 1991 will be briefly introduced in this section.
As for the 1998 case, a cloud system associated with
the SW vortex propagated from Sichuan basin on 28 June.
The low-level shear line extended eastward from there
after 12 UTC 28 June. New cloud clusters were generated
on that shear line (around 33 N/116 E, just near theJapanese Doppler Radar sites), and the Meiyu cloud zone
shifted northward to the Huaihe River Basin. After that,heavy rainfall area persisted from the morning of 29 to themorning of 30 June there, accompanied by the successive
generation and eastward propagation of meso- β-scaleecho systems as observed by the Fuyang Radar. Just
during that period, geopotential height at ~33 N/ 115.5 E
decreased rapidly and a meso-α-scale low was initiatedthere. It should be also noted that the center of asynoptic-scale surface low was located around 38 N/107 E(the eastern edge of the arid/semiarid region in China).
Similarly, a meso-α-low was initiated after the activationof the frontal-scale rainfall with organization into themeso-α-scale clouds also for the case in 1991.
In both cases, the enhancement of the rainfall activity inthe frontal zone was accompanied by the further northward
intrusion of strong low-level southerly wind across thetime mean position of the Meiyu front. It is noted that such
intrusion of the southerly wind component would result in
the increase in moisture transport and the generation of
convective activity there, in addition to the nearlystationary low-level southerly wind in the subtropical higharea toward the front. The intrusion of the relatively
strong southerly wind around Fuyang, where the meso-αlow was initiated, was due to the enhancement of eastward
or southeastward pressure gradient associated with theeastward extension of a synoptic-scale surface low along~40N, in both cases.
Climatologically speaking, the low-level southerly windcomponent in the subtropical high area toward the Meiyu
front is enhanced in late June, and the mean position of thefrontal zone shifts to Central China then. The subtropicalhigh area with strong southerly wind, and the area with thesurface-level heat low in the northwestern China arelocated rather adjacent to each other, in the mature stage ofthe Meiyu in Central China. Thus the present processes
could be seen in the limited stage in the seasonal march.
It is well known that the inflow of the dry air in the
middle troposphere is one of the important processes forenhancement of the severe rainfall systems. The arid orsemiarid region in China could provide such dry air into
the rainfall systems on the Meiyu front. In addition, theresult introduced here implies that the arid region in Chinamight play another interesting role in the evolution of ameso-α-scale low on the Meiyu front.
5. A possible effect on the time evolution of active
rainfall area on the Baiu front around the Japan

Islands area
Meso-α-scale lows are also sometimes initiated due tothe active convections on the Baiu front and are deepenedin the Baiu frontal zone with relatively strong baroclinicity
to the east of the Kyushu District (~130E, the westernedge of the Japan Islands). In such cases, meso-β-scale
rainfall systems often stagnate around Kyushu,
corresponding to its trailing portion of the low passed
eastward (Ninomiya et al., 1988a, b).
Around 29 June 1999, the heavy rainfall area around thenorthern Kyushu (~131E) expanded rapidly eastward to
the Chugoku District (132-135E, also in the western part of
the Japan Islands). That heavy rainfall area in Kyushu
was located in the eastern part of a meso-α-scale low
which propagated from the west. The eastward propagationof the heavy rainfall area was faster than that of the low.
This eastward expansion occurred when the heavy rainfall
area was still stagnated around Kyushu (~00 UTC 29
June) with the slight deepening of the low just to the westof that area. The area with the strong low-level southerly
wind component just to the south of the Baiu frontal zone
rapidly expanded eastward then.
It should be also remarked that both of the Pacific high(to the south of the Baiu front) and the Okhotsk high werestagnated to the east of Japan during the period from 28 to
30 July. The zone with a concentrated meridionaltemperature gradient was found around 30 to 40 N to theeast of the Japan Islands (to the east of ~ 140 E).
Nevertheless, the synoptic-scale eastward pressure gradient
was sustained even around that latitude (corresponding to
the mean position of the Baiu front there). Thus theresults of this case also imply an interesting role of mesoα-scale lows on the behaviors of the heavy rainfall area inJapan under the basic field with the continental-scale heat
low mentioned in Section3.
6. Large-scale thermodynamic conditions around the
Huaihe River Basin (seasonal and temporal variations)
6.1 Pre-Meiyu period in the Huaihe River Basin
Figure 3 shows the time series of the pentad mean air
temperature (T850) and specific humidity (Q850) at 850
hPa level averaged for 31.25-35N/110-120E for 1998
(roughly corresponding to the Huaihe River Basin). In
this year, although the Meiyu rainfall had persisted aroundthe southern part of the Changjiang River Basin since ~10 June, the first heavy rainfall event in this basin occurred
during the last decade of June with slight northward shiftof the Meiyu front after 20 June.
As pointed out by Kato (1985 and 1987), low-level air
temperature rose abruptly around 10 June in this year but
the specific humidity remained a small value in the HuaiheRiver Basin, in coincidence with the temperature risearound the arid region in China (not shown here). This
indicates the relative humidity decrease there, just before
the Meiyu stage in the Huaihe River Basin. In 1998, theMeiyu front once shifted further northward from the basin
and the subtropical high covered that area in the middle of

July. The specific humidity showed also the large value
then.

Fig. 3 Time series of pentad mean T850 (℃, solid line)
and Q850 (g kg-1,dotted line) at 850 hPa level for 31.2535N/110-120E for 1998, based on the GANAL. The
letters Hu and CJ2 indicate the Periods 3 (heavy rainfall
around the Huaihe River Basin) and 6 (that in the
Changjiang River Basin) in Fig. 2, respectively.

Fig. 4 Time series of the pentad mean (θe500−θe850) (solid line) and (θe500*−θe850) (dotted line)
averaged for 31.25-35N/110-120E in 1998 (K).
In the pre-Meiyu stage in the Huaihe River Basin in June,
the stability for moist convection seen from the potential
temperature difference between 500 and 850 hPa levels (θe500−θe850) decreased, due to the temperature increasein the lower layer (Fig. 4). However, the moisture content
was not so large there and the stability for the dry
convection (θ700−θ850) was not so small even at 12
UTC (20 BST, where the mixed layer shows still the
daytime character) (figure is not shown here). Thisimplies the potentially stable condition for moist
convection, as shown by the positive value of (θe500*−
θ e850), where θ e* denotes the saturation equivalentpotential temperature (Fig. 4).
6.2 After the onset of the Meiyu in the Huaihe RiverBasin
The Meiyu front shifted southward again and stagnatedaround the Changjiang River Basin in the last decade ofJuly 1998 to bring the second heavy rainfall event there. It
is interesting that the specific humidity at 850 hPa level
was not decreased so much in the Huaihe River Basin evenwhen the Meiyu front is located to the south of this region(Fig. 3). In that situation, (θe500−θe850) shows thelarge negative value (convective unstable), and the value of(θe500*−θe850) is nearly zero.
Relatively large amount of rainfall could moisten the
ground so that enough amount of latent heat together withsensible heat is supplied from the ground to the lower
atmosphere. In fact, although the latent heat flux to thenorth of the Meiyu frontal over China is rather small before
the beginning of July, it shows a considerable amount(about 100 Wm-2) after the middle of July (based on the
GAME reanalysis data) (not shown here).
In 1991, when the severe flood occurred around the
Huaihe River Basin in early July, the Meiyu season once
started in late May, rather earlier than in the normal year.
However, the intraseasonal variation of the location and theactivity of the Meiyu front were also very large, and thefour peak rainfall periods appeared around the HuaiheRiver Basin during May through the early August 1991.
In this year, the Meiyu front shifted rather southward tomerge the tropical convetive systems around the end ofJune. The specific humidity in the lower layer did notdecrease so much after it increased in late May.
Thus the present study suggests that the memory of the
Meiyu frontal rainfall might play an interesting role in thewater cycle just to the north of the mean position of the
Meiyu frontal zone. However, what kind of roles the land
surface processes and the temporal variation of the largescale atmospheric disturbances play on the water cycle
there, including their interaction processes, effects on therainfall characteristics there, is a remaining problem.