WORKSHOP ON MONSOONCLIMATE SYSTEMS
Toward Better Prediction of the Monsoon
BY KENNETH R. SPERBER AND TETSUZO YASUNARIT

he Earth’s monsoon systems are the lifebloodof more than two-thirds of the world’s population
through the rainfall they provide to the mainly
agrarian societies they inf luence. In some cases
the monsoon is remarkably regular, for example,
over India the interannual standard deviation of
the rainfall is about 10% of the annual mean, but
the perturbations are strong enough to lead tonatural disasters resulting from flood or drought andassociated land-use impacts. During the course of themonsoon season there can also be strong variations
in rainfall. Intraseasonal (30–60 day) oscillations(ISOs) influence the onset of the monsoon and giverise to protracted active (i.e., enhanced) and break(i.e., deficient) periods of rainfall. Foreknowledge ofthe active and break phases of the monsoon is important for crop selection, the determination of planting

AFFILIATIONS : S PERBER—Program for Climate Model Diagnosis
and Intercomparison, Lawrence Livermore National Laboratory,
Livermore, California; YASUNARI —FRCGC/JAMSTEC and HyARC/
Nagoya University, Nagoya, Aichi, Japan
CORRESPONDING AUTHOR : Kenneth R. Sperber, Programfor Climate Model Diagnosis and Intercomparison, Lawrence
Livermore National Laboratory, P.O. Box 808, L-103, Livermore,
CA 94550
E-mail: sperber1 @llnl.gov
DOI:10.1175/BAMS-87-10-1399
In final form 1 June 2006
©2006 American Meteorological Society

AMERICAN METEOROLOGICAL SOCIETYMONSOON CLIMATE SYSTEMS WORKSHOP
WHAT:

More than 60 experts gathered to assess thecurrent understanding of monsoon variability
and highlight outstanding problems simulating
the monsoon.
WHEN : 15–17 June 2005
WHERE : University of California, Irvine

times, and mitigation of potential flooding and shortterm drought. Poor simulation of the ISO has been apervasive problem in all scales of modeling. Though
there are exceptions to this latter statement, attempts
to translate “success” in simulating the ISO from one
model to another have met with limited success at
best. Additionally, the poor simulation of ISOs is alimiting factor for medium-range to seasonal forecasting. The success of empirical methods in forecasting
intraseasonal variability indicates that there is skill
to be gained through improved dynamical models.
Observational and modeling studies indicate that thediurnal cycle of radiative heating and surface fluxes
over the ocean are rectified onto the intraseasonal
time scale, indicating that a synergistic approachto studying monsoon variability is necessary. The
diurnal cycle of precipitation and clouds, which
directly inf luence radiative heating and surface
fluxes, are also poorly represented, especially in the
global models. Thus, it is anticipated that improving
the simulation of the diurnal cycle of precipitation
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| 1399and clouds in global models will contribute to an
improved ability to simulate ISOs. Improved understanding and simulation of the diurnal cycle is also
important because it influences low-levels jets and theassociated transport of moisture as well as the rainfall
over regions of complex topography. Furthermore,from a physical standpoint, study and improved
modeling of the monsoon is also important because
the associated large-scale energy exchange due to thecycling of water vapor in the atmosphere is central
to the development of the general circulation of theatmosphere.
Because of the potentially grave societal impactsof monsoon variability on numerous time scales, andbecause modeling monsoons remains a challenge,
the World Climate Research Programme (WCRP)
sponsored the first Pan-WCRP Workshop on Monsoon Climate Systems to promote closer interaction
between two WCRP projects—Climate Variability
and Predictability (CLIVAR) and the Global Energy
and Water Cycle Experiment (GEWEX). This effort
is part of a new WCRP strategic framework for Coordinated Observation and Prediction of the Earth
System (COPES). The urgent requirement of fullcoordination and cooperation of monsoon-related
research activities between these two projects forbetter understanding and seamless prediction of themonsoon systems was the background of the proposal
for this workshop. Improvement of multiscale interactions in global general circulation models (typicallyused by CLIVAR) is anticipated through the experience gained by GEWEX through its continental-scaleenergy and water cycle experiments and regionaland cloud-resolving modeling. Our workshop1 consisted of modeling and observations presented byrepresentatives of CLIVAR monsoon panels, GEWEX
projects, invited expert presentations highlighting thefundamental physics and dynamics of monsoons, andbreakout discussion groups to suggest the future pathof monsoon climate research.
Out of necessity, because of the longer time scalesconsidered (CLIVAR based), global climate models(GCMs) that are used for subseasonal, interannual,and climate studies are coarser in horizontal andvertical resolution and are less complex in termsof physical parameterizations than the (GEWEX

11400 |The meeting was chaired by Tetsuzo Yasunari and Kenneth
R. Sperber. Special thanks are given to S. Sorooshian and his
staff at University of California (UC) Irvine for hosting theworkshop. Funding for the meeting was provided by WCRP,
U.S. CLIVAR, and UC Irvine.
OCTOBER 2006

based) regional climate models (RCMs) and cloudresolving models (CRMs) that are used for diurnal tosubseasonal studies. The diurnal cycle of convection
emerged as an important component of the climate
system that could be addressed near term throughclose cooperation between CLIVAR and GEWEX.
The diurnal cycle is a forced mode of variability thatneeds to be adequately captured in order to properly
represent the moisture and heat budgets over bothocean and land. It is not uncommon for GCMs tohave phase errors of 2–6 h in the time of maximum
precipitation compared to observations. The phasing
and amplitude of the diurnal cycle of precipitation istypically better represented in RCMs.
CONCERNS. In global GCMs there are numerous limiting factors to the simulation of the diurnal
cycle of convection and clouds. Results indicate that
adequately resolving the planetary boundary layer(PBL) is important for the fidelity of the diurnal
cycle of convection because it is intimately related
to the surface diurnal heating that is essential forthe organization of cumulus cloud ensembles.
Simulations produced by a GCM with an embedded two-dimensional cloud-resolving model inplace of a typical convection scheme resulted in amore realistic diurnal cycle of convection. Results
suggest that, at a minimum, coupling between the
PBL and deep convection is necessary to improvethe diurnal cycle in GCMs. At present only the
Arakawa–Schubert convection scheme contains a
PBL submodel that tends to give a more realistic
representation of the diurnal cycle of precipitation.
Based on observations over the Amazon region andresults from a CRM, a realistic representation of thediurnal cycle of rainfall requires turbulent moisture
convergence to be adequately resolved at the top of
the PBL. In order to adequately realize this in a GCM
it was necessary to increase the vertical resolution
in the PBL. Added benefits included an improved
representation of the South Atlantic convergencezone and a reduction of SST errors over near-coastal
stratocumulus regions.
Increased vertical (and horizontal) resolution
will result in more adequately resolving complex
orography. This is necessary, for example, over the
Maritime Continent to capture diurnal land–seabreeze influences that extend hundreds of kilometers
offshore. Additionally, orography acts to anchor
rainfall at specific geographic locations (e.g., thewestern Ghats, the Burma Highlands, the Tibetan
Plateau, and the Philippines). Poorly resolving
these orographic anchor points contributes to er-rors in the simulation of the amplitude and spatial
distributions of rainfall. Thus, a potential benefit ofresolving complex orography is better simulation of
the mean state. Importantly, reduction of mean state
errors has resulted in improved variability [e.g., theMadden–Julian (30–60 day) ISO] in GCMs.
In coupled ocean–atmosphere GCMs, inadequate
vertical resolution of the upper ocean precludes
adequate representation of the diurnal cycle of SST.
In observations over the ocean under light windconditions, a strong diurnal cycle in SST tends to bepresent with the late-afternoon cumulus congestus
clouds. Thus, an inadequate representation of thediurnal cycle of SST in models can also lead to an
unrealistic radiation balance with poor phasing of thesurface fluxes compared to observations. This may
preclude adequate moistening of the atmosphere byshallow convection, which is a necessary precursor
to the deep convection that occurs during the active
phase of the ISO.
Errors in the amplitude and phase of the diurnal
cycle of precipitation also exist over land, indicatingthat a better understanding of land–atmosphere interactions is required. It was found that models withweak land–atmosphere coupling tend to exhibit onset
and peak diurnal rainfall earlier than in observations.
Possible reasons for weak soil moisture–precipitation
feedback in models include the inability of land surface schemes to correctly partition energy into thelatent and sensible heat components, inadequately
resolving the boundary layer (e.g., the need to include
weak mixing from above), and convection schemesthat are too sensitive to instabilities in the boundary
layer.
Low-level jets (LLJs) are an integral component of
monsoons that provide the conduit for the transportof moisture that fuels monsoon systems. LLJs exhibit
a strong diurnal component that also affects rainfall.
LLJs are intimately related to numerous processes, including (but not limited to) PBL processes, large-scale
upper-level synoptic influences, land–sea contrast,
surface friction, and land surface interactions. Modeling studies indicate that land–atmosphere couplingis particularly strong over India, East Asia, West
Africa, and the Great Plains of North America. The
complicated interactions involved in the formationand maintenance of LLJs provide an excellent test bed
for understanding interactions of a number of physical parameterizations. Improvement in the simulation of LLJs should lead to a better representationof the phase and amplitude of the diurnal cycle ofprecipitation and thus the warm-season rains. Thisis a severe test for models given the unique land–sea
AMERICAN METEOROLOGICAL SOCIETY

distributions, surface types, and orographic influences of the disparate monsoon regions.
As noted, RCMs are typically superior to GCMs in
representing the diurnal cycle of precipitation. Withbetter-resolved orography and more comprehensive
physical parameterizations, important aspects of themonsoon can potentially be realized. Given the much
longer time scales considered in GCM simulations,and in many cases the need for ensembles ofintegrations, it is not viable to simply transplant themore complex RCM physics directly into a GCM.
Rather, through a better understanding of the important physical processes it may be possible to develop
more complete parameterizations for GCMs.
OBSERVATIONAL PROJECTS. Observations
are crucial for understanding processes in the climate
system and they provide the foundation for future
model improvements. In addition, real-time observations are essential for assimilation into numerical
weather (and seasonal to interannual) prediction
models, generation of boundary conditions forregional and cloud-resolving models, and parameterization development to simulate unresolved scales. A
detailed analysis of the present observing system andobservationally based datasets (e.g., reanalysis) was
not possible given the time limitations of the workshop. However, concern was voiced over the decayof the present observing system, in particular, thedecrease in the number of sites from which routine
sondes are launched. New satellite products enablenear-global observations of the water and energy
cycle on diurnal and longer time scales, and haverevealed shortcomings in the representation of thevertical profile of moisture in reanalysis during thelife cycle of the MJO. It was stressed that in situ data
are needed despite the revolution in satellite dataacquisition that is presently underway.
The WCRP Coordinated Enhanced Observing
Period (CEOP) program is an example of the approach of multiscale evaluation in terms of time scale
and models. The CEOP observations obtained from2001–04 allow the validation of a hierarchy of models.
New concerns were voiced regarding the role of highlevels of aerosols over the Asian–African region, andtheir role in the water and energy cycle. In addition
to acting as cloud condensation nuclei (and therefore
as heat sources and sinks in the atmosphere), aerosols
also perturb the radiation balance. The extent towhich these effects feedback to influence the monsoon mean state and variability is an open question.
Over Africa the current sustained observing
system is not adequate to 1) evaluate and improve
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models of key phenomena and processes (e.g., diurnal
cycle, heat low, African easterly jet, etc.), 2) support
climate prediction and its impact, and 3) validate
satellite products. To begin to address these shortcomings the African Monsoon Multidisciplinary
Analysis (AMMA) project, a comprehensive observational campaign of the west Africa monsoon, withenhanced and special observing periods in 2005–07,
has been launched. In addition to better understanding and predicting the physical system, including
atmospheric, oceanic, biological, and chemical (aerosols) components, a major goal is to address socioeconomic impacts due to environmental change, landuse, and human activities. The results from AMMA
should also help to establish the configuration of asustained observing system over Africa.
Enhanced observations gathered during theNorth American Monsoon Experiment (NAME) in2004 provided an unprecedented set of atmospheric,oceanic, and land surface data for investigating therelationship between moisture transport, monsoon
onset, and surge events, as well as for understanding
the diurnal cycle of moisture over the ocean and complex terrain. Future NAME project milestones include
the assessment of global and regional model simulations of NAME2004, the reproduction of the diurnal
cycle of observed precipitation over the core monsoon
region, the evaluation of changed parameterizations,the quantification of the relative roles of ocean versus
land surface influences, and the improved skill ofwarm-season rainfall forecasts.
The South American Low-Level Jet Experiment(SALLJEX) was carried out in November 2002–
February 2003. Goals included an improved representation of the tropospheric flow on diurnal andlonger time scales, resulting from the heretofore
sparse sounding network, and a better understandingof the vertical structure of the LLJ and its relationship with mesoscale cloud systems and precipitation.
Hindcasts using SALLJEX data showed an improved
representation of regional rainfall anomalies in 24-h
forecasts.
While there was no formal presentation on the
Indian Ocean observing system of buoys, there wasstrong support for this developing resource for understanding air–sea interaction and the role the Indian
Ocean plays in the summer and winter monsoon.
The lack of routine Indian Ocean observations is seen
as an impediment to progress in understanding key
processes and forecasting of the monsoon. VASCO/CIRENE is a series of observational campaigns in
the western Indian Ocean just south of the equatorin which ocean–atmosphere observations have been1402 |OCTOBER 2006

or will be taken during the boreal winters of 2005–07.
Measurements include those associated with ocean
physics, air–sea fluxes, the atmosphere, and ocean
biogeochemistry. The experiment will revolutionize
our understanding of processes related to 1) the roleof the diurnal cycle in the warm-layer formation in
the upper ocean, 2) the origin of intraseasonal SST
variability, and 3) seasonal to interannual variationsof equatorial wave dynamics and ocean mixed layer
physics. In addition to process studies, the attendees
support and encourage the placement of a sustained
monitoring system in the Indian Ocean to improvethe basic understanding of the interactions that
occur in this basin and to improve predictability. An
implementation plan for sustained oceanic observations over the region has been developed by the joint
Global Ocean Observing System–CLIVAR Indian
Ocean Panel.
In early 2006 the Tropical Warm Pool International Cloud Experiment took place, with Darwin,
Australia, as the focal point. From surrounding
land and ocean locations, measurements fromradiosondes, precipitation and cloud radars, flux
sites, aircraft, and satellites were taken. It was thefirst time that coordinated measurements of cloudproperties and their effect on the local environment
had been taken in combination with measurementsof the heat, moisture, and momentum budgets. The
emphasis was on cirrus clouds because they are notwell understood. The new data will also be a resourcefor validating CRMs and parameterization development for global models.
New approaches for investigating systematic
model error include the Climate Change Prediction
Program Atmospheric Radiation Measurement
Parameterization Testbed (CAPT) in which station
observations are used to investigate error growth
in GCM hindcasts that have been initialized fromreanalysis data. Interest in expanding this approachto additional models and for the CEOP (and other)station data has been expressed.
THE FUTURE. Through the working group andplenary discussions, consensus was that more effort
and collaboration is needed for improving monsoon
modeling and prediction, including the following:1) correct simulation of diurnal cycles of precipitation and convection; 2) comprehensive modelingof surface interactions, the PBL, and convection;3) understanding intraseasonal oscillations; 4) impactof atmospheric moisture distribution and transport;5) the need for more process studies and modelingof the Maritime Continent and the Indian Ocean;6) sensitivity testing to determine the resolution
necessary in global models to simulate the multiscaleinteractions that dominate the Earth’s monsoon systems; 7) the need for improved and sustained observations over sparsely sampled regions of the Tropics;8) better observations of land surface conditions(e.g., soil moisture, snowcover, and snow depth); and
9) reversal of the decay of the present in situ observing
system. Initial implementation of these collaborative
tasks within WCRP will occur through a series of targeted workshops that will try to meet in conjunction
with (or as components of) planned meetings. A
more complete list of recommendations is in the fullmeeting report, which has been published as a WCRP
document and is available on the WCRP, CLIVAR,
and GEWEX Web site (e.g., online at http://eprints.
soton.ac.uk/19335/01/icpo_pub_103.pdf).
In the near term the emphasis will be on improving the diurnal cycle of precipitation in global
models. Given that regional and cloud-resolving
models perform much better in this respect, we
envision that a better understanding of the factors
involved (e.g., physics and resolution) in realistically simulating the diurnal cycle in these models

AMERICAN METEOROLOGICAL SOCIETY

can be translated to an improved representationin global models. This activity will require closeinteraction in monsoon-related modeling activities under CLIVAR and GEWEX as part of a new
Pan-WCRP initiative of monsoon research within
the COPES activity. The regional climate modeling studies and the efforts of numerous GEWEXCloud System Study (GCSS) working groups will
provide fertile ground for initiating collaborations
with CLIVAR for improving the diurnal cycle ofconvection in GCMs. Additional input from experts
in land–atmosphere interactions, for example, theGlobal Land Atmosphere Coupling Experiment(GLACE), will be an essential component to thistask. Anticipated benefits of improving the diurnal
cycle of convection include a better representationof intraseasonal oscillations, the seasonal cycle,
interannual variability, and the mean state.
ACKNOWLEDGMENTS. K. R. Sperber was supported
under the auspices of the U.S. Department of Energy Officeof Science Climate Change Prediction Program at the
University of California Lawrence Livermore National
Laboratory under Contract W-7405-Eng-48.

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