Regional Climate Modeling for East-Asian Monsoon System–A Review
Yuqing Wang
International Pacific Research Center and Department of Meteorology, School of Ocean and Earth
Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
e-mail: yuqing@hawaii.edu
Abstract
In this talk, the application of regional climate models (RCMs) to the study of East-Asian monsoon
system is reviewed, including the assessment of possible impacts of land-use and land cover changes onregional climate over East Asia, the regional impacts of global warming due to greenhouse effects, the studyof physical processes in the monsoon system, the applications of RCMs to seasonal predictions of EastAsian monsoon rainfall. Several directions that the regional climate modeling community could contribute toboth the monsoon studies and the global climate modeling of monsoon systems in general will be also
discussed.
Keyword: regional climate modeling, East Asian, monsoon system

1. Introduction
East Asia has a large latitudinal coverage withthe highest mountains on the earth and the longcoastline. The regional climate over the East Asia isdominated by both the complex monsoon systemand the rapid seasonal transitions. It involves
complex interactions among tropical, subtropical,
and mid-latitude weather and climate systems, andthe land-atmosphere-ocean interactions at various
spatial and temporal scales, ranging fromclimatological modulation of planetary generalcirculations to the local elements, such as terrain
slope, vegetation, and land-sea breeze. Regional
climate over East Asia and its variation exert
significant impacts on human lives and natural
ecosystems in the region, especially the available
water resources, flood/draught potentials, andagricultural productivity.
Because of the complexity of the East Asianmonsoon system, its realistic simulation by climate
models has been a challenging task for many years,
while it has attracted increasing attention of theclimate modeling community. In addition to thecontinuous improvements of global circulation
models (GCMs) in simulating the East Asianmonsoon system, one of the significant
advancements in the last decade or two has been theapplications of regional climate models (RCMs) tothe study of East Asian monsoon system. Comparedwith its global counterpart, the RCMs have beenshown considerable improvements in simulating not
only the mean but also the variability of the East
Asian monsoon system. In particular, the RCMs
reproduce better the extreme precipitation events
due to the use of higher-resolution that better
resolves the land surface properties and mesoscale
circulations. In this review, not only will the current
status of regional climate modeling for East Asianmonsoon system be discussed, but also the future
directions in this important area will be highlighted.

2. Historical background
Motivated by the need of regional climate
information to understand regional climate change
and its impacts, regional climate models (RCMs)

were first developed mainly as a dynamical
downscaling tool to address global change issues.
Since the first successful demonstrations of regional
climate modeling by Dickinson et al. (1989) and
Giorgi and Bates (1989), much effort has been
devoted to the development, evaluation, andapplication of RCMs. The principle behind regional
climate modeling is that given detailed
representations of physical processes and highspatial resolution that resolves complex topography,
land-sea contrast, and land use and land cover, alimited area model can generate realistic regional
climate information consistent with the driving large
scale circulation supplied by either global reanalysis
data or a general circulation model (GCM).
RCMs have been used to improve the simulations
and physical understanding of the East Asianmonsoon system since their emergence. Liu et al.
(1994) are among the first to simulate a summer
season climate over East Asia with the RCM
developed at the National Center for AtmosphericResearch (NCAR_RegCM). They showed that themodel could reproduce the basic characteristics ofthe East Asian monsoon climate and the associatedseasonal rain belt migration. The performance of
NCAR_RegCM in the simulation of June andJanuary climates over East Asia was verified by
Kato et al. (1999), who identified several
discrepancies of the model in the region, including
the simulated too strong weather systems, cold bias
in winter, and overestimated summer rainfall. The
NCAR_RegCM was coupled with a sulfate aerosol
model by Qian and Giorgi (1999) and was used to
study both the indirect and direct effects of aerosolson the regional climate over East Asia. Leung et al.
(1999) compared the performances of three RCMs
(all were based on the NCAR_MM4 but withdifferent model physical parameterization schemes)in simulating the heavy precipitation during the1991 summer monsoon over East Asia with thefocus on the effects of physical parameterizations onregional climate simulations. Hong et al. (1999)
evaluated the performance of the National Centers
for Environmental Prediction (NCEP) regional

spectral model in the simulation of the East Asianmonsoon in July 1987 and 1988. Sasaki et al. (2000)
evaluated the performance of the MeteorologicalResearch Institute (MRI) regional spectral model forthe cases of January and August 1994, with thefocus on the evaluation of a spectral lateralboundary coupling method.
The above-mentioned studies were followed by asurge of the applications of RCMs to the study of
the East Asian regional climate with the focus on
the monsoon system in the early 21st century, andalso several new RCMs have been developed for theregion (Fu et al. 2000; Wang et al. 2003). Some
major achievements in the study of East Asianmonsoon using RCMs will be reviewed in thefollowing sections.

3. Impact of land-use changes
Land use changes modify the exchange of energy,
momentum, moisture, and trace gases, affecting theearth’s climate. Land use changes are highly
inhomogeneous spatially. The high-resolution of
RCMs is ideal for assessing the impacts of land usechanges at different scales on regional climate. Landuse and land cover changes in East Asia have been
one of the largest regions in the world. More than
60% of the region has been affected by conversionof natural vegetation into farmland or grassland into
semidesert in the past 300 years. These land
use/cover changes alter the surface albedo,
roughness, leaf area index, and fractional vegetation
coverage, etc.
Kanae et al. (2001) found a long-term decreasing
trend in the 40 years precipitation observed in
Thailand. The trend is apparent only in the monthly
mean precipitation during September. Results fromthe CSU_RAMS simulations showed that,consistent with observations, the effect of
deforestation in the Indochina peninsula issignificant only in September, when the synoptic
wind is mild. In a recent study, Sen et al. (2004a)
studied both the local and remote effects of the
Indochina deforestation on the East-Asian summer
monsoon in the flood year 1998 using the RCM
developed at the International Pacific ResearchCenter (IPRC-RegCM, Wang et al. 2003). Since thedeforested Indochina Peninsula is subject to strong
monsoonal flow during summer months, in addition
to the local effect, there is a strong remote effect on
downstream summer monsoon rainfall over EastAsia.
Fu and Yuan (2001) studied the impact ofrecovered natural vegetation on the summer climate
using the regional integrated environmental model
system (RIEMS). They found that recovering thenatural vegetation in large scale could havesignificant effects on the summer climate over East
Asia, not only changing the surface climate, but also
modifying the summer monsoon circulation to someextent. Sen et al. (2004b) investigated the local andregional effects of vegetation restoration in northern

36o-42oN)
using
the
China
(90o-110oE,
IPRC_RegCM, and evaluated whether the changes
in rainfall induced by landscape change are large
enough to support a restored vegetation cover. Theirresults suggest that reclaiming the desertified lands
through vegetation restoration is quite difficult as
the local climate, even after a complete vegetation
restoration in a relatively large test area, can hardly
support vegetation. This is because rainfall increases
but the increase occurs in rainfall intensity and not
in rainfall frequency, the latter being probably more
important for the maintenance of the restored
vegetation in such an environment.
Zheng et al. (2002) studied the regional climate
effects of vegetation change in China using the
NCAR_RegCM2 and found that the increase inflood events over Yangtze-Huai valleys and theaggravation of north China droughts are likely due
to the combined effect of desertification of thenorthern grassland and southern degradation of
evergreen broadleaf trees, with the latter playing agreater role. Fu (2003) also found that the changes
in land cover have resulted in significant changes tothe East Asian monsoon, including the weakening of
the summer monsoon and enhancement of thewinter monsoon. As a result, precipitation, runoff,and soil water content are all reduced in many partsof the region, particularly over the northern China.
The significant impacts of land cover change on
East Asian monsoon climate have also been
demonstrated in several other studies (e.g., Wang etal. 2003; Shi and Wang 2003; Gao et al. 2003; Suhand Lee 2004).

4. Regional impact of global change
The use of RCMs in climate change research has
grown rapidly over the last decade as indicated by
the increasing volumes of literature cited between
the Second and Third Intergovernmental Panel on
Climate Change (IPCC) reports (IPCC 1996; IPCC
2001). The application of RCMs to climate change
has been recently reviewed extensively in IPCC
2001. Here only recent advancements in the study of
impacts of global warming on the East Asianmonsoon are briefly discussed.
Gao et al. (2001) investigated the impacts ofgreenhouse effects on climate change over China
with a revised version of the NCAR_RegCM nested
into the Australian CSIRO coupled GCM. In acontrol simulation, they showed that the simulationsof surface air temperature and precipitation over
China by the RCM are much improved compared tothe simulations by the global coupled model.
Results from the sensitivity experiment withdoubled CO2 showed remarkable increase in surface
air temperature, especially in winter season and innorth China, and a general increase in precipitationin most parts of China.
In a recent study, Chen et al. (2004) studied theregional climate change in East Asia due to doubled
CO2 with an interactive atmosphere-soil-vegetation

model nested in the NCAR climate system model(CSM). They found that bout a 20% increase inannual precipitation in coastal areas of central andnorthern China, but only by 8% in southern China.
The strongest warming of up to 4oC occurs insummer in northern China. The climate generally
tends to be warmer and wetter under doubled CO2
conditions except for inland areas of northern China,
where it becomes warmer and drier. Most of thesechanges are found to be associated with the more
intense East Asian monsoon under doubled CO2.
The above studies are mainly focused on themean climate change due to doubled CO2. Thechanges in extreme climate events should be equallyor even more important to society. Gao et al. (2002)
examined the possible changes in extreme climate
events over East Asia due to greenhouse effects.
They found that both daily maximum and daily
minimum air temperature increase in doubled CO2
conditions, but the diurnal temperature range
decreases due to the higher increase of minimum
temperature. The number of hot spell days insummer significantly increases while the number ofcold spell days in winter decreases. The number ofrainy days and heavy rain days increases over some
sub-regions of China, most noticeably north China
and southeast China.

5. Climate process studies
The use of high spatial resolution to resolve thecomplex lower boundary conditions and mesoscale
weather systems with the improved representationof model physics makes RCMs ideal for improving
our understanding of climate processes. Here wehighlight several studies relevant to the East Asianmonsoon system.
With the use of the NCAR_RegCM, Qian et al.
(2003) found that the increase in both snow coverand snow depth over the Tibetan Plateau in wintercan delay the onset and weaken the intensity of thesummer monsoon, resulting in a decrease inprecipitation in southern China and an increase in
the Yangtze and Huai River basins. They showedthat snow anomalies over the Tibetan Plateau
change the soil moisture and the surface temperature
first through the snowmelt process, and then alter
heat, moisture and radiation fluxes from the surface
to the atmosphere. The modified atmosphericcirculation in turn could also affect the underlying
surface properties. This interaction is the key to thesubsequent climatic changes downstream over
China.
The Meiyu/Baiu front genesis is considered to be
due to forcing from both land/sea contrast andorographic effect of Tibetan Plateau. Throughsensitivity experiment using the CSU-RAMS model,
Yoshikane et al. (2001) found that the Baiu front
could be generated by two factors: the zonal mean
field and the land/sea contrast. Although not anecessary condition, the orographic effect including
the Tibetan Plateau intensifies the low-level jet(LLJ) and the precipitation over the Baiu front.
Further, the condensational heating is necessary tokeep the LLJ to the south of the upper-level jet as
observed.
The presence of natural and anthropogenic
aerosols in the atmosphere can affect the climate
over the regions where aerosol loadings aresubstantial and the regions that are locateddownwind, such as the East Asia. Efforts to include
the interactive coupling of the climate and aerosols
were made by Qian and Giorgi (1999), Qian et al.
(2003), and Giorgi et al. (2002, 2003) using the
NCAR_RegCM and a simple radiatively active
sulfate aerosol model for climate simulations overEast Asia. Both direct and indirect aerosol effects
are represented and evaluated. It is found that theaerosol distribution and cycling processes show
substantial spatial and temporal variability. Bothdirect and indirect aerosol forcings have regional
effects on surface climate, with the indirect effectdominating in inhibiting precipitation. Because thesize distribution of the cloud condensation nuclei(CCN) is not explicitly simulated, the effect ofaerosols on the cloud radiative forcing is heavily
parameterized, based on an empirical relationshipbetween CDNC and the mass concentration of thesulfate aerosol.
Tropical cyclones (TCs) are strong synoptic
systems. Their representations in global GCMs arefar from realistic. Although RCMs can better
resolve TCs than GCMs, the intensity is usuallymuch weaker than the observations due to the model
resolution currently in use. The possible impact of
TCs on summertime circulation over the East Asia
was studied by Ahn and Lee (2002) by bogusing TC
vortices in a seasonal RCM simulation with the
NCAR_RegCM. The model with the bogus TCs
showed improvements in the East Asian summer
monsoon circulation, suggesting that part of thesystematic biases in RCM simulations over the East
Asia could be due to the unrealistic simulation ofboth the intensity and frequency of TCs over thewestern North Pacific.
The physical and biological interactions between
the terrestrial ecosystems and climate are an active
area of research in recent years. With a spectral
RCM coupled with a new land surface model which
includes the dynamical vegetation and carbon
dioxide cycle, Mabuchi et al. (2000) studied suchinteractions over the Japanese Islands andsurrounding area. They found that interannual
variations of downward short-wave radiation overthe land surface were mainly responsible for theinterannual variations of vegetation activity.
Variations of vegetation activity affected thevariations of the net carbon dioxide flux between theland surface and the atmosphere. In turn, thesevariations affected the interannual variations ofcarbon dioxide concentrations in the lower
troposphere over Japan. Chen et al. (2004) included

the interactions between climate and the ecosystem
in a study of the climate change over East Asia due
to increased atmospheric CO2 and found that thesimulated climate changes are only slightly
modified by vegetation feedbacks.

6. Seasonal prediction
During the last decade, significant progress indynamical seasonal prediction with global coupledatmosphere-ocean GCMs (or AOGCMs) has been
made by the meteorological services and scientificcommunity around the world. With AOGCMs
potentially making skillful seasonal prediction of thelarge-scale fields, some investigations have recently
started to examine the use of RCMs nested within
the AOGCMs to improve seasonal climate
predictions at the regional scale. Leung et al.
(2003c) recently suggested that seasonal climate
forecasting may be a useful framework for testing
the value of dynamical downscaling because unlike
climate change projections, climate forecasts can beverified.
Recently, Ding et al. (2003) presented some
initial results of the experimental use of the nested
RCM for seasonal prediction since 2001. In theirexperiments, an AOGCM provides the boundary
and initial conditions for the RCM with horizontal
resolution of 60 km. To verify the performance ofthe nested RCM, two 10-year (1991-2001)
integrations for the summer (June-August) driven byobserved and AOGCM simulated large-scaleconditions, respectively, were performed to produce
model climatology and hindcasts. Preliminary
results have shown that the RCM has some skill in
simulating and predicting the seasonal rain belts,
showing more areas with positive anomaly
correlation coefficient (ACC) than the AOGCM
simulations. The best predicted regions with high
ACC are located in west China, northeast China,
and north China where the AOGCM also has
maximum prediction skill. One significant
improvement derived from the RCM is the increaseof ACC in the Yangtze River valley where the
AOGCM shows a very low, or even negative ACC.
This improvement is likely related to the more
realistic representations of the large-scale terrains in
the RCM. Real-time experimental predictions forthe summers since 2001 using this nested RCM
were made on April 1 of each year.

7. Future directions
Regional climate modeling has proven to be able
to improve climate simulations at the regional scales,
especially in regions where forcings due to complex
orographic effect, land-sea contrast, and land use,regulate the regional distribution of climate
variables and variations, such as East Asia. The
regional climate modeling approach has also beenshown to be useful for improving our understandingof various climate processes, such as landatmosphere interaction, cloud-radiation feedback,topographic forcing, and land use change, as

discussed in section 4. Significant progress has been
made in the area of the application of RCMs toglobal change research and seasonal climate
predictions as dynamical downscaling tools during
the last decade. Progress has also been made in bothunderstanding and improving the regional climate
predictability.
The issue of climate change impacts, adaptation,and mitigation requires the use of regional climatechange scenarios. RCMs will remain an important
dynamical downscaling tool for providing theneeded information. Since society is more
vulnerable to changes in the frequency or intensityof extreme climate events (e.g., drought and flood,
extreme high/low temperature) rather than the mean
climate states, future applications of RCMs inclimate change study will require demonstration of
the skill in simulating extreme events. The use of
ensemble simulation technique will be important toestablish the statistical significance of changes
associated with events that have low probability ofoccurrence.
Several areas need to be further developed or
improved, such as (1) the inclusion of cloud-aerosolinteractions based on higher moments mixed-phase
cloud microphysics parameterization so that moreaccurate cloud-radiation-climate interactions can bemodeled with high-resolution RCMs; (2)understanding the model behaviors at high-spatial
resolutions to address issues that could arise inapplying physics parameterizations, in particular the
cumulus convective parameterization schemes
beyond the spatial scales originally intended; (3)examination of the relationships among various
variables at different temporal and spatial scales toprovide more complete and process-based
understanding of the model biases and the failure tocapture certain climate features; (4) improvement ofmodel physics parameterizations so that the daily
precipitation intensity distribution and diurnal cycleof clouds and precipitation can be simulated
realistically; (5) use of ensemble simulations toimprove signal detection of climate change or
climate sensitivity; and (6) development andapplication of regional earth system modelingsystems, so that the complex feedbacks among
various climate system components can beconsidered consistently and interactively. It is ourbelief that internationally coordinated efforts can be
developed to advance regional climate modelingstudies for global monsoon system. Finally, since
the final quality of the results from nested RCMs
depends in part on the realism of the large-scale
forcing provided by GCMs, the reduction of errors
and improvement in physics parameterizations inboth GCMs and RCMs remain a priority for the
climate modeling community.

References (omitted, provided by request)