Study on Nocturnal Development of Cloud Clusters During the Meiyu
Period in Eastern ChinaZhuxiao Li 1, Masayuki Kawashima 2 and Yasushi Fujiyoshi 2(1.Graduate School of Environment Earth Science, Hokkaido University. 2. ILTS, Hokkaido University.)
Graduate School of Environment Earth Science, Hokkaido University, Japan, 060-0819,
N19W8, North District, Sapporo, Japan. E-mail: lzx@lowtem.hokkudai.ac.jp
Abstract
The evolution of 61 long-lived mesoscale cloud clusters that developed in Eastern China during theMeiyu period in 1998 and 1999 were traced and analyzed by hourly GMS data in this study. It was foundthat 2/3 of them tend to peak at nighttime (Nocturnal type), others peaked in the evening (Evening type).
Most of the Nocturnal type clusters peaked immediately south to the Meiyu front. The nocturnal
strengthened LLJ and the resultant low-level moisture convergence immediately south to and within theMeiyu frontal zone was suggested to mainly contribute to the dominant convection peak of the Nocturnaltype.

1. Introduction
Cloud clusters are composed of deep
convective clouds with an organized structure thatcan persist long time. The cloud shield that tops
the convection is noticeable in satellite imagery as
a circular or oval area of low blackbody
temperature (TBB). Cloud clusters cause heavy
rainfall and greatly influence energy and water
cycles on earth (Maddox 1980).

contrast with the many studies on the diurnal
variation of cloud clusters in America and tropicaloceans, few studies have noted the diurnal
variation of cloud clusters over Eastern Asia.
Presentstudy

investigatedwhetherthedevelopment of cloud clusters in Eastern China has
a diurnal variation and explored the mechanism
under the diurnal cycle.

2. Data and methods of analysis

Many studies (Trier and Parsons, 1993, et al.)In this study, 61 cloud clusters with life time

indicate that cloud clusters over Tropical oceans

longer than 12 hours and maximum area exceeds

and Central America show clear diurnal variation

60,000 km2 developed in eastern China (25-40N,

with a nocturnal convection peak. In Eastern Asia

110-120E) during the Meiyu period in 1998 (June,during Meiyu period, many cloud clusters with aJuly and August) and 1999 (June and July) were

meso-αor βscale and multiscale structure occur

traced and analyzed. The movement and evolution

and propagate along the Maiyu-frontal zoneof individual cloud clusters was traced manually(Akiyama, 1984; Iwasaki and Takeda, 1993), theyby hourly TBB distributions in GMS-IR images.

can cause heavy rainfall and floods over theThe grid size of the TBB data was 20km in 1998

Yangtze and Huaihe River valleys. However, inand 10km in 1999.The GAME Reanalysis 1.5 that1basedonHUBEX

Basin

It was found that most (38/41) of theExperiment) observation data with a spatial

Nocturnal-type cloud clusters tend to developresolutiontemporallargely just over or immediately south of theresolution of 6 hours (2, 8, 14 and 20 LST

Meiyu front at nighttime and there is a significanteveryday) was used to investigate the diurnal

southwesterly synoptic-scale LLJ (S-LLJ, > 10 mvariations-1) existed in the south of the Meiyu front,of50(HuaihekilometersRiverandof atmospheric conditions inthedevelopment environment of cloud clusters.

3. Diurnal variation of cloud clusters
The 61 analyzed cloud clusters can bedivided into two types by their dominant peak timeextends from the South China Sea to Korea andKyushu.

4.1 Diurnal variation of the low-level windfieldas follows: (I) Nocturnal-type: More than twoWe

mainlyexaminedthediurnalthirds (43/61) of the cloud clusters peaked between

variation in the low-level wind fields over Eastern

midnight and early morning (21 to 06 Local

China during the period from 1 to 15 August inStandard Time (LST)), and more than half of the1998 when frequent Nocturnal-type cloud clusters

nocturnal peaks (23/43) occurred between 00 andand almost stationary Meiyu front that can02 LST. Some long-lived clusters showed a

simplify the study of time variation of wind fields.

secondary evening peak or two midnight peaks.
(II) Evening-type: The remaining cloud clusters(18/61) peaked mainly from afternoon to early
evening (from 15 to 18 LST).

different

fromlow-level (925 hPa) wind field. A distinct diurnal
variation exists in the horizontal wind field at 925
hPa over the domain south of the Meiyu frontal

Nocturnal-type cloud clusters havecharacteristics

Fig.1 indicates the diurnal variation in thezone (25-32N,hereafter the South Domain.). The

Evening-type

low-level wind velocity, especially its southerlyclusters. Table1 shows the main characters of thecomponent, was much larger at 02 LST than that atcloud clusters of these two types. The Nocturnal

other times, especially than 14 LST (about 2type clusters are much more intense with larger

times).

area of inner deep convection (TBB below

southwesterly wind within the Meiyu frontal zone–70°C) that related to the heavy rainfall more

was small at all times with slight time change. The

directly (Mapes and Houze, 1993), longer lifetime

nocturnal strong southerly wind over the Southand more frequent occurrence than the Evening

Domain weakened dramatically after it arrived thetype ones. In present study, we studied thearea immediately south to and within the Meiyu

developed environment of the convection peak offrontal zone. The southerly wind at 14 and 20 LST

the Nocturnal type cloud cluster.

also decreased over that domain, but in a much

4. Diurnal variation of wind and moisture

By

contrast,

thevelocityofthesmaller range.

field over Eastern China during MeiyuPeriod2

The

nocturnal

strengtheningofthesouthwesterly low-level wind (LLJ) over theSouth Domain was attributed to the distinct
clockwise direction shift of the ageostrophic wind(not shown) from easterly to southerly atnighttime over there. Blackadar (1957) attributed
the nocturnal veering of an ageostrophic LLJ to
an inertial oscillation in wind initiated by thesudden cessation of strong turbulent mixing in theespecially that at 14 and 20 LST.
Similar diurnal variation with maximum at 2
LST in the horizontal wind field over the South
Domain and in the resultant moisture convergence
field over the MFCZ occurred at 850 and 700hPa, but with a smaller amplitude. Diurnal
variation in the low level (at 925 hPa) wind fieldand moisture convergence fields similar to that inAugust also occurred in June and July of 1998.

boundary layer after sunset.
The nocturnal intensification of southerly
component of low-level (925hPa) winds over theSouth Domain, acted with the increased low-level
water vapor from evening time over there caused
intothelarge

enhancementoflow-level

northward moisture flux at night time (2 LST).

The nocturnal strengthened LLJ and theformation of low level (925hPa) moisture
convergence over MFCZ was suggested tocontribute to the convection peak of individual
Nocturnal type cloud cluster. The low-level
moisture

convergencelargely increased theamount of water vapor at low level and

4.2 Diurnal variation in the low-level
moisture convergence
The nocturnal large southerly moisture flux(925hPa) over the South Domain dramaticallyweakened followed the low level southwesterly

strengthened the potential instability over MFCZ
and also force weak synoptic-scale lifting tocontribute to the nocturnal convection peak ofcloud cluster.

5. Summary

wind at that time and resulted in low-level
moisture convergence with large area andintensity as shown in Fig.2 that indicates thediurnal variation in the moisture convergence at
925 hPa. The large convergence at 2 LST
occurred over the domain with southern border
immediately (100km) south to the Meiyu frontal
zone and including its southern part (from31-34N), it was named Meiyu Front Convergence
Zone hereafter (MFCZ). Moisture convergenceappeared within this region at the other times, butcovered a much smaller area with weaker intensity,

There is a clear diurnal variation in theevolution of the cloud clusters occurred in Eastern
China during Meiyu period. It was found thatmost of them tend to peak at nighttime (Nocturnaltype), others peaked in the evening (Eveningtype). Most of the Nocturnal type clusters peaked
immediately south to the Meiyu front. The
nocturnal strengthened LLJ over South Domain
and its resultant low level moisture convergenceover the MFCZ are proposed to mainly contributeto the nocturnal development of the cloudclusters.

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Table1 Characters of midnight and evening type cloud clusters
Type of

Peaktime

Peak time

NumbeLife

Peak area

Peak area

Cloudofdeepof Cluster

rtimeofof very Deepcluster

convection

Nocturn

2.3

alLST

typeEvening

17.4 LSTofCluster

clusters
4 LST

Cloud

114,000

41

convection
33000 km2

km2
17.4 LST2014 h109,000

21,000 km2

km2

Type

Fig.1 Diurnal cycle (8, 14, 20 and 2 LST) of

Fig.2 Diurnal cycle (8, 14, 20 and 2 LST) of

horizontal wind distribution (left panel) and its

horizontal moisture convergence distributionlongitudinal mean value (right panel) at 925 hPa

at 925 hPa. The area within the solid lines inaveraged for 1-15, August of 1998. Area withineach panel indicates the Meiyu frontal zone

two solid lines indicates the Meiyu frontal zone

and within the dash lines is the Meiyu(32-35N, 110-120E).

FrontalConvergenceZone(31-34N,110-120E

References:
Akiyama, JMSJ, 62, 485-504,1984.
Blackadar, BAMS, 38, 283-290, 1957.
Maddox, BAMS 61, 1374-1387, 1980.
Trier and Parsons, MWR, 121, 1078-1098,1993.
Iwasaki and Takeda, JMSJ, 71, 733-747,1993.

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