SOLA, 2005, Vol. 1, 117 120, doi: 10.2151/sola. 2005 031
117
Structure of Mesoscale Convective Systems during the Late Baiu
Season in the Global Warming Climate Simulated
by a Non-Hydrostatic Regional Model
Sachie Kanada1, Chiashi Muroi2, Yasutaka Wakazuki1, Kazuaki Yasunaga1,
Akihiro Hashimoto1, Teruyuki Kato2, Kazuo Kurihara2,
Masanori Yoshizaki2 and Akira Noda2
1
Advanced Earth Science and Technology Organization, MRI, Tsukuba, Japan
2
Meteorological Research Institute, Tsukuba, Japan
Mesoscale convective systems (MCSs) that bring
rainfall in the vicinity of Kyushu Island, Japan during
the late Baiu season in the present and global warming
climates are examined by a non-hydrostatic regional
climate model (NHM) with the horizontal grid of 5 km.
In the global warming climate, two types of MCSs
appear in the vicinity of Kyushu Island. One travels
from the Chinese Continent and the other from the
southern part of the East China Sea to Kyushu Island.
These two MCSs often merge over the sea southwest of
Kyushu Island, and they rapidly develop to bring heavy
precipitation to the vicinity of Kyushu Island. Among
the latter, MCSs with low cloud-tops below 4 km MSL
(Mean Sea Level) are found.
In the comparison with the present climate, the
averaged cloud and rain water mixing ratios in the
vicinity of Kyushu Island become much larger, and the
peak altitude of the mixing ratios are about 0.5 1.0 km
higher in the global warming climate. The cloud water
mixing ratio between 2 4 km MSL increases in the
global warming climate, corresponding to MCSs with
low cloud-tops.
These results suggest one of the processes to
produce heavy rainfall in the vicinity of Kyushu Island
in July in the global warming climate.
1. Introduction
The impact of global warming on the present climate
is one of the most highlighted subjects from the viewpoints of economic activity and the global environment
(IPCC 2001). There have been many attempts to
approach this subject by numerical simulations.
However most of these simulations were performed by
hydrostatic models with relatively coarse resolution,
focusing on the large-scale circulation and the global
climate change. In order to reveal the impact of global
warming on the present regional climate change, a highresolution non-hydrostatic regional model (NHM) which
treats cloud microphysics explicitly should be adopted.
Such model can represent not only the horizontal distributions of rainfall in detail but also the 3-dimensional
structures of mesoscale convective systems (MCSs).
Yoshizaki et al. (2005) presented an overview of such
NHM results with the horizontal grid of 5 km, and they
reported that the monthly mean precipitation in the
vicinity of Kyushu Island in July would become much
larger under the conditions of the global warming than
in the present climate. It is a great concern which
factors of the following cause this increase: the strucCorresponding author: Sachie Kanada, Meteorological
Research Institute, 1-1, Nagamine, Tsukuba 305-0052, Japan.
E-mail: [email protected]. ©2005, the Meteorological
Society of Japan.
ture-change of MCSs, the increase of supplied water
vapor or the change on the appearance frequency of
MCSs.
Multi-scale features of MCSs observed during the
Baiu season have been statistically examined by
Ninomiya and Akiyama (1992). The MCSs described in
this study belong to the meso-- and meso--scale cloud
systems. The main purpose of this study is to investigate changes in the structures of MCSs that bring the
heavy rainfall in the vicinity of Kyushu Island during
the late Baiu season as a first step to reveal the impact
of global warming on the regional climate by using a
non-hydrostatic regional model with the horizontal
resolution of 5 km.
2. Comparison of accumulated precipitation
in the present and global warming climates
The NHM time-slice climate simulations in the
present and global warming climates were performed
from June to July for ten years, since the Baiu frontal
activity in East Asia was one of our main targets. The
NHM with a horizontal grid of 5 km was nested in the
results of a hydrostatic global climate model (AGCM)
with a horizontal grid of 20 km. The model domain of
the NHM was 800 × 600 horizontal grids (4000km ×
3000km) and 48 vertical layers (top height: 22 km),
covering the wide region from the eastern part of the
Chinese Continent to the Japan Islands. The detailed
designs of the AGCM and the NHM are described in
Kusunoki et al. (2005) and Yoshizaki et al. (2005), respectively.
According to Yoshizaki et al. (2005), the precipitation in the vicinity of Kyushu Island increases in the
global warming climate. The Baiu season also often lasts
until August or has no end in the extreme cases. The
horizontal distribution of the monthly mean daily precipitation in July in the global warming climate is
shown in Fig. 1a. An intense precipitation becomes concentrated around Kyushu Island and on its downwind
side in the global warming climate. The temporal variations of 10yr-averaged daily precipitation averaged in
the vicinity of Kyushu Island in the present and global
warming climates from June to July are shown in Fig.
1b. The daily precipitation in the present climate increases at the beginning of June and decreases in the
middle of July, showing the onset and the end of the
Baiu season, respectively. On the other hand, the daily
precipitation maintains a high intensity of nearly 10
mm day 1 even at the end of July in the global warming
climate. Note that the averaged daily precipitation
during the first half of July becomes much larger in the
global warming climate. The MCSs that bring this
heavy rainfall are investigated in detail in the following
sections.
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Kanada et al., Heavy Rainfalls using a Cloud Resolving Model
days over Kyushu Island in July are chosen for the
analysis. Rainfall events associated with typhoons are
excluded. A total of 12 cases are found in the present
climate. The same number of 12 cases is selected from
the results of the global warming climate. A timeaveraged horizontal distribution of the cloud water
mixing ratio qc, averaged vertically around the melting
layers for each climate is shown in Fig. 2. The regions
with moderate qc (6 × 10 5 kg kg 1 ) are located around
northern Kyushu in the present climate (Fig. 2a), while
the qc is less than 5 × 10 5 kg kg 1 over the southern part
of the East China Sea. On the other hand, two regions
with large qc are found in the global warming climate
(Fig. 2b): one extends from the eastern coast of the
Chinese Continent and the other from the southern part
of the East China Sea to Kyushu Island. They merge
over the sea southwest of Kyushu Island, on the
downwind side of which an abrupt increase of qc exceeding 8 × 10 5 kg kg 1 is found.
These regions with large qc are caused by 1) MCSs
traveling from the eastern coast of the Chinese
Continent (hereafter referred to as Type-1 MCS), 2)
MCSs traveling from the southern part of the East
China Sea (referred to as Type-2 MCS), and 3) the
merger Type-1 and Type-2 MCSs (referred to as Type-3
MCS). Among the 12 selected cases in the global
warming climate, the merger of Type-1 and Type-2
MCSs was found for 5 cases, while only one merger
occurred in the present climate.
Fig. 1. (a) Horizontal distribution of monthly mean accumulated daily precipitation in July in the global
warming climate, and (b) temporal variations of 10yraveraged daily precipitation from June to July in the
present climate (black dotted line) and the global
warming climate (red solid line) averaged in the rectangle shown in Fig. 1a.
3. Time-averaged characteristics of MCSs
that bring rainfall in the vicinity of Kyushu
Island in the present and global warming
climates
Fig. 2. Time-averaged horizontal distributions of the
mixing ratios of cloud water (× 10 5 kg kg 1) around the
melting level: (a) the present climate and (b) the global
warming climate. Vertical average was made between
4.20 5.52 km and 5.06 6.50 km for (a) and (b), respectively.
The time-averaged characteristics of MCSs that pass
over Kyushu Island during the late Baiu season are
compared between the present and global warming
climates.
Yasunaga et al. (2005) showed that the number of
MCSs that appear in the vicinity of Kyushu Island in
July increases in the global warming climate. MCSs, appearing in the global warming climate, travel from both
the eastern coast of the Chinese Continent and the
southern part of the East China Sea to Kyushu Island.
They concentrate in the vicinity of Kyushu Island and
on its downwind side. However, in the present climate,
most of MCSs travel from the eastern coast of the
Chinese Continent through the northern part of the East
China Sea to Kyushu Island.
Since our focus is on the relation between the
increase of precipitation in the vicinity of Kyushu
Island during the late Baiu season and the abovementioned MCSs, rainfall events that last longer than 2
Figure 3 shows the time-averaged longitude-height
cross-sections of qc averaged between 30 and 31 N in the
present and global warming climates. In the present
climate, qc larger than 4 × 10 5 kg kg 1 is found only over
the northern part of the East China Sea (120 130 E). On
the other hand, the qc in the global warming climate
starts to increase around 124 E and attain 5 × 10 5 kg
kg 1 in the southern part of Kyushu Island (130 134 E).
Other area with moderate qc larger than 4 × 10 5 kg kg 1,
corresponding to Type-1 MCSs, is found over the
northern part of the East China Sea. In the global
warming climate, the peak altitude of qc is about 0.5 1.0
km higher than that in the present climate. The depth of
the high qc layer (qc > 1 × 10 5 kg kg 1 ) in the global
warming climate extends widely from a height of 2.5
km to 6.5 km MSL in the vicinity of Kyushu Island.
Figure 4 shows the vertical profiles of qc and the rain
water mixing ratio qr averaged in the rectangle shown
SOLA, 2005, Vol. 1, 117 120, doi: 10.2151/sola. 2005 031
in Fig. 1a. Both qc and qr in the vicinity of Kyushu Island
in the global warming climate become much larger than
those found in the present climate, and the peak altitudes of qc and qr become about 0.5 km higher than
those of the present climate. Note that a remarkable
increase in qc is found between 2 4 km MSL in the
global warming climate.
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a region with qc larger than 5 × 10 5 kg kg 1 appears at a
height of approximately 3 km over the East China Sea
(127 129 E, 29.5 N). This region will hereafter be called
a low cloud-top (LCT) system. Another region with
moderate values of qc widely extends to the west (123
127 E) of the LCT system at a height of 6 km. The LCT
system travels slowly to the northeast, maintaining its
low cloud top lower than 4 km until 3 hr. At 6hr, these
two regions merge over the sea southwest of Kyushu
Island (129 E, 30.4 N), and after then they rapidly
develop while increasing qc and qr (6 and 12hr). At 9hr,
the MCSs reach Kyushu Island, and at 12hr, they extend
over Kyushu Island, accompanying with the wide precipitation.
Fig. 3. Time-averaged longitude-height cross-sections of
cloud water mixing ratios averaged between 30 and 31
N in (a) the present climate and (b) the global warming
climate.
Fig. 4. Time-averaged vertical profiles of (a) cloud water
and (b) rain water mixing ratios averaged over the rectangle shown in Fig. 1a. The black lines show the
present climate and the red lines show the global
warming climate.
4. Temporal variations of Type-3 MCS in the
global warming climate
In this section, one case of the Type-3 MCSs defined
in the last section, is selected to study the merger of
Type-1 and Type-2 MCSs in detail. This MCS corresponds to the meso--scale cloud system, described in
Ninomiya and Akiyama (1992).
Figure 5 shows the horizontal distributions of qc and
the temporal variations of vertical cross-sections of qc
and qr for the selected case. Each vertical cross-section
displays the most intense part in qc of the MCSs. At 0 hr,
Fig. 5. (a) Horizontal distributions of cloud water mixing
ratios at heights of 6.0 (color shade) and 3.4 km MSL
(contour lines), respectively, where the contour lines are
drawn for 0.5, 1, 3, 5 and 10 (× 10 5 kg kg 1 ). (b) The
temporal variation of vertical cross-sections of cloud
(color shade) and rain water (contour lines) mixing
ratios (× 10 5 kg kg 1), respectively, where contour lines
are drawn for 1, 5, 10 and 50 (× 10 5 kg kg 1). Arrows in
(a) and in the top panel of (b) indicate the target LCT
system. The vertical cross-section in (b) at 0hr is taken
along the line A B in (a).
These results show one of the possible processes to
produce heavy rainfall in the vicinity of Kyushu Island
120
Kanada et al., Heavy Rainfalls using a Cloud Resolving Model
during the late Baiu season in the global warming
climate. A remarkable increase of qc between 2 4 km
MSL in Fig. 4a supports the frequent appearance of LCT
systems in the global warming climate.
Now, the formation and maintenance mechanisms of
the LCT systems are considered. The annual mean
surface air temperature and the sea surface temperature
over the East China Sea in the global warming climate
are expected to become about 2 3 degrees higher than
those in the present climate (Uchiyama et al. 2003), and
abundant water vapor is supplied from the East China
Sea in the global warming climate (Wakazuki et al.
2005). However, LCT systems maintain their characteristics for more than 3 hrs, while the other MCSs with
cloud tops exceeding a height of 6 km also appear in the
global warming climate (Fig.5).
Figure 6 shows the longitude-height cross-section of
the vertical velocity along 28N in the vicinity of the
East China Sea before a MCS with high cloud-tops
catches up with the LCT system and the LCT system described in the last section intensifies. Several shallow
cores of the cloud water mixing ratio are located at a
height of 3 km (125 127 E and 131 133 E). Downdrafts
widely prevail above a height of 3 km, while weak
downdrafts and updrafts intermingle below that height.
Some cloud water begins to be formed around 3 km
MSL, just on the boundary of these layers. According to
Kusunoki et al. (2005), the North Pacific high in July
tends to extend westward in the global warming
climate, covering a wide area over the southern part of
the East China Sea. Therefore the subsidence in the
North Pacific high suppresses the activity of MCSs
there. The places where the downdrafts above a height
of 3 km found in Fig. 6 correspond to the periphery of
the subsidence. However, more detailed studies are
needed to reveal the formation and maintenance mechanisms of the LCT system by using the results of both
the present and global warming climates simulations.
vicinity of Kyushu Island become much larger and their
peak altitudes become about 0.5 1.0 km higher in the
global warming climate. A remarkable increase of the
cloud water mixing ratio is found between 2 4 km MSL.
In the global warming climate, two types of MCSs
appear in the vicinity of Kyushu Island. One travels
from the Chinese Continent and the other from the
southern part of the East China Sea to Kyushu Island.
These two MCSs often merge over the sea southwest of
Kyushu Island, and after then they rapidly develop to
bring heavy rainfall in the vicinity of Kyushu Island.
Among the MCSs from the southern part of the East
China Sea, MCSs with low cloud-tops below a height of
4 km MSL are found. In the present climate, however,
fewer MCSs travel from the southern part of the East
China Sea, and little merger events are seen over the sea
southwest of Kyushu Island.
These results suggest one of the processes to
produce heavy rainfall in the vicinity of Kyushu Island
in July in the global warming climate. Further studies
are needed to clarify whether or not the characteristics
and structures of MCSs are changed in the global
warming climate. The formation and maintenance processes of the low-cloud top systems and their influence
on heavy rainfall in the vicinity of Kyushu Island
should be also clarified in near future.
Acknowledgments
This study is supported by the Ministry of Education, Culture, Sports, Science and Technology, and
numerical simulations are performed at the Earth
Simulator. The authors are grateful to Dr. T. Matsuo
(JMA), Dr. T. Aoki, and Kyosei-4 modeling group for
carrying out the project and two anonymous reviewers
for constructive comments.
Fig. 6. Longitude-height cross-section of vertical
velocity and mixing ratio of cloud water at 0 hr along
28 N before a MCS with a high cloud-top from the west
catches up with the LCT system. Color shades and
contours indicate the vertical velocity (cm s 1 ) and the
cloud water mixing ratio in the global warming climate,
respectively. Contours are drawn from 0.2 to 0.8 × 10 5
kg kg 1 with an interval of 0.2 × 10 5 kg kg 1.
5. Summary
In this study, mesoscale convective systems (MCSs)
in the vicinity of the East China Sea and Kyushu Island
during the late Baiu season are compared between the
present and global warming climates by using the
results of the NHM time-slice climate simulations with
the horizontal grid of 5 km.
In the comparison with the present climate, the
averaged cloud and rain water mixing ratios in the
IPCC (Intergovernmental Panel on Climate Change), 2001:
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Kusunoki, S., J. Yoshimura, H. Yoshimura, A. Noda, K. Oouchi
and R. Mizuta, 2005: Change of Baiu in global warming
projection by an atmospheric general circulation model
with 20-km grid size. J. Meteor. Soc. Japan, (submitted).
Uchiyama, T., A. Noda, S. Yukimoto and M. Chiba, 2003: Study
of the estimate of new climate change scenarios based on
new emission scenarios-IPCC AR4 Experiments, CGER’s
Super Computer Activity Report, 12 2003, CGER-I0612005, Center for Global Environmental Research/National Institute for Environmental Studies, Japan.
Ninomiya, K., and T. Akiyama, 1992: Multi-scale features of
Baiu, the summer monsoon over Japan and the East Asia.
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Changes of the Baiu frontal activity in the global
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Yoshizaki, M., C. Muroi, S. Kanada, Y. Wakazuki, K. Yasunaga,
A. Hashimoto, T. Kato, K. Kurihara, A. Noda and S.
Kusunoki, 2005: Changes of Baiu (Mei-yu) frontal activity
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Manuscript received 28 April 2005, accepted 29 June 2005
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