Structure of Moist Layer and Sources of Water
over the Southern Region Far from the Meiyu/Baiu Front∗
Taro Shinoda1 , Hiroshi Uyeda1 , and Kei Yoshimura2(1: HyARC, Nagoya University, 2: IIS, University of Tokyo)* HyARC, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, JAPAN.
e-mail: shinoda@rain.hyarc.nagoya-u.ac.jp

Abstract
We investigate structure of a moist layer and sources of water in the southern region far from the Meiyu/Baiu
front using GAME reanalysis data. In the period from July 4 to 15, 1998, the Meiyu/Baiu front was locatedover northern China and the Korean Peninsula. Both regions over both the plain area of mainland China (theeastern region of 112◦ E line) and the East China Sea had moist air masses; the total precipitable water was
about 45kgm−2. However, the structure of these moist layers is quite diﬀerent. The moist layer over mainland
China is deep, reaching 600 hPa. On the other hand, the moist layer over the East China Sea is shallow, witha thickness of up to 800 hPa. This diﬀerence in the structure of the moist layers can be attributed to thedevelopment of a moist convective mixing layer and the generation of shallow convective clouds in the daytime
over mainland China as a result of the supply of abundant latent and moderate sensible heat ﬂuxes from theland surface.
We also examine the sources of water supplied into the Meiyu front using Colored Moisture Analysis (CMA).
Though water vapor to generate the deep moist layer over mainland China is mainly transported from theupstream regions of the Asian summer monsoon, which include the Indian Ocean, the Indochina Peninsula,and the South China Sea, the rate of the contribution of water from the southeastern region of mainland China
is also over 15% of the total precipitable water, when the Meiyu front is located over northern China. Thisshows that the land surface over southeastern China is also regarded as a major source of water.

1

Introduction

The Meiyu/Baiu front is a boundary between wet anddry regions in the eastern Asia. Water vapor thatcauses heavy rainfall events around this front originates in the southern region far from the front (wet region) and is transported northward by the southwesterly monsoon (e.g., Akiyama 1973). Akiyama (1973)
also considered that the horizontal transport of water
vapor is mainly restricted to the lower troposphere(below 700 hPa) both over mainland China and theEast China Sea. Shinoda and Uyeda (2002) have
hypothesized that the development of the convective mixing layer and shallow convective clouds transported water vapor from the atmospheric boundary
layer to the middle troposphere; this process should
form a deep moist layer over mainland China. However, we could not conﬁrm the structure and horizontal extension of the moist layer in the lower troposphere not only over mainland China, but also overthe East China Sea. The ﬁrst purpose of this study
is to conﬁrm the structure of moist layers over bothregions.
Shinoda and Uyeda (2002) also indicated that thelatent heat ﬂux from the land surface over the wet
region of mainland China, especially from the paddy
ﬁelds that are widely distributed around the Yangtze
and Huaihe River Valleys, was responsible for moistening the lower troposphere and would be the sourceof water into the Meiyu front. The second purpose ofthis study is to quantitatively conﬁrm the contribu-tion of the land surface over mainland China to thesource of water into the Meiyu front.

2Data

The GAME (GEWEX Asian Monsoon Experiment,
GEWEX: Global Energy and Water Cycle Experiment) conducted intensive observations around theeast and southeast Asian regions in summer season
in 1998. The results of these intensive observations
are included in GAME reanalysis data. Thus, we use
6-hourly GAME reanalysis data version-1.5 developedby Yamazaki et al. (2000) in this study. The GAMEreanalysis data are on a 1.25◦ × 1.25◦ grid and cover
the period from April to October 1998. In this study,
we deﬁne a term of a “moist layer” as relative humidity is larger than 60% in the lower and middle
troposphere.
In order to explore the source of water supplied
into the Meiyu front, we employ a “Colored Moisture Analysis (CMA)” developed by Yoshimura et al.
(2004). The CMA is an analytic method that allows
for the estimation of atmospheric moisture advection
from speciﬁc source regions in daily time scale. The
CMA’s water transport model includes atmosphericwater balance equations and vertically well-mixed assumption, and uses external meteorological forcings(precipitation, evaporation and horizontal advection)
obtained from GAME reanalysis. Water evaporatingfrom earth surface is tagged as one of 16 kinds of wa-Fig. 1 Surface precipitation rates (mmday −1 ) averaged from July 4 to 15, 1998, using GAME reanalysis data. The values above 5mmday −1 are shaded.

ter and is mixed into precipitable water. The total
precipitable water in each grid can be presented by
the sum of tagged water from any sections. The calculation is started at 00 UTC April 1, 1998. Thus,
we can identify the source of water at the speciﬁc grid
over mainland China near the Meiyu front.

3Diﬀerence in the structure of
the moist layers

This study will focus on the water circulation in thesouthern region far from the Meiyu/Baiu front from
July 4 to 15, 1998 (Fig. 1). The Meiyu front is locatedover the Yellow River Valley and the Korean Peninsula during this period. Figure 2 shows the horizontal
distribution of total precipitable water (vertically integrated water vapor amount) and of the vertically integrated horizontal moisture ﬂux averaged from July
4 to 15, 1998. There is a steep gradient of precipitablewater along the Meiyu front. The horizontal moisture
ﬂux shows moisture transport toward the Meiyu front
from the South China Sea through to the plain regionof mainland China (the eastern region of 112◦ E line)
and the East China Sea. The total precipitable water
values over the plain region of mainland China andthe East China Sea are both larger than 45kgm−2.
This result shows that there are moist air masses overboth the plain region of mainland China and the East
China Sea.
However, there are signiﬁcant diﬀerences in thestructure of moist layers over mainland China and theEast China Sea. Mixing ratio of water vapor near thesurface over mainland China is less than that over theSouth China Sea and over the East China Sea (notshown). That above 800 hPa over mainland China
is larger than that over the East China Sea. Figure
3 shows vertical cross sections of relative humidity
along 116◦ E (over mainland China) and 125◦ E (overFig. 2 Horizontal distribution of total precipitablewater (kgm−2 ) and vertically integrated horizontal
moisture flux vectors (kgm−1 s−1 ) averaged from
July 4 to 15, 1998, using GAME reanalysis data.
The total precipitable water values above 30kgm−2
are shaded.

the East China Sea) averaged from 4 to 15, 1998.
There is a signiﬁcant deep moist layer; that is, relative humidity is larger than 60%, between 18◦ N and25◦ N along 116◦E in this period (Fig. 3a). This region of the moist layer corresponds to two tropicaldepressions that are recognized in the distribution ofsurface precipitation (Fig. 1). The moist layer overthe plain region of mainland China (from 25◦ N to37◦ N along 116◦ E) reaches up to 600 hPa. On theother hand, the moist layer over the East China Sea(from 25◦ N to 33◦ N along 125◦ E) reaches up to 800
hPa, except for the deep moist region extending fromthe South China Sea, that is, the area between 15◦ Nand 24◦ N along 125◦ E (Fig. 3b).
In order to develop a deep and moist convective
mixing layer, the supply of the sensible and latent
heat ﬂuxes from the surface is necessary. Figure 4ashows the horizontal distribution of the latent heatﬂux averaged from July 4 to 15, 1998. Over the East
China Sea, the value of the latent heat ﬂux is about 80
to 100W m−2, and there is no diurnal variation. In thesouthern region far from the Meiyu front over mainland China, the latent heat ﬂux exceeds 180W m−2.
Surprisingly, this value is twice that over the East
China Sea. A large amount of latent heat ﬂux fromthe land surface over mainland China should moisten
the atmospheric boundary layer and increases theamount of total precipitable water over this region(Fig. 2). Figure 4b shows the horizontal distributionof the sensible heat ﬂux averaged in the same period.
Over the East China Sea, there is no sensible heatﬂux throughout the day. On the other hand, the sensible heat ﬂux is about 40 to 60W m−2 over mainland
China. The supply of the sensible heat ﬂux from theland surface over mainland China should contributeto the development of the convective mixing layer.

Fig. 3 Vertical cross sections of relative humidity along 116◦ E (a) and 125◦ E (b) averaged from
July 4 to 15, 1998, using GAME reanalysis data.
Relative humidity values above 60% are shaded.

The supply of abundant latent and moderate sensible
heat ﬂuxes from the surface should form the deep andmoist convective mixing layer over mainland China.

4Source of water vapor into
the Meiyu front

As shown in the previous section, a large amountof latent heat ﬂux from the land surface causes thegeneration of the moist layer over mainland China.
In this section, we conﬁrm whether the land surface
over mainland China can be regarded as a source ofwater into the Meiyu front. In order to explore thesources of water supplied into the Meiyu/Baiu front,we employ the “Colored Moisture Analysis (CMA).”
We organized 16 sections that are closely related to
the water circulation in the east Asian region.
Figure 5 shows a daily time series of the water
sources of the total precipitable water in a grid including the center of the GAME-HUBEX region overmainland China. As the Meiyu front approached overthis region after June 23, 1998 (days of year = 174),water sources dramatically change from those in theearlier period. Instead of the remarkable decrease of
the contribution of the northern part of the EurasianContinent (northeastern and northwestern China andthe Central Eurasian Continent) to the total precip-Fig. 4 Horizontal distributions of the latent heat
flux (a) and the sensible heat flux (b) from thesurface averaged from July 4 to 15, 1998, using
GAME reanalysis data. The latent heat flux above
40W m−2 and sensible heat flux above 20W m−2 areshaded, respectively.

itable water, water from the upstream regions of theAsian summer monsoon; that is, the Indian Ocean,
the Indochina Peninsula, and the South China Sea,
increases over the GAME-HUBEX region. The sumof the contribution amount (rate) of these regions on
July 14 (days of year = 195), 1998 occupies 25.54 mm(41.99%). This result shows that water that generates
the moist layer over mainland China is mainly supplied from the upstream regions of the Asian summermonsoon.
However, the contribution amount (rate) of thesoutheastern region of China to the total precipitablewater is about 10 mm (over 15%) after the passingof Meiyu front. This shows that the land surface
over southeastern China is one of the most importantsources of water to form the moist layer over this region and can be regarded as a major source of water
into the Meiyu front.

5Concluding Remarks

The structure of the moist layer in the lower
troposphere in the southern region far from theMeiyu/Baiu front over mainland China and the East
China Sea are investigated by using GAME reanaly-

(a) Over mainland China (33.125 N, 115.625 E)
70

Precipitable Water Amount (mm)6050403020100155160165170175 180 185 190
Time (Days of Year)195

200205210

InitialSE. China

SW. China

NE. China

NW. China

Indochina Pen.

Indian Subcont.

C. Eurasian Cont.

S. China Sea

E. China Sea

N. Indian Oce.

S. Indian Oce.

NW. Pacific O.

Other Pacific O.

Other Seas

Other Lands

Fig. 5 Daily time series of water sources of the total precipitable water at the grid including Fuyang(33.125◦ N, 115.625◦ E) over mainland China.

sis data. In the period from July 4 to 15, 1998, theMeiyu/Baiu front was located over the Yellow River
Valley and the Korean Peninsula. Both regions overboth the plain area of mainland China (the eastern region of 112◦ E line) and the East China Sea had moist
air masses; their amount of the total precipitable water was larger than 40kgm−2. However, the structureof these moist layers, which is deﬁned as the relative
humidity is larger than 60% in the lower and middle
troposphere, is quite diﬀerent as shown in Fig. 6.
The moist layer over mainland China is deep, reaching 600 hPa. This deep moist layer is probably formedby the development of the convective mixing layerand shallow convective clouds in the daytime. Themoist convective mixing layer is formed by the supply of abundant latent heat ﬂux from the land surface, especially from the paddy ﬁelds that are widelydistributed over southeastern China, and by the supply of moderate sensible heat ﬂux. Shallow convective clouds generated around the top of the convective
mixing layer transport water vapor from the convective mixing layer to the middle troposphere (Shinodaand Uyeda 2002); consequently, the moist layer becomes deep. Though water vapor to generate thisdeep moist layer is transported from the upstream
regions of the Asian summer monsoon, the land surface over southeastern China is also regarded as amajor source of water. The amount (rate) of thecontribution of water from the southeastern regionof mainland China is about 10 mm (over 15%) of thetotal precipitable water. Finally, this deep moist layer
merges into the Meiyu front and supplies water vapor
into it.
On the other hand, the moist layer over the East
China Sea is shallow, reaching 800 hPa. There is nosupply of the sensible heat ﬂux over the East ChinaFig. 6 Schematic illustration of the transport ofwater and the structure of the moist layer overmainland China (a) and over the East China Sea(b). Arrows show the transport of water.

Sea; thus, the deep moist layer cannot be formed.
The mixing ratio of water vapor near the surface islarger than that over mainland China. Finally, thisshallow moist layer also merges into the Baiu frontand supplies water vapor into it.

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