A numerical study of mesoscale precipitating system along Meiyu Frontobserved during GAME-HUBEX 1998
*Kozo Nakamura1, 2, Yasushi Fujiyoshi1, 3(1: Frontire Research Center for Global Change, 2: Ocean Research Institute, University of Tokyo,
and 3: Institute of Low Temperature Science, Hokkaido University)* Ocean Research Institute, University of Tokyo, Nakano-ku, Tokyo, Japan, 164-8639.
e-mail: nakamura@ori.u-tokyo.ac.jp
Abstract
In order to improve our understanding of the mesoscale organization process, we performed a numerical experiment of
the disturbance observed near Fuyuang radar site (115.83 E, 32.93 N) on 2 July 1998, during GAME/HUBEX 98. The
model used in this study is Non Hydrostatic Model (NHM) developed by MRI/JMA. We performed two runs using
two datasets, i.e., GAME-Reanalysis dataset and ECMWF dataset, for initial and boundary conditions, and weinvestigate the organizing process by comparing the results of two runs. In both runs, the model successfully reproduced
the development of the large-scale horizontal shear, convergence, and the precipitation along the Mei-yu front. However,the disturbance of the GAME-Reanalysis run was too strong and moved too fast to the northeastward, and after its
movement, there is no intense convection. On the other hand, in the ECMWF run, there occurred many convection
systems along the front region and properly reproduced the persistent heavy rainfall area. We preformed several
experiments (the wetness parameter w=0.1 and w=1.0) to investigate the effect of the surface conditions. It is shownthat although the effect of the wetness parameter on the boundary layer structure is large, its effect on the convective
activity seems to be small.
Keyword: mesoscale convection, heavy rainfall

1. Introduction
One of the most important research subjects of the
GAME-HUBEX Project is to understand and to model
the mesoscale organization process of the cloud andprecipitation system along the Meiyu front, because
precipitation does not occur homogeneously but occurs
in some mesoscale organized convection system along
the front. There are several processes which areconsidered to be important for the development of themesoscale disturbances, such as, large-scale convergenceand northward moisture flux in the lower layer,southward flow of cold dry air in the middle layer, upper
layer disturbance along the jet stream, and sensible andlatent heat flux from the surface. In order to improve
our understanding of the development process of
mesoscale disturbance, we must investigate what is
important in these processes for each event.
There occurred several heavy precipitations during
the Intensive Observation Period (IOP) of the
GAME/HUBEX experiment. In this paper, some resultsof the simulation of the mesoscale disturbances occurredon 2 July during GAME/HUBEX IOP 98 are reported.
It is shown that the precipitation occurred in the two
successive mesoscale disturbances, which moved along
the Meiyu Front. Therefore, it is important to simulate
the successive development of the mesoscale system inorder to simulate the heavy rainfall. We will simulate theheavy rainfall using several analysis datasets. By
comparing the results of the runs of the different datasets,
we will examine the development process of the
disturbances.

as is investigated by many authors (e.g., Xu and Xu,
1999; Fujiyoshi et al, 2000; Uyeda, 2000; Maesaka andUyeda, 2000). During this period, Meiyu front was
located along the southwest-northeast direction, andseveral mesoscale precipitation systems moved along the
Meiyu front one after another.
The precipitation
systems brought about a heavy rainfall along the Meiyu
front as is shown in Fig. 1. The Fuyuang radar (115.83
E, 32.93 N) is located in the heavy rain band region.
We used two analysis data sets for the initial andboundary conditions. The two data sets are
GAME-Reanalysis dataset, and ECMWF dataset. Fig. 1
shows the analyzed data of daily precipitation of
ECMWF dataset integrated for 24 hours from 07/01/00
UTC (Universal Time Coordinated), i.e., 07/01/08 BST(Beijing Standard Time), to 07/02/00 UTC, i.e., 07/02/08
BST.
Fig. 2 shows the hourly rainfall estimated from
Fuyuang radar observation. From 20 BST (12 UTC)
to 04 BST (20 UTC), a rain band along from thewest-southwest to the east-northeast moved overFuyuang radar site. Another rain system developed

2. The precipitation system observed on 2 July
1998
The rainfall we simulated in this study is the one
observed near Fuyuang radar site (115.83 E, 32.93 N) on
2 July 1998. This is one of the several heavy rainfalls
during the HUBEX Intensive Observation Period 1998,

Fig. 1: Analyzed daily precipitation on 1 July 1998 in
ECMWF analysis data. The model domain of thenumerical simulation is indicated in the figure.

Fig. 3: Two analyzed data of 700 hPa wind and verticalvorticity, 06UTC 1 July. GAME –Reanalysis (left)
and ECMWF (right) data.

Fig. 2: Hourly rainfall estimated from Fuyuang radarobservation from 20 BST (12 UTC) 1 July to 08
BST (00 UTC) 2 July.
about 00 BST (16 UTC) at 114E and moved after theprevious system.
The time sequence of TBB(equivalent blackbody temperature) observed by GMS
shows two cold region corresponding each precipitation
system (Figure is not shown.).
It indicates that the precipitation does not occur
continuously, but occurs in the mesoscale organized
precipitation systems which continually occur one after
another. As is shown in these figures, it is important tosimulate the subsequent occurrence of the mesoscaledisturbances in order to simulate the heavy precipitation
occurred on 2 July 1998. In the simulation of thesubsequent occurrence of the mesoscale disturbances, we
can investigate the mechanism of the organization of theconvection.

3. ModelThe model used for the simulation in this study is
the NHM (Non-Hydrostatic Model) developed by MRI(Meteorological Research Institute) / JMA (Japan
Meteorological Agency). The details of the model are
shown in http://www.mri-jma.go.jp/Dep/fo/mrinpd/
INDEXE.htm. The domain of the model is shown in
Fig. 1. It is a square about 2500km×2500km wide inorder to include the whole lifetime of the mesoscale
systems, and its center is located about at (113E, 33N)
near to the Fuyuang radar. The horizontal grid interval
is 13.5 km, and there are 38 layers in the vertical. We
used the convective adjustment scheme for cumulus
parameterization. There are four soil layers and theland surface temperature is predicted by the heat balance
equation with wetness parameter denoted by w. In thecontrol runs, we set w=0.1 for land surface and w=1.0
for sea surface.
In this study, we used two data sets in order to makeinitialandboundary
conditions.
One
isGAME-Reanalysis data sets made by JMA, and the otheris the analysis data by ECMWF. The horizontal
resolution of the two data sets is 0.5 deg, and their timeinterval is 6 hours. We used the data at 06 UTC, i.e., 14
BST on 1 July for the initial conditions and the model
was integrated for 18 hours.
Fig. 3 shows horizontal vorticity and wind at 700hPa
at 06 UTC 1 July in the two analysis data. In both data,

they analyzed a subtropical high to the south of Japan,
and strong southerly wind in the southern part of China,
which went to the large shear zone at the Meiyu front.
In the lower atmosphere, the wind transports a large
amount of humidity toward the Meiyu front zone.
Around the Korean peninsula, there is a low pressure,which brought about the heavy rain on 29-30 June.
After the passage of the low pressure, there occurred
northerly flow in the northern part of China. Therefore,
the Meiyu front zone is also a convergence zone between
the southerly and northerly flows. The difference in thewind and vorticity fields between the two data sets does
not seem to be so large to make large difference in thedevelopment of the disturbances.
The horizontal
convergence in the lower layer is about 5x10-5 at its
maximum in ECMWF data and it is a little larger than in
GAME-Reanalysis data. On the other hand, the verticalvorticity in the lower layer is about 1.5x10-4 at its
maximum in both GAME-Reanalysis data and ECMWF
data.
There is a large difference between the two data inthe surface air temperature at 06 UTC. It is much
warmer in the southern part of China in GAME
Reanalysis data than in ECMWF data. It seems to
produce active convection that might enhance themoisture transport to the front area. However, thespecific humidity in the region is smaller in GAME
Reanalysis data than in ECMWF data. So, although theactivity of the convection in the lowermost layer is activein the results using GAME Reanalysis data, theconvection does not penetrate to the upper level toproduce active condensation.

4. Results and discussions
As a whole, the model successfully reproduced thedevelopment of the large-scale horizontal shear,convergence, and the precipitation along the Meiyu front.
In both runs, there occurred convective rain around theFuyuang radar area at t=8-9UTC, i.e., 2-3 hours after the
initial time. (Figure is not shown.) However, thedisturbance of the GAME-Reanalysis run moved to thenortheastward, and after its movement there was nointense convection around the region, because theconvection, which occurs at the convergence zone
between the outflow from the previous convection andthe southerly flow in the southerly region of the previous
convection, also moved toward the northeastward along
the flow. On the other hand, in the ECMWF run,
although the disturbance first developed around theFuyuang radar moved to the northeastward, there
developed several disturbances one after another along
the Meiyu front, and the rainfall lasted after the firstFig. 4: Simulated precipitation from 17 UTC to 18 UTC
1 July. Arrows indicate horizontal wind, andcontour lines indicate specific humidity at
GAME-Reanalysis (left),
18UTC, z=1.5km.
ECMWF (right)
convection moved eastward.
The convection in the GAME-Reanalysis run moved
toward northeastward, and the velocity of the movement
is only a little smaller than the wind velocity. Therefore,
the relative wind to the convective motion, whichindicates the inflow to the convection, is from north andthe convection at the point can be maintained only forfew hours. On the other hand, in the ECMWF run, thespeed of movement of the convection is much slower
than the wind speed. The relative wind is from the south,
and the downdraft makes the cold pool, over which thesoutherly wind goes upward. The southerly air is very
moist and the convection is maintained more than a few
hours at the same position.
Fig. 4 shows the one-hour rainfall from 17 UTC to
18 UTC, i.e., from 11 hours to 12 hours after the initialtime, and horizontal wind at z = 1.5 km at the time of 18
UTC. Although there is a large southerly wind andlarge convergence along the front area, there is nointense rainfall at the southwestern region of this figurein the GAME-Reanalysis run (left figure). On the other
hand, in the ECMWF run, there occurred many
convection systems along the front region and they
moved along the front region. Therefore, in the
ECMWF run, the "heavy rainfall area" is well simulated,
as is shown in Fig. 4 (right).
The difference between the two runs is wellindicated in Fig. 5, which shows the 18-hour integrated
precipitation in the two runs. In the ECMWF run(center), there occurred many rain systems frequently
along the Meiyu front and the total amount becomes asmuch as was observed.
However, in the

GAME-Reanalysis run (left), the total rain is large onlyin the eastern region in the figure, and it is smaller thanthe observed in the western part along the Meiyu front.
(The right figure in Fig. 5 is the result of the modified
GAME-Reanalysis data run, and will be discussed later.)
It is often said that such meso scale cloud system isoften associated with a large vorticity on the rightforward (moving direction) region of the precipitation
area. In this case, the right forward region is thesoutheast region, where intense vorticity was observed,
and the relation between the precipitation and vorticity
agrees well with the usual cases. This simultaneous
occurrence of the precipitation and vorticity is wellindicated in Fig. 6, which shows the longitude-time
section of the maximum values of vertical vorticity
below z<4.2km and rain intensity between latitudes of
31N and 33.5N. In both runs (upper and middle), theheavy rain system which appeared around 115E at 09
UTC moved eastward at the velocity of about
100km/hour. The system was associated with the large
vorticity. The maximum value is about 7x10-4 s-1 inboth runs. After the movement of the first system toward
to the east, the vorticity around 110E-115E became small
in the GAME-Reanalysis run, while it remained a large
value in the ECMWF run. It seems that the vorticity isimportant for the successive occurrence of the many
convective systems along the Meiyu front, while thevorticity is produced by the large heating due to theconvection. This kind of interaction between theconvection and the vorticity is important for the heavy
precipitation that is brought about by the several
mesoscale disturbances occurring one after another.
There are several processes which are considered to
be important for the mesoscale organization process ofthe convective systems. One of them is condensational
heating and its effect on the producing depression.
The heating is brought about from the surface sensible
and latent heating, and/or moisture supply by thelarge-scale southerly wind. And another process is theeffect of upper layer trough, which brings about thelarge-scale upward motion and enhances the convective
activity. We performed several sensitivity runs in whichsurface and lower moisture conditions are changed.
At first, we discuss the results of the runs performed
to investigate the effect of the wetness parameter w in the
NHM. In NHM, the land surface condition is represented
by the wetness parameter, w. The latent heat flux at theFig. 5: Simulated precipitation integrated during 18-hour from 06 UTC 1 July to 00 UTC 2 July.
GAME-Reanalysis(left), ECMWF(center), and modified GAME-Reanalysis(right) runs.

precipitation, increasing humidity is not enough for theconvection to be organized to bring about the large
precipitation observed, because the increased humidity
enhances the convective activity in the wide area of thesouth China region, and it does not lead to theenhancement of the mesoscale depression/vorticity along
the Meiyu-front region which is important for theconvections to be organized.

5. SummaryFig. 6: Longitude-Time section of maximum verticalvorticity (shading) and maximum rain intensity(contour). The contour interval is 10 mm/hour.
GAME-Reanalysis (upper), ECMWF (lower).
surface is calculated by the bulk formula using the"moisture difference between the air and saturation
surface value" multiplied by w. We preformed several
experiments to investigate the effect of the surface
conditions.
Although the effect of the wetness parameter on theboundary layer structure is large, its effect on theconvective activity appears to be small. When thesurface wetness is large, the moisture becomes large inthe boundary layer. However, the surface temperaturedoes become lower because of the cooling due to theevaporation (latent heat flux). Because the effects ofthese two processes on the convective activity areopposite with each other, the convection does not
becomes so active. As a result, the effect of the surface
wetness parameter on the convective activity becomes
small in the experiment.
The effect of the dryness in the boundary layer in
GAME-Reanalysis data on the weak precipitation is
examined by another sensitivity run, in which theartificial moisture is added to GAME-Reanalysis
humidity data. The additional humidity is 2 g/kg below
z*=1.2km, and it decreases to zero at z*=5km. The valueof the 2 g/kg is the value of the humidity difference
between GAME-Reanalysis and ECMWF datasets
averaged over the whole domain.
The result partly shown in the right in Fig. 5
indicates that (1) there is a rain region along the frontline, although the intensity is weak, (2) the total
precipitation is much increased by the modification of
the humidity profile, however, the rain is distributed in
the wide area, and the generation of vorticity is not soenhanced, and (3) the maximum value of the
precipitation at each point is not so increased as in the
result in ECMWF run.
It is shown that although the total precipitation
averaged over the whole domain becomes as large as in
the result of the ECMWF run, the precipitation intensity
does not become so large. It is because the convection
is not so organized as in the run of the ECMWF run.
The results indicate that although the effect of theboundary layer humidity is important for the large

A numerical experiment of a mesoscale disturbanceobserved on 2 July 1998 along Meiyu front was
performed using the two (GAME Reanalysis and
ECMWF) datasets.
The simulated mesoscaleorganization largely depends on the analysis data. In
the ECMWF run, there occurred successive convection
along the Meiyu-front region and the heavy rainfall iswell simulated. However, in the GAME-Reanalysis run,the successive convection is not well simulated. The
effect of the land surface condition seems to be small.
The effect of the lower layer humidity on the
precipitation amount is large, however, the increase of
the humidity in the lower layer is not enough for theconvection to be organized to bring about the heavy
precipitation observed, because the increased humidity
enhances the convective activity in the wide area of thesouth China region, and it does not lead to theenhancement of the depression/vorticity of the
Meiyu-front region.
The results indicate that theinitialization including the meso-scale disturbance isimportant for the simulation of the heavy precipitation
because the disturbance, including the spatial distributionof humidity, is important for the successive developmentof convection that brings about the heavy precipitation.

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