Role of Low-Altitude Moist-Layer for Development ofPrecipitation Systems over East China Sea during the Baiu
Hiroshi Uyeda, Manami Nomura, Taro Shinoda, Kazuhisa Tsuboki,Katsuhiro Nakagawa*, Kenji Nakamura and HyARC Radar Observation Group
Hydroshperic Atmospheric Research Center, Nagoya University, Nagoya 464-8601, Japan
Tel.: +81-52-789-3492, FAX: +81-52-789-3436, e-mail: uyeda@rain.hyarc.nagoya-u.ac.jp* Okinawa Subtropical Environment Remote-Sensing Center, National Institute of Information
and Communications Technology, 4448 Aza-Onna, Onna, Kunigami, Okinawa 901-0411 Japan
1. Introduction
The Meiyu/Baiu frontal precipitation
system is one of the most prominent
phenomena in Asian monsoon. Structureand development processes of theprecipitation systems have been studied forbetter understanding of the system andimprovement of numerical models.
It is notable that intensive field
experiments were carried out overcontinental China recently on precipitation
systems in June and July (Zhao and Takeda,1998; Ding, 2001).
The field experimentson the Meiyu frontal precipitation systems
with Doppler radars in the network ofsounding were carried out in 1998 and 1999over the Huaihe River Basin during theGAME/HUBEX (GEWEX Asian Monsoon
Experiment/ Huaihe River Basin Experiment).
Maesaka and Uyeda (2004a,b) showed twotypes of precipitation systems identified withthe radar data of IOP in 1998: one without atemperature gradient in the subtropical air
mass and the other with a temperature
gradient as a result of a merger with the coldfront. The role of the low-altitude moist
layer in the south of the Meiyu front isemphasized.
Succeeding the part of the heritage ofGAME/HUBEX, intensive field experimentsin the downstream of Yangtze River werecarried out in 2001 and 2002 with Doppler
radars and wind profilers in the network ofsounding (Geng et al., 2004; Yamada et al.,2003). Yamada et al. (2003) revealed oneof the mechanisms on the development of
the meso-α-scale convective system along
the east coast of China: convergence ofstrong southwesterly inflow to the Meiyu
frontal system and the low altitude easterly
flow from ocean.
At the same time intensive field
experiments in the east part of East China
Sea and the west part of Kyushu werecarried out from 1998 to 2002 with Doppler
radars, wind profilers, airplanes and research
vessels on board radar (Yoshizaki et al.
2002). One of the interesting results is thefinding of the water vapor front (Moteki et al,2004a,b). They found a convergence linewith the large water vapor gradient distinct
from the Baiu front about 100 km south of theBaiu front. This gave an approach to reveal
the Baiu frontal precipitation systems overthe East China Sea.
These recent studies and previous
studies in the west part of Kyushu Island
facing to East China Sea are suggesting theimportance of studies on the role of
low-altitude moist-layer for development ofprecipitation systems around the Baiu front.
An example of the roles is studied with radar
data obtained in Okinawa.
2. Radar observation network in Okinawa
In LAPS (Lower Atmosphere andPrecipitation Study), field experiments with
Doppler radar were carried out from 13 May
to 6 June in 2003 at Okinawa and Miyako inNansei Islands. In Okinawa intensive field
experiments were carried out in cooperation
with Okinawa Subtropical Environment
Remote-Sensing Center, NICT.
X-bandDoppler radar (Nagoya University) was set atKatsuren and C-band polarimetric radar
COBRA (NICT) is installed at Nago. (Fig.1)

Fig.1 Radar observation ranges of X-band
Doppler radar at katsuren and polarimetricradar, COBRA, at Nago.

3. Rainband on 25 May 2003
In Okinawa, a rainband passed over
Okinawa was observed on 25 May as shownin Fig. 2. Wind fields in the precipitation
system were analyzed with the X-band
Doppler radar and COBRA for the area
shown in Fig.3.
Vertical wind fields
averaged for X-axis in Fig. 3 are shown in Fig.
4: at 05 JST (before passage), 06 JST (onisland) and 08 JST (after passage).
Tracking the radar reflectivity shown by
contours in Fig. 4, it is shown that convectionis the strongest in the rear of the systembefore landing. Stratiform region is in thedownstream of the system movement.
Updraft starts from the rear side of thesystem and becomes taller at the peak of theconvection (reflectivity core).
Wind fields are analyzed for the dual
Doppler radar analysis area as shown in Fig. 2.
Before the landing of the rainband on
Okinawa Island, wind direction of the low
altitude in the south of the rainband at 1 kmASL was southerly (Fig.4a). The moist flow
reached the northern end of the rainband(Fig.4b).
Sounding at Naha, Okinawa,
coincided with the analyzed wind fields thatwind is south – southeasterly and close tosaturation below 1 km ASL (not shown).
When the rainband was on Okinawa Island
convection was weak. Water vapor supply
to the rainband in the low altitude seems to
be blocked by Okinawa Island. After thepassage, updraft in the convection became
stronger again. Moist airflow in the low
altitudes is considered to be important for thegeneration of convection.

Fig.2 Precipitation band around Okinawa
at 03 JST on 25 May 2003 by JMA radar.
A prominent rainband extends fromsouthwest to northeast with the length
more than 400 km. (JST = 9 hrs + UT)

Fig.3 Analysis area (solid box) of verticalstructure of the precipitation system.
Dual Doppler radar analysis areas areshown by dotted circles.

Fig.4 Time variations (at 05, 06 and 08
JST 25 May 2003) of vertical crosssection of reflectivity (contour) and updraft
fields (color shades) along the Y-axis ofFig.3. Values are averaged along theX-axis in Fig.3. 06 JST correspond to
the time when the rainband passed overOkinawa Island. Contours are every 3
dBZ from 15 dBZ. Warm color shows
updrafts and cold colors downdrafts.
Updrafts are at the low altitudes in thenorthwestern edge of the precipitation
system.

The rainband was composed ofprecipitation cells aligned parallel to the bandin the northwestern edge of the precipitation
system.
The system moved southeast
passing over Okinawa Island. Stratiform
region was ahead of the system movement
and the convective region was in the rearedge of the system. Stratiform region did
not change much before and after thepassage. On the other hand, the height ofthe convective core of 3.5 km ASL before thepassage decreased to the level below 2.0 km
ASL after the passage at 0548 JST.
The development of the convective cells
was constrained after the passage when theconvergence in the low altitude decreased
because of the weakening of thenorthwesterly behind the Okinawa Island. A
few hours after the passage, whennorthwesterly recovered the strength at thenorthwest edge of the precipitation system,
the rainband re-strengthened.
Theseobservational results revealed that themaintenance mechanism of the rainband andthe precipitation system is continuous
formation of not so tall convective clouds ona weak convergence line in the moist
environment of the low atmosphere.
4. Discussions
The maintenance mechanism of therainband is considered as follows (schematic
illustration in Fig.6). Low convective clouds
developed along the weak convergence
below 1 km ASL using the abundant water
vapor of the southwesterly. Cloud droplets
and rain drops formed rapidly using the moist
air in the low altitude. Stratiform region
extended to the downstream of the system
movement and the downdraft there did notreach to the ground surface then low altitude
moist air reached to the northwest edge ofthe linear precipitation system. In this way
the rainband maintained its linear shape forlong time.

(a)

(b)

Fig. 5 Wind fields at 0436 JST 25 May
2003. a) At 1 km ASL and b) verticalcross section along the line A-A’ in a).

Fig. 7 Schematic illustration of the structure and maintenance mechanism of
the rainband in the system formed over East China Sea.

Present study revealed a part of thedetailed structures of the Meiyu/Baiu frontal
precipitation system, which is a uniquesystem in the mid-latitude Asia. However
the unified understanding of the system fromthe west of Continent China to Japan is stillon the way. Understanding on the role ofthe convection in the moist environment in
the south of the Meiyu/Baiu front would help
the understanding on the precipitation
systems around monsoon Asia. As for thedevelopment of convective precipitation
system in the moist environment, treatmentof boundary layer would be very important asshown by Shinoda and Uyeda (2002).
5. Concluding remarks
Recent field experiments on theMeiyu/Baiu frontal precipitation systems
revealed the development processes of thesystem in the moist environment. Detailed
studies on the systems showed that rapid
development of convective cloud below themelting layer in the south of the Meiyu/Baiu
front plays an important role for developmentof meso-β and meso-α scale precipitation
systems along the Meiyu/Baiu front.
Studies on the annual variation ofprecipitation systems along the Meiyu/Baiu
front would help the understanding onmonsoon. Further studies on developmentof precipitation systems with cloud resolving
model are expected. Observational data ofLAPS, in particular in Okinawa and Miyako
would contribute to understanding on roles of
low-altitude moist-layer for development ofprecipitation systems during the Meiyu andBaiu
Acknowledgement
The authors express their thanks toKatsuren Town Office and Hirara City Office forproviding the radar observation sites.

References
Ding, Y., Y. Zhang, Q. Ma and G. Hu, 2001:
Analysis of the large-scale circulation
features and synoptic systems in East Asia
during the intensive observation period ofGAME/HUBEX. J. Meteor. Soc. Japan, 79,277-300.
Geng, B., H. Yamada, K. K. Reddy, H. Uyeda
and Y. Fujiyoshi, 2004: An observational
study of the development of a rainband on
a Meiyu front causing heavy rainfall in thedownstream region of the Yangtze river. J.
Meteor. Soc. Japan, 82, 1095-1115.
Maesaka, T., H. Uyeda, T. Kato and M.
Yoshizaki,
2004a:
Structureandmaintenance process of the precipitation
systems in the Meiyu frontal zone overcontinental China during GAME/HUBEX
1998. Part I: Meiyu front in the subtropicalair Mass. J. Meteor. Soc. Japan(submitted)
Maesaka, T., H. Uyeda, T. Kato and M.
Yoshizaki,
2004b:
Structureandmaintenance process of the precipitation
systems in the Meiyu frontal zone overcontinental China during GAME/HUBEX
1998. Part II: Meiyu front merged with acoldfront. J. Meteor. Soc. Japan(submitted)
Moteki, Q., H. Uyeda, T. Maesaka, T. Shinoda,
M. Yoshizaki and T. Kato, 2004: Structureand development of merged two rainbands
observed over the East China Sea during
X-BAIU-99 Part I: Meso-β-scale structureand development processes. J. Meteor.
Soc. Japan, 82, 19-44.
Moteki, Q., H. Uyeda, T. Maesaka, T. Shinoda,M. Yoshizaki and T. Kato, 2004: Structureand development of merged two rainbands
observed over the East China Sea during
X-BAIU-99 Part II: Meso-α-scale structure
and build-up processes of convergencein the Baiu frontal region. J. Meteor. Soc.
Japan, 82, 45-65.
Shinoda, T. and H. Uyeda, 2002: Effective
factors in the development of deep
convective clouds over the west region ofeastern China during the summer
monsoon season. J. Meteor. Soc. Japan,80, 1395-1414.
Yamada, H., B. Geng, K. K. Reddy, H. Uyeda
and Y. Fujiyoshi, 2003: Three-dimensional
structure of a mesoscale convective
system in a Baiu-frontal depression
generated in the downstream region of theYangtze River. J. Meteor. Soc. Japan, 81,1243-1271.
Yoshizaki, M., T. Kato, C. Muroi, H. Eito, S.
Hayashi and CREST Observation Group,
2002:
Recentactivitiesoffieldobservations on mesoscale convective
systems (MCSs) over East China Sea andKyushu in the Baiu Season and over theJapan Sea in winter. International Conf. on
Mesoscale Convective Systems andHeavy Rainfall/Snowfall in East Asia,Tokyo, Japan. 80-85.
Zhao, B. and T. Takeda, 1998: Huaihe River
Basin Experiment (GEWEX/HUBEX).
Beijing University, 24pp.