Maintenance Processes of moisture field in the Northern and theSouthern regions adjacent to the Meiyu front over China in 1998
Shoichiro Ikeda*, Kuranoshin Kato***The Graduate School of Natural Science and Technology, Okayama University,
dns17108@cc.okayama-u.ac.jp**Faculty of Education, Okayama University, Tsushima-naka, 3-1-1, Okayama 700-8530
JAPAN, kuranos@cc.okayama-u.ac.jp

1.

Introduction

front over the continental part of China?

The rainfall activity around theMeiyu/Baiu

frontinEast

AsiaisWe

havealreadypresented

prelim results at the LAPS Workshop

sustained by the huge moisture transport

2004, etc.

from the subtropical high area.

on the results of the second topic.

On thetheThus we will report mainlyAndother hand, the region to the north of thefinally, we summarize the results for both

front over China is very dry.

areas.

It isinteresting that the moisture content is
It is also pointed

2. Seasonal evolution of the Meiyu
rainfall belt in 1998

out, for example, that the cloud clusters

As shown in Fig. 1, the extremelywith remarkable diurnal variation often

heavy rainfall event occurred around theappear

Changjiang River basin in late June 1998

especially large in China although it islocated over the land.

aftertheofJune,

according to Kato et al. (1995).

This ispartlyresulted

increase

inin

middlebyseasonal

rainfall belt moved to stagnate aroundthe Huaihe River basin at the end of

although that area was still to the northJune to the beginning of July (Period 3).

of the Meiyu front.

However, it has not

The Meiyu rainfall belt shifted to thebeen clarified yet how the moisturenorth, and the subtropical high had

content there increase seasonally, evencovered on the Huaihe River basin in thethe Meiyu front is situated to the south ofmiddle of July (Period 4).

that area.

Based on these facts, we havethe Meiyu rainfall belt had shiftedinvestigated on the problems below, forsouthward again and the second heavy

the case in 1998 (the GAME/HUBEX IOP

rainfallperiod), by using the GAME reanalysis

Changjiang River basin in late Julydata, and so on.

(Period 6).

Whyisthecontent

The Meiyu

there,(1)

moisture

thereferred to as Period 2.

moisturecontent

especially large in the subtropical higharea over China land area?
(2) How does the moisture increaseseasonally even to the north of the Meiyu

eventoccurred

After that,aroundtheThe Huaihe River basin waslocated to the north of the Meiyu rainfall
belt then.

Rainfall amount (mm/day)Continental side (112.5〜120E)(N)
Heavy Rainfall in the
Huaihe river Basinarea(mm day-1)

Huaihe River Basin, Period 4
Heavy Rainfall in the
Changjiang River Basin,Period 6

Period 2

China.

The activation of theMeiyu Front to the north of the

Heavy Rainfall in theChangjiang River Basin,aroundshallowconvective

cloudsduetodestabilization
Ridge(500hPa) axis
at 115E

Area １

associated with suchsituation

Area ３would

transport the moisture
upward to sustain theJuly 1998

JunePeriod123

54large6layer there.

(over the eastern China) based on the GAME-reanalysis data (value is transformed

On the other hand,

The thick solid lines indicate the latitude of the ridge lineof the subtropical high at 500 hPa level on the daily Weather Maps, JMA.

although

Thehigh area also overrainfall event around the Changjiang River basin in 1998 corresponding to Period 2China was small inin this figure.

Firstly,

wesummarize

themaintenance processes of huge moisturecontrast in the subtropical high area thesouth to the Maiyu rainfall belt.

Inorder to examine the temporal variationof moisture content “in the subtropical

Period 4, not only thelatent heat flux but also the sensible heat
flux at the ground were relatively large
there.

the moisture more upward, which would
result in the large PW with thick moist
layer there even in such situation.

4.

It is interestingthat the larger PW was sustained in thesubtropicalhighareain

Chinathroughout there periods, while PW
sometimes showed rather small value
was the East China Sea area.
In Period 2, the northward moisture
fluxpassingtroughthatarea

wasespecially large, in the subtropical highThemoisturemaintenancePrecipitable Water Content (mm)on the 600km South of the Precipitation peakwater content (PW) referred to the 600
are shown in Fig. 2.

So, the dry convection due to theheating from the ground could transporthigh area”, time series of the perceptible
km south to the Meiyu/Baiu rainfall belt

moisture

flux in the subtropicalThe main branch of the Changjiang River is around 30 N. The first heavy

3.
The moisture maintenanceprocesses in the subtropical high areain 1998

thenorthward

Huaihe River basin is located around the latitude indicated as “CREST Observation
Area”.

withrelatively thick moist

Fig. 1. Time-latitude section of 6-hourly rainfall amount averaged for 112.5-120E

into the unit of mm/day).

PW

PW (mm)80706050
4030

116.25E

126.25E

Continent

East China Sea

20 Period 2
100
6/20
6/25

6/30

543

7/5

7/10

7/15

67/20

7/25

1 1000hPag7/30
1998= ∫ content
qdp (PW)
Fig. 2. Time series of precipitablePWwater
300 hPain the subtropical high area at 116.25E or 126.25E,
integrated from 300 hPa to 1000 hPa level (mm).
for the detail, please refer to the text.

As5-day Mean Sensible and Latent Heat Fluxes

5-Day mean T850（℃） and Q850（g/kg）

in Huaihe River Basin

Huaihe River Basin

5-day Mean SH

31.25-35N/110-120ET850(℃)

2(W/m )

2018161412108Heavy rainfall
6around Changjiang
4Rainfall aroundRiver Basin
Huaihe River Basin
2025

Huaihe River Basin 31.25-35N/110-120E

120

Q850(g/kg)(W m-2)and LH

Period 6100

40
20
8/21

8/1

8/11

7/22

7/12

7/2

6/22

6/12

6/2

5/23

5/13

0
5/3

0

4/3
4/13
4/23
5/3
5/13
5/23
6/2
6/12
6/22
7/2
7/12
7/22
8/1
8/11
8/21

560

4/23

10

4/3

1580

4/13

20

Q850 (g/kg)

T850 (℃)

Temperature increase

1998

1998

Fig. 3. Time series of pentad mean T850 (℃ doted

Fig. 4.

line) and Q850 (g/kg solid line) at 850 hPa level for(SH thin line) and latent heat flux (LH thick line)

31.25-35N/110-120E in 1998

averaged

process in the north to the Meiyu
front in 1998
In the Huaihe River basin, while thetemperature at 850 hPa level even to thenorth of the Meiyu front increased
rapidly in the middle of June, specifichumidity did not increase until the end ofJune, as pointed out by Kato (1995).
TheMeiyu

rainfallbelt

shiftedsouthward to the Changjiang River Basin
again in Period 6.

However, it is notedthat specific humidity in the region to thenorth of the Meiyu front was sustained
the high value.

Although the figure isnot shown here, the stability for deep
convection has decreased since Period 3.
Time-latitude section of wind field at
850 hPa level along 116E shows that,
although the strong southwesterly windprovided to the south of the Meiyu front,
the relatively weak northerly wind wassustained to the north to the front, in
Period 6 (the figure is not shown here).
Thus, in this case, the moisture transport
from the south could not contribute to themaintenance of large moisture content in
the Huaihe River Basin.
Time series of 5-day mean sensible andlatent heat fluxes averaged for the HuaiheTime series of 5-day mean sensible heat flux

fortheHuaiheRiverbasin(31.25-35N/110-120E), in Wm -2 .

River Basin is shows in Fig. 4.

Latentheattoflux

fromthegroundtheatmosphere has increased rapidly at thebeginning of July (just after the heavy
rainfall event in the Huaihe River Basin).
It is also interesting that the large amountof sensible heat flux is sustained even after
the rapid increase in the latent heat inearly July.

This might suggest that theground surface temperature was sustained
relatively high value due to the solar
insulation even after the moistening of theground.

The time latitude section oflatent heat flux (not presented here) showsthat the area with large latent heat flux tothe north of the Meiyu front extended
widely northward to 40N after the middleof June, corresponding to the northern
limit of location of the surface Meiyu front.
Figure 5 shows the time series of dailymean rainfall amount and latent heat flux
averaged for this region.

Latent heat flux

increased just after the heavy rainfall in
the Huaihe River Basin.

We should notethatheatthelarge

latentflux

wassustained during middle and late July,although the rainfall amount was rather

Daily Mean Latent Heat Flux (LH) and Daily Precipitation (PR)

Heavy rainfall in
Changjiang River Basin

Period 6 – Period 2

Heavy rainfall in
Changjiang River Basin

25140
120
100

2080

7/30

7/25

7/20

7/15

7/10

7/5

6/30

0
6/25

20

0
6/20

40

5
6/15

60

10

6/10

15

Latent heat flux（Ｗ/ｍ２）

Heavy rainfall in Huaihe River Basin

30

Fig. 5.

Time series of daily mean latent heat flux anddaily mean precipitation in the Huaihe River basin.

small then.
Figure 6 shows the latitude-heightsection of difference of mean specifichumidity in Period 6 from that in Period 2.
The specific humidity around the HuaiheRiver basin (to the north of the Meiyu
frontal zone) is larger in Period 6 than in
Period 2, not only around the surface but
also up to the 700 hPa level.

Recallingthat the sensible heat flux as well as thelatent heat flux was larger in this region in
Period 6 than in Period 2, it is suggested
that the moisture supplied from the ground
was transported upward to maintain thethick moist layer also there.
As for the summery for the northern
region, the ground gets wet once the region
experienced the heavy rainfall period byMeiyu front.

This would enable thepersistent large latent heat flux after that.
In addition, the hot ground also results in
the relatively large sensible heat flux there.
These situations could contribute to themaintenance the thick moist layer there
through the upward transport of moistureby the dry convection in Period 6.

5.

Δq (g kg-1) 112.5−120ＥHuaiheRiver Basin(31.25-35N/110-120E)
淮河流域
潜熱と降水量1998年
Daily mean rainfall amount
Daily mean latent heat flux

35
Rainfall amount（mm/day）(hPa)

Results in the other years
The similar situation was found also in1991, although the large scale evolution of
the Meiyu front was rather different fromFig. 6.

Latitude-height section of difference ofspecific humidity in Period 6 from that in Period 2
averaged for 112.5-120E (Δq) (g kg -1 ).

Contours forthe negative values are shown in dotted lines.

that in 1998.

At the end of May in 1991,

the Meiyu front shift to the Huaihe RiverBasin and shifted southward again toSouth China at the end of June.

But, thelarge moisture content was maintained inthe Huaihe River basin even in that period.
As for the case in 2004 (the LAPS
special observation was curried out aroundthe Huaihe River basin), southerly wind at
850 hPa sometimes invaded to this area
after the middle of June, although theFuyang (located in the Huaihe River basin)
was located to the north of the surface front.
Specific humidity showed the increasing
trend, not only at the surface level but also
at the 850 hPa level during the latter halfof June.

6.

Concluding results
We would like to emphasize that the“wet and hot ground” plays an importantrole in the maintenance processes ofmoisture field in both the northern andsouthern regions of the Meiyu frontal zone
in China.

However, detailed examination

about the role of “memory” of the rainfall
the latent heat supply from the ground,and so on, is a remaining problem.