Large-Scale Water Cycle Processes in East Asia Associatedwith Seasonal Evolution of the Meiyu/Baiu Front*Kuranoshin Kato
(Faculty of Education, Okayama University, Okayama, 700-8530 JAPAN)*Phone:+81-86-251-7735, FAX:+81-86-251-7755*E-mail: kuranos@cc.okayama-u.ac.jp

1. Introduction
The rainfall activity around the Meiyu/Baiu front is sustained by the hugenorthward transport of moisture from the subtropical high region due to intense
low-level southerly wind. On the other hand, the temperature gradient in thelower layer across the front is relatively weak, especially over China. Such weakbaroclinicity, however, plays an important role in characterizing the rainfall
features and its activity there, such as the structure change in a meso-α-scale
low with its eastward propagation, effects of the southeastward intrusion of aupper-level cut-off cold low, and so on. The effects of the heating from theground around the arid and the semiarid region China are also important.
However, these characteristics show some remarkable seasonal transitions fromspring to summer. Thus roles of the arid/semiarid region in China coupled withthe humid monsoon system in the water cycle around the Meiyu/Baiu front
should be examined by considering their seasonal evolution. Thus we will
present some topics relating to the water cycle processes around the Meiyu/Baiu
frontal zone, in association with the seasonal evolution of the Asian Monsoon
system.
2. Maintenance of the extremely humid air mass in the subtropical highregion to the South of the Baiu front (a case for subtropical high over the East
China Sea)
The moisture content in the subtropical high area to the south of theMeiyu/Baiu front shows rather large variation. According to the case study for 6–15 July 1988 (Kato, 1992), the specific humidity (q) in the layer from 900 to 700
hPa levels increased greatly around 11 in the subtropical high area in East China
Sea (Fig. 1). Interestingly, the rapid increase in q was in coincidence with theenhancement of low-level southerly wind and that of the shallow convection there.
This suggests a role of the shallow cumulus convection in sustaining the thickmoist layer (up to about 3 km level), resulting in providing the extremely humid
air to the Baiu front. However, further studies are needed on the maintenanceprocesses of such moist air there, including detailed observations in the1cloud-capped boundary layer.
3. Seasonal evolution of the large-scale features of the Meiyu/Baiu front
According to the previous studies, the large-scale features of the Meiyu/Baiu
frontal present the following seasonal transition, although the details are notshown here.
(a) Change from the path of the baroclinic disturbances into thequasi-stationary cloud zone around the beginning of May.
(b) Abrupt increase in the deep convective clouds in the frontal zone inSouth China and the Southwest Islands area in the middle of May,
simultaneously with the onset of the Southeastern Asian Monsoon.
(c) Abrupt weakening of the low-level baroclinicity across the Meiyu front in
China around the end of May.
(d) Abrupt enhancement of the low-level southerly wind toward the front,
and the northward shift of the frontal zone to Central China and the Japan
Islands.
(e) Abrupt northward shift of the Meiyu/Baiu front to North China and thearea to the northern part of Japan.
While the event (b), (d) and (e) are related to the transition of tropicalmonsoon systems, those of (a) and (c) are affected by the warming in the higher
latitudes over the continent. Especially for the event (c), the heating from theground over the arid and semiarid region in China plays an essential role. Kato(1987) pointed out that the polar frontal zone characterized as a path of thebaroclinic disturbances shifts northward to around 60 N in the eastern part of theEurasia. The major circulation systems in the higher latitudes and their effects
on the rainfall features on the Meiyu/Baiu front change considerably at that time.
4. Effect of the arid region in China on generation of meso-α-scale lows
around Central China in the mature stage of the Meiyu there
Meso-α-scale lows often appear on the Meiyu/Baiu front in association withthe heavy rainfall events there. Although so-called the southwest vortices(meso-α-scale) are often originated from the eastern foot of the Tibetan Plateau
to propagate eastward, the activation of rainfall by the large-scale processes
sometimes results in the generation of meso-α-scale lows on the front in anothersituations. Kato et al. (2001) made a case study for 2-3 July 1991 and 29-30
June 1998 (GAME/HUBEX workshop in Kunming, China).
As for the case in 1998, a cloud system associated with the SW vortex
propagated from Sichuan basin on 28 June. The low-level shear line extended2

eastward from there after 12 UTC 28 June. New cloud clusters were generated onthat shear line (around 33 N/116 E, just near the Japanese 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 the morning of 30
June there. Just during that period, a meso-α-scale low was initiated there. It
should be also noted that the center of a synoptic-scale surface low was locatedaround 38 N/107 E (the eastern edge of the arid/semiarid region in China).
Similar initiation process of a meso-α-low was found also for the case in 1991.
In both cases, the enhancement of rainfall activity in the frontal zone wasaccompanied by the further northward intrusion of strong low-level southerly
wind across the time mean position of the Meiyu front. It is noted that such
intrusion of the southerly wind, coupled with the nearly stationary one in thesubtropical high area toward the front, would result in the huge moisturetransport and the generation of convective activity there. .
The further northward intrusion of the relatively strong southerly wind wasdue to the enhancement of eastward or southeastward pressure gradientassociated with the eastward expansion of a synoptic-scale surface low along 〜
40N. A quasi-stationary surface-low is formed around the arid region in China(Fig. 2), maybe associated with the heating from the ground.
It is well known that the inflow of the dry air in the middle troposphere is
one of the important processes for enhancement of the severe rainfall systems.
The arid or semiarid region in China would provide such dry air into the rainfall
systems on the Meiyu front. In addition, the above mentioned cases indicatethat the arid/semiarid region in China might play another interesting role in theevolution of a meso-α-scale low on the Meiyu front. However, interannual
variability of the effects on the middle and high latitude systems including theheat low, and its influence on the Meiyu/Baiu frontal activity should be examinedfurther.
5. Further northward invasion of water vapor across the mean position ofthe Meiyu front
According to the case study for 1979, the moisture content in North China,
which is located to the north of the mean position of the Meiyu front, increased at
the onset of the Meiyu in Central China (Fig. 3). Frequency of the rainfall
events in North China Plain to the Huaihe River Basin increased then, associatedwith the periodic passages of the Meiyu frontal disturbances. While themoisture inflow into Changjiang River Basin is mainly due to the seasonal mean
low-level southerly wind, the inflow into further northern region is due to thetemporal one.
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Once the moisture penetrates into the northern region by the transient
southerly wind and brings precipitation there, the air could be moistened by there-evaporation from the ground even after the temporal rainfall events. If then,the moisture could be transported further northward at the passage of the next
disturbance. It is not so clear how such re-cycling of the water vapor contributes
to the increase in moisture in the northern region, compared to the simple
northward transport by the disturbances.
It is also noted that a part of the disturbances on the front (in meso-α-scaleor synoptic-scale) are formed after the activation of the Meiyu/Baiu front.
Furthermore, such activation of the front is sometimes influenced by the heat low
around the arid region in China (Section 4). Thus, roles of the arid region inChina in the water cycle around the Meiyu/Baiu front, including the interaction
between the time mean field and the temporal systems, are a remaining problem.

4Fig. 1(Left) Time-height section of q (g/kg) at the ship Keifu Maru (around 32.5N/127 E)

in the subtropical high area in East China Sea throughout the analysis period.

Sequence ofwind vectors at 840 hPa level there is also presented at the top of this figure, where a fullbarb denotes 2 m/s, a half barb 1 m/s and a flag 10 m/s.
(Right) Time sequence of amount of the clouds whose tops are higher than 700 hPa level (%),
averaged for 27 N – 33 N/ 120 E – 130 E (Kato, 1992).
Fig. 2Distribution of the sea level pressure (hPa) averaged for the period from 28 June to4 July 1998, in solid lines.

The areas with the southerly wind component more than 4 m/s

are also shown in dotted lines (m/s).
Fig. 3

Latitudinal distribution of specific humidity at 850 hPa level averaged from 110 E

to 120 E for each period, in g/kg.

Lines for “Pre-Meiyu” (1 – 19 June 1979), “Meiyu” (20June – 22 July 1979) and “Midsummer” (23 July – 17 August 1979) are shown in adotted-broken line, a dotted line and a solid line, respectively (Kato et al., 1995).

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