ADVANCING GLOBALAND CONTINENTAL-SCALEHYDROMETEOROLOGYContributions of GEWEX Hydrometeorology Panel
BY R. G. LAWFORD, R. STEWART, J. ROADS, H.-J. ISEMER, M. MANTON, J. MARENGO,

T. YASUNARI, S. BENEDICT, T. KOIKE, AND S. WILLIAMSThis article introduces the GEWEX Hydrometeorology Panel, describes the participating
continental-scale experiments, outlines its current activities and priorities, and identifies new
areas of science to which atmospheric scientists and hydrologists are invited to contribute.

The GHP* (refer to appendix for acronym expansions) was formed in 1995 to coordinate the activities of the CSEs and related global initiatives.
Many of the GHP activities had their roots in projects
that began in the late 1980s. At that time the space agen-

AFFILIATIONS: LAWFORD—International GEWEX Project Office,Silver Spring, Maryland; STEWART—McGill University, Montreal,
Quebec, Canada; ROADS—Scripps Institution of Oceanography,
San Diego, California; ISEMER—GKSS Forschungszentrum
Geesthacht GmbH, Geesthacht, Germany; MANTON—Bureau of
Meteorology, Melbourne, Victoria, Australia; MARENGO—Centro dePrevisao de Tempo e Estudos Climaticos, Cachoeira Paulista,
Brazil; YASUNARI—Nagoya University, Nagoya, Japan, and FRSGC,
Yokohama, Japan; BENEDICT—University of California, San Diego,
La Jolla, California; KOIKE—University of Tokyo, Tokyo, Japan;
WILLIAMS—UCAR/JOSS, Boulder, Colorado
CORRESPONDING AUTHOR: R. G. Lawford, International GEWEX
Project Office, Suite 450, 1010 Wayne Ave., Silver Spring, MD
20910
E-mail: lawford@umbc.edu
DOI:10.1175/BAMS-85-12-1917
In final form 1 June 2004
©2004 American Meteorological Society

AMERICAN METEOROLOGICAL SOCIETY

cies and the WCRP became interested in supporting
the newly proposed GEWEX.
When GEWEX commenced in 1990 with a focus on
global products and data, its lead scientists recognizedthat the global datasets needed to be evaluated at regional scales. At the same time significant improvements were being made to land surface models as aresult of intensive regional experiments being carriedout by the ISLSCP and BAHC initiative under the IGBP.
In particular, ISLSCP carried out intensive field campaigns that focused on relatively homogeneous areasof 10,000 km2 (approximately the size of a climate
model grid square) and involved intensive observational periods for (generally) 2–4 weeks several times
a year. However, there was also a growing recognitionby the climate community of the need for continentalscale land surface studies. The concept of a continentalscale hydrologic experiment was developed in 1990 asa result of the convergence of GEWEX interests for aregional test bed, the need to scale up ISLSCP intensive studies to larger geographical areas, and the de*Information about the GHP can be found online at http://
ecpc.ucsd.edu/projects/ghp/.
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| 1917

sire of IAHS to involve hydrology more actively in climate research. The proposed regional experiment was
based on the hypothesis that water and energy budgets over a large basin would not be as sensitive to random errors as they are at a point or for a small watershed and therefore they could be examined in ameaningful way because there was an increasing likelihood of closing continental-scale water and energy
budgets to acceptable limits using the newly available
datasets and models.
In 1990, a group of international experts recommended the Mississippi River basin as a focus area andthen worked with others to draw up a science plan for
the GEWEX GCIP. As planning progressed, however,a consensus emerged that areas with other important
processes, such as permafrost and tropical forests,
should also be studied. In addition, a number of countries could make stronger national contributions to
GEWEX if they studied a basin that included their national territories. As a result, five mature continentalscale experiments and two relatively new experiments
were developed to cover 10 large land areas. Together,
they have accumulated many years of unique data,
research results, and experience.
GEWEX established the GHP in 1995 to coordinate
the wide range of interests and activities involved in thesecontinental-scale experiments. The GHP also takes responsibility for coordinating the activities of the ISLSCP,
GRDC, and GPCC. The overall GHP mission is to “demonstrate the capability to predict changes in water resources and soil moisture at time scales up to seasonaland interannual as a component of the World Climate
Research Programme’s prediction goals for the climate
system.” To this end, it maintains an overview of research in the CSEs and influences the priorities of each
CSE. The GHP also initiates, synthesizes, reviews, andrecommends joint activities that promote a common
research agenda in each of the CSEs and integrates theresults from the individual CSEs. The GHP further
promotes and coordinates interactions with the GRP and
GMPP. The GHP has developed numerous summaries and reports to GEWEX, WCRP, and the public on
the role of macroscale hydrometeorological processes
in the climate system and in water resources planning.
GHP members, appointed by the GEWEX SSG, include international scientific experts and the principal
contacts for each of the CSEs. The GHP reviews proposals for new CSEs and makes recommendations to
the GEWEX SSG based on an evaluation of their ability to meet the following criteria:
1) regional data collection activities,
2) a substantial land surface modeling effort,
1918 |DECEMBER 20043) linkages with a NWP center,4) free and open data exchange, and5) linkages with resource management agencies (generally water resources) that would use results fromthe experiment.
The mature experiments, including GAPP in the
United States (which extends GCIP in both space andtime), BALTEX, GAME, LBA, and MAGS, comply withthe criteria listed above. These CSE areas are outlinedin Fig. 1, along with MDB. The latest approved CSE,
LPB, along with the AMMA initiative are collaborative
experiments with WCRP’s CLIVAR program. It should
be noted that GAPP and GAME are also developing
strong ties with CLIVAR.
With the maturation of the CSEs and their datasets,
GHP has placed a new emphasis on collaborative research linking the individual CSEs. New initiatives, including WEBS (1998) and WRAP (1999), werelaunched. The requirements for free and open dataexchange to support these activities led to the formation of a data committee in 1998. The eight-step GHP
strategy is to1) develop a consensus on the best surface characterization fields available for use by GHP;
2) establish access to this information for special casestudies;3) study regional climates and develop statements ofprogress by the CSEs and by the GHP;4) develop a strategy for examining the processes,
datasets, and remote sensing capabilities relevantto the CSEs and other regions (This should resultin clearer priorities for future research and data collection activities.);5) carry out demonstration projects offline with uncoupled hydrological–land surface models and withthese models coupled to an atmospheric model;
6) select a specific observational period in one CSE as
the basis for validation transferability studies
among the CSEs;
7) produce summary statements regarding the influence of land on large scales for each of the CSEs; and
8) prepare a summer school on regional climate andwater resources and publish a GHP overview article.
The following sections provide additional highlights
of GHP and CSE activities.
CSEs. CSEs serve as the regional building blocks forthe global initiatives led by the GHP. Each of the CSEs
deals with a range of observational and scientific issues and unique environmental and socioeconomic

conditions (see Table 1). The
number of nations participating insome CSEs (e.g., BALTEX andGAME) has posed major coordination and logistics challenges.
A science plan and documented
research results can be found ateach of the CSE Web sites foundin the following paragraphs.
Table 2 provides a summary of
their chief geographical characteristics. Highlights from each CSE aresummarized below.

FIG. 1. Continental-scale areas included in the GEWEX Hydrometeorology Program.

BALTEX. The BALTEX project(Raschke et al. 2001, and online atwww.gkss.de/baltex/) has been studying the entire
drainage basin of the Baltic Sea, including the sea itself,
since 1994. The basin, which involves land in 14 nations,is in the transition zone between maritime and continental climates. The atmosphere/land surface/ocean
model coupling and the development of regional climate models is a particular focus in BALTEX, and two
different regional coupled model systems have been
established (e.g., Hagedorn et al. 1999; Döscher et al.
2002; Jacob et al. 2001) and are currently being validated

and improved. BALTEX is unique for its strong marine research component comprising both modelingactivities (e.g., Lehmann et al. 2002) and field experiments (e.g., Launiainen and Vihma 2001). Data evaluation and analysis of enhanced observing periods, including PIDCAP (August–October 1995) and BRIDGE(October 1999 to February 2002) is ongoing. BALTEX
is currently defining enlarged objectives for its second
phase (2003–12), including issues of climate and climate
impact research, as well as water quality issues.

TABLE 1. Summary of the countries and unique features of each of the CSEs.
Lead country

Other countries

Unique aspects

BALTEX

Germany

Denmark, Sweden, Finland,
Russia, Poland, Belarus,
Estonia, Latvia, Lithuania,
the Netherlands, Austria,
United Kingdom

Involves an oceanographic component
over the Baltic Sea

GAME

Japan

China, Korea, Thailand, Malaysia,
India, Nepal, Sri Lanka, Russia,
Mongolia

Transect of study areas from tropics
to the Arctic

GAPP

United States

MexicoInvolves well-instrumented
midlatitude regions, land surface
processes, and water resource
applications

LBA

BrazilColombia, Peru, Bolivia,
Venezuela, Ecuador,
European Union, United States

Addresses role of tropical rain
forests and the consequences oftropical deforestation for regionaland global climate

LPB

Brazil,Argentina

United States, Uruguay, Paraguay,
Chile, Bolivia, Organization ofAmerican States

Addresses the role of low-level jetsand land cover changes in a humid
basin

MAGS

CanadaFreshwater inputs to the Arctic
Ocean; cold region processes

MDB

Australia

Hydrologic regimes in semi-arid areas

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| 1919TABLE 2. Some physical characteristics of the CSEs (from GHP Web site and other sources).
AREA (106 X km2)

DISCHARGE (103 m3)

CLIMATEBALTEX

2.0

470

Continental but modified by the Atlantic Ocean

GAME-HUAIHE

0.1

30

Continental

GAME-LENA

2.49

na

Continental-Arctic

GAME-THAILAND

0.16

37

TropicalGAME-TIBETnana

Continental-High Elevation

GAPP/GCIP

3.2 (GCIP)

570

Continental

LBA

6.2

6,300

LPB

3.3

820

Extratropical Humid Basin

MAGS

1.8

300

Continental-Arctic

MDB

1.0

na

Continental-Semiarid

AMMA

na

na

Continental-Semiarid

GAME. The GAME (information online at www.hyarc.
nagoya-u.ac.jp/game/) initiative is being undertaken infour large areas in eastern Asia, three of which arestrongly influenced by the summer occurrence of theAsia–Australian monsoon. Moisture and energy fluxes
in humid environments over rice paddy fields andforests are being studied in the Chao Phraya River basinin Thailand. The important role of vegetation and land
use in the seasonal cycles of energy and water and theirchanges, such as deforestation, have been documented through these experiments (e.g., Tanaka et al.
2003; Toda et al. 2002). Observational programs in
HUBEX in China have focused on a better understanding of energy and hydrological processes in cloud andprecipitation systems along the Meiyu front and theirinteractions with land surface processes. GAME
research on the Tibetan Plateau, a flat plain located at
or above 3500 m elevation and intersected by an east–west mountain chain, examines the effects of thiselevated thermal source on the monsoons. In cooperation with Chinese TIPEX, two observational networks have been deployed on the plateau, which aresupplemented by special field campaigns, automaticweather stations, and a 3D Doppler radar system. The
GAME Lena River basin study area, with a focus on
taiga and tundra regions in Siberia, provides intensive
observations of circumpolar Arctic processes.
Research themes for studies in this area involve seasonal and interannual variations in one-dimensional
water and energy fluxes over different landscapes,
water balances in tundra watersheds, and the distribution and effects of ground properties and vegetation (Ohta et al. 2001).
1920 |DECEMBER 2004

Tropical

The GAME experiment has been enhanced by similar initiatives taking place in China (TIPEX, SCSMEX,
and KORMEX). The GAME reanalysis data for the IOP
1998 has been released, and new aspects of the energy
and water cycle processes in the Asian monsoon region are being clarified using this dataset (e.g., Ueda
et al. 2004). The scientific results of GAME are documented in a special issue of the Journal of the Meteorological Society of Japan (Yasunari 2001).
GAPP. After extensive planning (Schaake 1993; WMO
1992) and pilot projects in 1993 and 1994, GCIP, theforerunner of GAPP (online at www.ogp.noaa.gov/
mpe/gapp/), was fully implemented in 1995 in the Mississippi River basin. A major component of this research was directed toward understanding the regional
and local-scale hydrometeorological processes during
different seasons, and their role in understanding andclosing regional energy and water budgets for the basin.
GCIP undertook systematic studies throughout thebasin to determine hydrologic balances over a wide
range of regional climates. The basin was divided into
four LSAs (IGPO 1993), each with its own regional andseasonal scientific focus. Soil moisture processes werestudied in the Southwest, cold season processes in thenorth-central, precipitation and streamflow regimes inthe East, and the annual cycle in the Northwest. These
areas were studied sequentially moving from theSouthwest (1995–97), to the north-central (1996–98),
East (1997–99), and Northwest (1998–2001). GCIP relied mainly on data from existing meteorological andhydrological networks across the Mississippi Riverbasin, including the unique research datasets from theARM CART facility. GCIP benefited from the strong
interactions between the research component of the
program and NOAA/NCEP’s OHD. The strategies
and successes of GCIP are detailed in NRC (1998), andoverview articles by Coughlan and Avissar (1996) and
Lawford (1999).
GAPP was initiated in 2001 as an extension to GCIP.
GAPP extends the area of study to the Pacific coast andinto the northern parts of Mexico. GAPP places moreemphasis on monthly to seasonal prediction issues andprovides a basis for studying orography and precipitation and the role of land in the monsoonal circulations
associated with the Gulfs of Mexico and California.
GAPP is partnering with CLIVAR in NAME, which
addresses the role of monsoon systems and remote
forcing on summer precipitation patterns in the UnitedStates (Higgins et al. 1998).
LBA. LBA (online at http://lba.cptec.inpe.br/lba) studies the natural variability of energy and water budgets
over the Amazon River basin and examines the effectsof rapid deforestation on these budgets. LBA is an international project led by Brazil and involves active participation by, and support from, other Amazon countries, the United States, and the European Union. LBA
is highly interdisciplinary and links hydroclimatologicalresearch to the project’s broad ecological researchgoals. LBA relies upon the measurement of key parameters in selected areas over long periods (3–5 yr)that is supplemented by intensive observing periods
during the wet (WET) and dry (DRY) seasons. Data
from an LBA-WET AMC (wet season of 1999) and the
LBA-DRY to LBA-WET AMC (September–October2002), which took place in Rondonia, Brazil, have beenused to determine the diurnal cycle in different circulation and rainfall regimes, the effects of land surfacechange on the planetary boundary layer and rainfall,
and the effects of aerosols from biomass burning on
the physics of rainfall and the onset of rainfall (Marengo
2004a). Data from SALLJEX, which took place under
the auspices of LBA and the VAMOS program in thewet season of 2003, are being used to document thecharacteristics of the low-level flow from the Amazon
basin to southern Brazil and northern Argentina.
Results from LBA have led to improved weather
and climate prediction capabilities at CPTEC in Brazil.
The project is contributing to sustainable development
goals through studies of the transferability of models
and techniques to the Sao Francisco River basin in
northwest Brazil.
MAGS. The goal of MAGS (online at www.msc.ec.gc.ca/_toc/gewex_e.html) is to assess the role of the
AMERICAN METEOROLOGICAL SOCIETY

Mackenzie River basin in the climate system, including its freshwater flux contribution to the Arctic
Ocean. The basin is very sensitive to climate change,
as indicated by the warming of 2°C per decade thathas been observed there since the mid-1970s (Caoet al. 2001). The role of northern lakes and their freeze–
thaw cycle and permafrost in regional water balances
are also being explored. These local processes must
be studied in the context of more remote processes
because the large-scale circulation and synoptic storms
play critical roles in determining the moisture fluxes
into the basin (Stewart et al. 1998; Rouse et al. 2003).
Important land surface processes operating in thebasin include snowfall interception by boreal forest
canopies, blowing snow, snowpack evolution, snowmelt, spring breakup, lake processes, permafrost melt,
runoff processes, and evapotranspiration. The paucity of operational data being collected in the basin,
particularly north of 60°N, has been a major challenge
in implementing MAGS.
MDB. The MDB (online at www.bom.gov.au/bmrc/csr/
gewex/) project examines water and energy budgets
in a semiarid catchment where the potential evaporation rate is more than twice that of the precipitation
rate and the interannual variability of precipitation isvery large. Irrigation is a major factor in the basin’s
water budget. Information from MDB is being used
locally to help in the management of groundwater andbasin salinity.
LPB. LPB, which was approved in January 2004, will
examine a highly productive La Plata River basin (information online at www.joss.ucar.edu/platin/) where
the SALLJ transports moisture from the Atlantic
Ocean and the Amazon River basin into the basin ona year-round basis (Berbery and Collini 2000).
Precipitation patterns are strongly dependent upon
adjacent ocean SST anomalies. The project will consider the roles of the Andes Mountains, low-level jets,
and the Pantanel on regional water budgets.
Emerging study area. AMMA. The AMMA (online at
http://medias.obs-mip.fr/amma/index.en.html) project
consists of intensive study areas in two contrasting partsof the rain regime over southwest Africa, including two
CATCH sites, the dry Niamey site in Niger and the relatively more humid site in the Oueme basin in Benin.
The local and remote causes for long-term trends in precipitation patterns over these regions are being examined and modeled. Both AMMA and the newly approved
LPB initiative support the goals of the second phase of
GEWEX through their strong linkages with CLIVAR.
DECEMBER 2004

| 1921REGIONAL CLIMATE SYSTEMS. Each CSE represents a particular climate regime and interacts withthe global climate system in a unique way. Figure 2
shows the range of climate regimes covered by the experimental areas. The characteristic climates range
from dry polar conditions to wet or arid tropical conditions and span most of the larger land climate regimes. In spite of the differences among the basins, thelarge-scale teleconnection patterns of climate anomalies suggest that these CSE areas do not operate independently. For example, SST anomalies over the equatorial Pacific and Atlantic Oceans lead to large changes
in the precipitation regime over the Amazon as a result of changes in the Walker circulation. These circulation changes influence other areas, such as the precipitation regimes in the Mississippi River basin, andthe southern GAME and MDB study areas. Anomalies
in the extratropical CSEs also appear to be connectedwith sea surface temperature anomalies in the North
Pacific Ocean. It has been shown that many aspects ofregional climate in northern Europe, including the
BALTEX region, vary in concert with the NAO.
Through WEBS and other regional studies, GHP
continues to develop a better understanding of the regional climate of each CSE, the interactions between
each land area and its moisture sources, and the transport mechanisms, including dominant large-scale circulation patterns. Evaluations of the ability of model
simulations and data assimilation systems to close regional water budgets in different climate- and dataintensive regimes provides a benchmark for the modeling community.
Local processes also have a large influence, particularly during the summer and the transition months.
For example, Stewart et al. (2002) analyzed the recordlow-flow year of 1994/95 on the Mackenzie River andfound that a number of surface and atmosphericfactors, including below-average atmospheric convec-

FIG. 2. Range of the mean annual temperature and meanannual accumulated precipitation characterizing each of
the CSEs (Roads 2002).

1922 |DECEMBER 2004

tion over the basin, combined in a mutually reinforcing way to produce this low-flow year. Droughts over
the Mississippi River basin also have been associatedwith lower-than-average moisture convergence overthe basin, particularly those related to reductions inthe strength of the low-level jet from the Gulf of
Mexico (Dirmeyer and Brubaker 1999). As noted earlier, this nocturnal summer low-level jet dominates
moisture fluxes into the Mississippi River basin andaccounts for much of the east–west gradient in precipitation that occurs across the basin and the wellknown nighttime precipitation maximum that occurs
in the central part of the basin. A low-level jet also acts
as a conveyer belt for moisture leaving Amazonia andentering the La Plata basin. This jet operates all year
round, but is most intense during the austral summer(Liebmann and Marengo 2001). Ohta et al. (2001) haveshown that the taiga forest and permafrost exert animportant influence on the seasonal cycle of surface
energy and water budget of Siberia. GHP has facilitated many of these interbasin comparisons of important processes by bringing together experts from different countries to develop common approaches totheir analysis.
LAND SURFACE PROCESS UNDERSTANDING. The success of regional (and global) land surface
models depends upon effectively representing the critical land–atmosphere interactions in models. Many research activities within the CSEs have been directed at
improving our understanding of key land surface processes that appear to contribute to predictability. A
number of special observing periods in support of process studies have been carried out by the CSEs (Table 3).
Vegetation. Growing season moisture fluxes to the atmosphere through transpiration can be as large as 300–
500 W m-2 in the midlatitudes (Baldocchi and Meyers1999). Plant type influences the magnitude and seasonal
distribution of transpiration, while plant density also
affects the amount of transpiration and the surface albedo (Baldocchi and Meyers 1999). These moisture inputs can lead to increased low-level instability and convective precipitation in continental areas (Xue et al. 1996).
Modeling studies indicate that the scale and patterns ofhetereogeneity in vegetation distribution influence local
cloud and precipitation patterns (Avissar and Liu 1996)
and seasonal climate (Lu et al. 2001). GHP is placing
increasing emphasis on the development of dynamic
vegetation models that incorporate plant phenology.
Soil moisture. Viterbo and Betts (1999) showed that the1993 Mississippi floods were intensified by earlier pre-

TABLE 3. GHP Enhanced Observing Periods and Web Sites for accessing the special data sets.
Continental Scale Experiment
Special Data Periods (IOPs)

Web site for accessing data

AMMA (CATCH)

www.lthe.hmg.inpg.fr/catch/database.htm

BALTEXPIDCAP (Aug – Oct 95)
BRIDGE (Oct 99 – Feb 02)

www.gkss.de/baltex/data.html
www.gkss.de/baltex/data.html
www.gkss.de/baltex/data.html

GAME
HUBEX (May-Aug 98)
Tibet (summers 98, 99)
Tropics (summers 98, 99)
Siberia (May-Sep 98, 99)
KORMEX (Summer 98)

http://gain-hub.mri-jma.go.jp/
http://gain-hub.mri-jma.go.jp/hubex/Hubex_data.html
http://gain-hub.mri-jma.go.jp/Tibet_data.html
http://gain-hub.mri-jma.go.jp/Tropics_data.html
http://gain-hub.mri-jma.go.jp/siberia/Siberia_data.html
http://gain-hub.mri-jma.go.jp/kormex/Kormex_data.html

GAPP
GCIP LSA-SW (Apr-Sep 95,96)
GCIP LSA-N (Oct 96-May 98)
GCIP LSA-E (Oct 97-Sep 99)
GCIP LSA-NW (Apr 99 – Mar 01)
NAME (prop. May 04-Sep 04)

www.joss.ucar.edu/gapp/www.joss.ucar.edu/gapp/project/gcip
www.joss.ucar.edu/gapp/project/gcip
www.joss.ucar.edu/gapp/project/gcip
www.joss.ucar.edu/gapp/project/gcip
www.joss.ucar.edu/name/dm/name_dm_index_html

LBA
TRMM-LBA (Nov 98- Feb 99)
LBA DRY AMC (Jan-Feb 99)
LBA DRY-WET AMC (Sep-Oct 02)
SALLJEX (Dec 02-Feb 03)

http://lba.cptec.inpe.br/lba/lbadis/lbadis.htm
http://lba.cptec.inpe.br/lba/lbadis/lbadis.htm
http://lba.cptec.inpe.br/lba/lbadis/lbadis.htm
http://lba.cptec.inpe.br/lba/lbadis/lbadis.htm
http://lba.cptec.inpe.br/lba/lbadis/lbadis.htm

MAGS
CAGES: 1998-1999

www.usask.ca/geography/MAGS/lo_Data_e.htm
www.usask.ca/geography/MAGS/lo_Data_e.htm

MDB

www.bom.gov.au/bmrc/csr/gewex

cipitation events. The saturated soils, in turn, suppliedmoisture to produce precipitation downwind from thearea of the initial rain. Land surface models have thedifficult task of representing soil wetness in the upper
few centimeters, where the water leaves the soil andenters the atmosphere, as well as the movement ofmoisture in the deeper soil layers. The soil moistureterm in climate models is important because it accumulates errors from inaccurate precipitation inputs.
Finescale heterogeneity in soil texture and hydraulicproperties add to the complexity of the parameterizations in these models. Studies have shown thateven soil moisture measurements must undergo extensive calibration to account for this heterogeneity
before they are used to validate regional and global
climate models.
Cold season processes. Cold season processes are important for GAME, MAGS, BALTEX, and GAPP.
Datasets have been developed to facilitate research on
AMERICAN METEOROLOGICAL SOCIETY

ground freezing; snow formation, accumulation, andmelt; winter surface albedo; and ice formation. These
new unique cold season datasets include temperature
profiles in the soil throughout the annual cycle, freezing and thawing patterns, permafrost distribution,snow cover and its heterogeneity, and surface ponding.
Advances have included better representation of multilayer snowpacks in land surface models (Yanget al. 1997), parameterizations based on vegetation andtheir associated subgrid-scale variability (Liston et al.
1999), the heterogeneity of albedo in shallow snow cover(Kunkel et al. 1999), frozen ground processes (Koren
et al. 1999), and vegetation greenup in permafrost regimes (Ohta 2001). Increased understanding of theseprocesses has led to the improvement of a number ofland surface models and improved simulations andpredictions of surface temperatures in winter.
MODELS AND DATA ASSIMILATION.
Uncoupled and coupled models. Neither models nor data
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by themselves are adequate for accurately simulatingwater budget components (Ropelewski and Yarosh1998). This limitation has motivated the development
of better assimilation and prediction systems, including modeling systems that accommodate the horizonal
movement of surface water through streamflow.
Models are also used for process studies, to develop
data assimilation products in a consistent framework,and to produce experimental predictions. GHP studies have relied on global models, regional mesoscalemodels, land surface schemes, and distributed andlumped hydrologic models. Regional mesoscale models are being run as independent climate models, using reanalysis datasets for boundary conditions, andare being nested in global models in a prediction modewith considerable success.
Mesoscale model development activities have relied on the intercomparison of models and on observations for establishing parameter values within models and for assessing model performance (Jacob et al.
2001). By participating in PILPS (Wood et al. 1998;
Lohmann et al. 1998) and PIRCS (Tackle et al. 1999),
modelers have identified many weaknesses in the way
different land surface and atmospheric schemes handle
individual water budget components, such as runoff
and evapotranspiration.
Land surface scheme improvements have beenshown to lead to better predictions of precipitation.
The proper representation of soil moisture in models
for precipitation and flood prediction was importantat both ECMWF and NCEP, especially in studies involving GCMs (Vieterbo and Betts 1999). More accurate representations of snow processes have also improved temperature and precipitation simulations andforecasts. Further work is needed to understand how
future improvements could arise from a better representation of energy balance processes.
Large-scale hydrologic modeling. Both lumped and distributed hydrologic models have been used in GHP to
explore the partitioning of rain into runoff and infiltration. Errors in model outputs arise not only fromuncertainties in the values of the input variables (e.g.,precipitation), but they can also result from problems
in model structure and parameter estimates.
Distributed hydrologic models (e.g., VIC model) are
attractive because they facilitate the use of highresolution distributed forcing data produced by radar and satellites and utilize high-resolution distributed hydrologic parameters derived from satellite dataor other sources (Wood et al. 1997). They also often
have grids that are easier to interface with climate models—an ultimate GHP goal. In addition, these models
1924 |DECEMBER 2004

allow for the explicit representation of the lateral movement of water through the areal sub-elements of thebasin.
Data assimilation. Data assimilation is an important tool
for both climate studies and numerical weather prediction. The development of systems that can assimilate specialized datasets has been important for
BALTEX, GAPP, and LBA. For example, within
BALTEX water vapor fields derived from GPS(Emardsen et al. 1998) are being incorporated into prediction systems. Although these new atmospheric
datasets pose challenges for data assimilation, their
inclusion in initial fields produces significant improvements in the forecasts. A regional LDAS (Mitchell et al.
1999), developed through GAPP, allows a number ofdifferent land schemes to be used in uncoupled andcoupled modes including the Noah, CLM, VIC, and
MOSAIC land models. Through collaboration with
NCEP, NASA is extending this system to cover the
globe (Houser et al. 2001). The Eta Model has also been
upgraded to assimilate precipitation gauge data andradiances from the GOES. This modeling system recently has been used by NCEP to produce a 32-km
resolution regional reanalysis over North America fora 25-yr period.
WEBS. One of the major objectives of the CSEs has
been to assess the accuracy with which water and energy budgets could be characterized and “closed” on acontinental scale. Results from a WEBS for the Mississippi River basin (Roads 2002, Roads et al. 2003) suggest that the water budget for the Mississippi Riverbasin can be closed to within approximately 15%.
Studies by J. A. Marengo (2004b), Costa and Foley(1999), and Matsuyama (1992) suggest that the Amazon basin is a sink for moisture where precipitation
exceeds evaporation. A more global study, using reanalysis products reported in Roads (2002) and Roads
et al. (2002), showed that the annual mean percentage
error in closing the budgets (as measured by the difference between estimated runoff and moisture convergence to the runoff) is coincidentally small in somebasins/regions, such as the Mississippi River basin(20%), Lena River basin (4%), BALTEX (3%), Tibet(16%), and GAME Tropics (6%), despite large errors,
which cancel, in individual terms (like precipitation andevaporation). The closure is less over the Mackenzie(76%), GAME HUBEX (84%), Amazon (46%), and
CATCH (in the AMMA region)(116%), where the errors can be as large as the associated runoff.
WEBS is continuing in the individual CSEs, as wellas other GEWEX projects, and will make use of over-lapping and updated GEWEX products such as the
ISCCP cloud and radiation products, GVaP water vapor products, GPCP precipitation products, and SRB
radiative fluxes. Datasets from other GEWEX projects
will aid future GHP WEBS. A comprehensive archiveand analyses of global streamflow and runoff data havebecome available at GRDC. Emerging projects in
GMPP are developing improved land surface simulations and data assimilation, which will further aid in
GHP’s research.
WRAP. The GEWEX WRAP (established in 1999)
promotes interaction between the GEWEX and water resources communities. GEWEX has produced
many products that could assist water resource managers with more efficient water use. WRAP members
have a wide range of expertise related to water management and represent many international water programs, as well as the CSEs. WRAP relies on physically
based hydrology and “application” or decision supportmodels, and the coupling of these models with regional
climate models. Some applications studies require
downscaling large-area (model grid square) precipitation forecasts and observed averages, statisticalanalyses of the relationships between SST anomalies
and seasonal streamflow, and analysis of the value andutility of seasonal forecasts in water management decisions. In future studies the hydrologic and applications modeling communities are expected to identify
the degree of complexity required for input data andoutputs in seasonal and interannual prediction studies.
A number of the CSEs are carrying out regional
workshops and studies to determine the information
requirements of water resource managers. One workshop showed that water managers would like to see
more projects demonstrating the use of GEWEX products in decision making. WRAP will explore how theconcerns about forecast accuracies and uncertainties
can be addressed through GEWEX initiatives.
Currently, the MDB and GAPP projects involve water
agencies and include trial delivery of tailored forecast
and assimilation products from NWP centers for routine water management. Future WRAP activity will
focus on building better hydrologic prediction capabilities through collaboration with IAHS PUB. In addition, WRAP, through GAPP and MDB, is supporting
studies of the Red–Arkansas, San Pedro, andMurrumbidgee basins in the UNESCO/WMO HELP
program. In the future WRAP will also actively contribute to the GWSP of the ESSP.
CEOP. In 1997, GHP developed the concept of a
CEOP for the time frame of 2002–04, when the CSEs
AMERICAN METEOROLOGICAL SOCIETY

would be fully functioning and the next generation ofEarth Observation Satellites would be producing newmeasurements of global water and energy cycle variables. CEOP has been accepted as an independent
WCRP project, although GHP still provides leadershipfor the implementation of CEOP and facilitates research to address CEOP science goals (IGPO 2001).
Through CEOP, the influence of continentalhydroclimatic processes on the predictability of global
atmospheric circulation patterns, the role of land inmonsoonal systems, and the changes in water resources up to seasonal time scales are being studied.
Through the involvement of the CEOS and its
members (particularly the space agencies), extensivearchives of satellite remote sensing data and products
are becoming more accessible. A number of major
NWP global models are now providing three-dimensional model output fields, which are being archived at
MPI, associated with the BALTEX project. Each of the
CSEs is also supplying data from a total of 36 global
reference sites (Fig. 3) to the CEOP data center at
UCAR. A global data-integration system at the University of Tokyo will archive remote sensing data. Data
integration is also being carried out by the GLDAS infrastructure being developed at NASA Goddard and
NCEP. (For more detail on CEOP plans, visit the CEOP
Web site at www.gewex.org/ceop.htm/.)
GHP DATASETS. In addition to the CSE reference
sites, individual GHP projects and centers have in situ
data and products that are useful for global water cycle
research. ISLSCP provides support through the generation of global datasets. GRDC developed a global
monthly climatology of runoff, which is being used as
the basis for many global water balance studies. GPCC,
which contributes precipitation products to GPCP studies, also develops global products. In addition, BALTEX
has developed a GPS for analyzing three-dimensional
water vapor fields (Emardsen et al. 1998), as well as aprecipitation dataset covering much of the BALTEX
region, integrating Doppler radar and in situ gauge data
(Koistinen and Michelson 2002). In collaboration with
the FAA, GCIP facilitated the development of a water
vapor sensor mounted on commercial aircraft(Fleming 1996) and has produced 5-yr datasets for theMississippi River basin of hourly radar rain amounts(at 4-km resolution) and shortwave radiation fromsatellite data.
Each CSE has its own active data collection andmanagement program focused on priority regional
scientific issues. As the GHP began to initiate studies
along specific themes, it became evident that a more
effective means for sharing datasets among CSEs was
DECEMBER 2004

| 1925

FIG. 3. CEOP reference sites.

needed. A data committee now works toward this goalby developing protocols for data collection and exchanges and developing a common data policy.
A number of the CSEs are providing multiyear
datasets from special networks and extensive shorterperiod datasets from field campaigns for satellite product development. GHP datasets are used to assess theuncertainties in data products derived from new satellites. CSE data and their analyses have led to the development and refinement of satellite algorithms for soil
moisture and precipitation. Datasets are also being
developed to support “a priori” parameter estimation
for land surface and hydrologic models.
EMERGING ISSUES. As GHP matures, morecross-cutting themes will emerge. Some of these newthemes are described below. In addition, extreme
events will be another theme considered by the GHP
in the future.
Predictability studies. Predictability studies identify theprocesses, areas, seasons, and global circulation regimes in which predictive skill exists at longer (seasonal) time scales. Land surface processes that contribute to persistent atmospheric conditions are being
quantified and appropriately represented in coupledland–atmosphere models. To date, GHP predictability studies have helped to assess the sensitivity of mod1926 |DECEMBER 2004

els to anomalies of different sizes, determined
whether errors in initial states have long-term effects
on predictions, and established what feedbacks canaccount for the autocorrelations that are observed insoil moisture and other water cycle variables. In the
United States, for example, studies of the lowfrequency variability of soil moisture by Oglesby et al.
(2002) have shown that the influences of land surface
processes and remote ocean temperature effects are
hard to separate, due to their interdependencies. In
LBA, CPTEC is studying seasonal forecasts in the
Amazon basin. Validation and predictability assessments for the basin are made using global datasets
such as ISCCP, GVaP, and GPCP products. As datasets
become available, CPTEC plans to extend these studies to other parts of the world and to coordinate predictability studies with other CSEs in collaboration with
CLIVAR and WRAP studies.
Orographic precipitation. The effects of mountains onregional hydroclimate and water supply are an increasingly important focus for a number of CSEs.
Geography determines the distribution of precipitation and snowpacks in many parts of the world.
The rate and timing of the melt of snowpacks and glaciers often determine the availability of water throughout the summer on the surrounding lowlands.
Furthermore, the influence of mountains, such as theTibetan Plateau and the western Cordillera in thewestern United States, on the overall atmospheric circulation through barrier and elevated heat and moisture source effects needs to be better understood.
Datasets from GAME-Tibet and GAPP are expected
to contribute to the required studies. In addition,modeling studies to assess the importance of model
resolution in simulating precipitation processes in complex terrain and to correct for the influences of elevation and aspect are being initiated.

BAHC

GHP’s FUTURE. In summary, the GHP is applying
its new scientific understanding to analysis and prediction questions being developed in many regions. GHP
has made many contributions to international environmental programs and has, therefore, advanced the ability of GEWEX and WCRP to reach their own prediction goals. GHP also has contributed to the integrationof WCRP’s programs. GHP has facilitated the linkages
between the hydrologic and meteorological science
communities and between NWP organizations andhydrologic forecast services. Although many global
programs have international participation, few canclaim as much success in having as broad a multinational base of scientific leadership and financial investment as GHP.
As GHP contributes to the second phase of GEWEX,
it is shifting its emphasis from analysis to prediction.
This shift will allow individual CSEs to contribute more
effectively to the GHP mission of “demonstrating a capability to predict changes in water resources and soil
moisture on time scales up to seasonal and interannual
as an integral part of a climate prediction system.” It
will also require CSEs to develop more uniformity toensure stronger contributions to the global themes of
GHP and GEWEX. CEOP contributions also will enable GHP to explore the wide range of innovative observing capabilities that will be available with the newsatellite systems being launched during the first few
years of the twenty-first century.

CliC
CLIVARCLM
CPTEC

ACKNOWLEDGMENTS. The authors would like to express their gratitude to their many GHP colleagues from morethan 50 countries and to the space agencies, meteorologicalservices, and governmental environmental organizations
that have funded and otherwise supported the activities of
the GHP and its component experiments.

BALTEXBRIDGE
CART
CATCH
CEOP
CEOS

CSE
ECMWF
ESSP
FAA
GAME
GAPP
GCIP
GCM
GEWEX
GHP
GLDAS
GMPP
GOES
GPCC
GPCP
GPS
GRDC
GRP
GVaP
GWSP
HELP
HUBEX
IAHS
IGBP

APPENDIX: LIST OF ACRONYMS
AMC
Atmosphere Mesoscale Campaign
AMMA
African Monsoon Multidisciplinary
Analysis
ARM
Atmospheric Radiation Measurement
AMERICAN METEOROLOGICAL SOCIETY

IGPO
IOP
ISCCP

Biological Aspects of the Hydrological
Cycle
Baltic Sea Experiment
the Main BALTEX Experiment
Cloud and Radiation Testbed
Coupling of the Tropical Atmosphereand Hydrological Cycle
Coordinated Enhanced Observing
PeriodCommittee on Earth Observing
Satellites
Climate in Cold Regions
Climate Variability
Common Land Model
Centro de Previsao de Tempo e
Estudos Climaticos (Brazil)
Continental Scale Experiment
European Centre for Medium-Range
Weather Forecasts
Earth Science System PartnershipFederal Aviation Administration
GEWEX Asian Monsoon Experiment
GEWEX Americas Prediction Project
GEWEX Continental Scale International Project
Global climate model
Global Energy and Water Cycle
Experiment
GEWEX Hydrometeorology Panel
Global Land Data Assimilation System
GEWEX Modeling and Prediction
Panel
Geostationary Operational Environmental Satellite
Global Precipitation Climatology Center
Global Precipitation Climatology
Project
Global positioning system
Global Runoff Data Center
GEWEX Radiation Panel
Global Water Vapor Project
Global Water System ProjectHydrology for Environment, Life andPolicyHuaihe Basin ExperimentInternational Association of Hydrological Sciences
International Geosphere–Biosphere
Programme
International GEWEX Project Office
Intensive observing period
International Satellite Cloud Climatology Project
DECEMBER 2004

| 1927ISLSCPKORMEX
LBA
LDAS
LPB
LSA
MAGS
MDB
MOSAIC
MPI
NAME
NAO
NASA
NCEP
NOAA
NRC
NWP
OHD
PIDCAP
PILPS
PIRCS
PUB
SALLJ
SALLJEXSCSMEX
SRB
SSG
SST
TIPEX
UCAR
UNESCO
VAMOS
VIC
WCRP
WEBS
WMO
WRAP

International Satellite Land Surface
Climatology Project
Korean Monsoon Experiment
Large Scale Biosphere–Atmosphere
Experiment in Amazonia
Land Data Assimilation System
La Plata Basin
Large-scale area
Mackenzie GEWEX Study
Murray Darling Basin
A NASA land surface scheme
Max Planck Institute for Meteorology
North American Monsoon Experiment
North American Oscillation
National Aeronautics and Space
Administration
National Centers for Environmental
Prediction
National Oceanic and Atmospheric
Administration
National Research Council
Numerical weather prediction
Office of Hydrologic Development
Pilot Study for Intensive Data Collection and Analysis of Precipitation
Project for the Intercomparison ofLand Surface Schemes
Project for the Intercomparison ofRegional Climate Simulations
Project on Ungauged Basins
South America Low-Level Jet
South America Low-Level Jet ExperimentSouth China Sea Monsoon Experiment
Surface radiation budget
Scientific Steering GroupSea surface temperature
Tibetan Plateau Experiment (Chinese)
University Corporation for Atmospheric Research
United Nations Educational, Scientificand Cultural Organization
Variability of Americas Monsoon
SystemVariable Infiltration Capacity (model)
World Climate Research Programme
Water and Energy Budget Study
World Meteorological Organization
Water Resources Applications Project

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