Multi-year simulations of the East Asian monsoon
by using the regional climate model(RegCM-NCC)* Yihui Ding 1,Qiaoping Li 2
(1: National Climate Center, China Meteorological Administration,Bejing,100081,China
2: Institute of Atmospheric Physics, Chinese Academy of Sciences,Bejing,100029,China)* e-mail:Dingyh@cma.gov.cn
Abstract
By using the regional climate model(RegCM-NCC), the East Asian mean circulations during 1998 to 2002 aresimulated, and results show that the model can well reproduce the patterns and the intensity as well as seasonal march of theEast Asian monsoon. The seasonal variation of regional rainfall and temperature are reproduced in the model, with threenorthward shift time and intensity of the rain belts over the sub-regions well corresponding to the observation, but rainfall in
North China overestimated. Error evaluation shows that there is a systematic bias in the simulated air temperature in thelower troposphere in summer, which may be a main responsible factor for the stronger simulated summer monsoon, thus
resulting in the overestimated rainfall in North China.
Keyword: regional climate model, East Asian monsoon, numerical simulation

Model Introduction
The regional climate model (RegCM_NCC) used in
the study is developed based on the second generation of
the regional climate model of NCAR/RegCM2 (Giorgi etal., 1993a, b). By modifying and assembling the various
physical process parameterization schemes in the RegCM2,
the new regional climate model has been establishedduring 1996-2000 and has been used in climate simulationand seasonal prediction (Ding et al., 1998). Table 1 listed
the details of the physical schemes options for this study,
the lateral boundary data are provided every 12 hours, thesea surface temperature data is updating per 7 days.
Figure1 shows the simulated domain and the terrain
height. .
Tible1 Physical process schemes
Model Configuration
RegCM_NCC
Horizontal grids
60km
Vertical layers (top)16 sigma layers (50 hPa)
Cumulus convection
Betts-Miller(Betts A K,1986)
Moisture scheme
implicit
PBL
Holtslag(Holtslag et al.,1990)
Radiation
CCM3(Kiehl et al.,1996)
Land surface
LPMⅡ( Shi et al.,2000)
Boundary condition
Exponential relaxation
Initial, boundary data NCEP/NCAR Reanalysis II(SST)(NOAA:OI.V2)
Simulation period
1997.12~2002.12

Fig.1 Model domain and terrain height

Simulation analysis
The improved RegCM-NCC has been used in somecase of extremely weather or climate events study (Shi, etal., 2001), relatively little work was performed over East
Asia for long-term climate simulation. In this study, a5-year simulation over East Asia is conducted to examine
the model’s capability in reproducing seasonal variation ofEast Asian monsoon and precipitation in China. The
simulation period is selected from 1997 to 2002, comparedto the 30yrs climatological field, winter mean temperaturein the 5yrs is warmer in most regions of China, especiallyin 1998 and 2002, and summer mean precipitation is wet insouth of China and droughty in north of China, so simulate
result can help us to examine the model’s capability toreproduce the abnormal climate characteristics.
Evaluation has been conducted through comparing
the simulation with NCEP reanalysis data. Results showthat the model well reproduces seasonal variation of East
Asian circulation, such as the simulated location andintensity of South Asia high and the onset or retreat time ofWest Pacific subtropical high are consistent with theobservation. The simulated seasonal transition andintensity of winter monsoon is closer to the fact. The

characters of Siberia cold high and outbreak frequency andmain regions of “cold surge” over East Asia are also
simulated with realistic manner. However, the simulated
summer monsoon is stronger compared with NCEP
reanalysis field. Figure 2 presents the height field at500hPa, winter mean circulation is simulated very well,and the model also reproduces two major large-scale
circulation patterns in summer: one is the subtropical highover the northwestern Pacific Ocean and the other is a
mid-latitude westerly trough. However, the subtropicalhigh is more intense and the mid-latitude trough is
weakened compared to the reanalysis field. The locationand intensity of the subtropical high not only control theprogress of the East Asian monsoon but also influence thedistribution of precipitation over china. Further
investigation shows that the simulated skip time is similar
to the observation, but the location of subtropical high ismore intensive and being farther to north by 2-3 degrees
than normal.

Fig.2 500hPa height field (unit: gpm)
The model also well reproduced seasonal characterof the lower troposphere wind, in winter, there is strong
northwest wind from Siberian and northeast china to Japan,
in summer, a strong southwest monsoonal wind flow
northward along the subtropical high, but the flow is more
intense compared with the observation(sees in figure3).
The model well indicates the location and intensity ofupper-level jet in winter and south Asia high. The seasonal
character of air temperature and the spatial distribution andtransport of moisture in each layer are also well simulated.

Fig.3 850hPa wind (unit: m/s) (shaded area is Tibetan)
Bias analysis found that summer mean bias ofgeopotential height on lower troposphere(below 500hPa) is
positive over the ocean and negative over the land, on thecontrary, temperature bias being negative over ocean andpositive over land, and the systematic bias is not
remarkable in other season. The systematic bias
exaggerates the summer temperature difference between
the land and ocean, which may be one of the main
responsible factors for the stronger summer monsoon in themodeling.
The model well presents the seasonal spatial
characteristics of surface air temperature andprecipitation(sees in figure 4), the simulation in winter isbetter than that in summer. The simulated summer
precipitation in south China, north-west China are exactly
consistent with the observation, but in north china, theprecipitation is overestimated, which is consistent with thesimulation bias for subtropical high and summer monsoon.
On the other hand, there are illusive rain belts in someareas with high topography.

Fig.4 mean surface air temperature (unit:ºC)

in East Asia. In Development of Operational Short-term Climate
Model series.2. Y.H., Ding, ed. China Meteorological Press,Beijing, 217-231.

Shi Xueli, Ding Yihui, Liu Yiming, 2001: Simulation ofeastern China summer climate with a regional climate
model. Climate and Environment Research, 6,249-254

Fig.5 mean precipitation (unit: mm)

Conclusions and discussion
1. The model can well reproduce seasonal variation ofmean circulation in East Asia, the patterns and seasonal
march of the East Asian monsoon are well identified in themodeling, the simulated transition and intensity of wintermonsoon is closer to the observation, but the simulated
summer monsoon is stronger compared with NCEP
reanalysis field, with the location of subtropical high being
farther to north by 2-3 degrees than normal.
2. The spatial distribution of rainfall and airtemperature are well reproduced in the model, especially,
rainfall in mid-lower Yangtze River basins and south China
well corresponding to the observation, but rainfall in north
China is overestimated.
3. Error evaluation shows that there is a discernible
systematic bias in the simulated mean air temperature inthe lower troposphere in summer. The systematic bias
exaggerates the summer temperature difference between
the land and ocean, which may be a main responsible
factor for the stronger summer monsoon in the modeling,
thus resulting in the overestimated rainfall in North China.
The simulation bias may be also related to thecomplex topography and cloud-radiation parameterization
scheme, which would be the keystone for us to improve themodel. In addition, due to the limited calculation condition,
the simulation domain is too small to study the whole East
Asia monsoon.

References
Giorgi, F., M.R. Marinucci, G. De Canio and G.T. Bates, 1993a:
Development of a second generation regional climate model(RegCM2) I, Boundary layer and radiative transfer processes.
Mon. Wea. Rev., 121, 2794-2813.
Giorgi, F., Marinucci, M.R. and Bates, G.T. 1993b: Developmentof a second-generation regional climate model (RegCM2), Part II:
convective processes and assimilation of lateral boundaryconditions. Mon. Wea. Rev., 121:10, 2814-2832.
Ding, Y.H., Qian, Y.F., Yan, H., Gao, K., Shen, T.L., Miao, M.Q.,
Li, W.L., Liu, Y.M., Zhao, N., Yi, L. and Shi, X.L., 1998:
Improvement of high-resoulution regional climate model and its
application to numerical simulation of prolonged heavy rainfalls