GLASS : GLOBAL LANDATMOSPHERE SYSTEM STUDYJan Polcher1, Peter Cox2,
Paul Dirmeyer3, Han Dolman4,Hoshin Gupta5, Ann Henderson-Sellers6,Paul Houser7, Randy Koster7, Taikan Oki8,
Andy Pitman9, and Pedro Viterbo10
1

Laboratoire de Meteorologie Dynamique du CNRS,
Hadley Centre, 3Center for Ocean-Land-Atmos. Studies,
4
Alterra, 5University of Arizona, 6ANSTO,
7
NASA/Goddard Space Flight Center,
8
University of Tokyo, 9Macquarie University
10
European Centre for Med.-Range Weather Forecasts

2A new generation of land surface schemes(LSS) is emerging. The schemes are evolvingfrom general circulation model parameterizations thatprovide fluxes to the atmosphere into independent
models that are increasingly being compared tomodels of hydrology, biogeochemistry and ecology.
This expanding scope is driven by the growth ofinterdisciplinary studies of the earth system. GLASS
aims to encourage these developments by coordinating the evaluation and intercomparison of thisnew generation of LSSs, and applying them to scientific queries of broad interest.
Workshops organized by GEWEX, the European
Centre for Medium-Range Weather Forecasts(ECMWF), the International Geosphere-Biosphere
Programme (IGBP), and the Institut National desSciences de l'Univers (INSU) during the past few
years have pointed out a number of directions inwhich innovation in the next generation of LSSs
will be necessary. The first point identified was
the inclusion of bio-physical processes. There is aneed to include the carbon cycle in order to beable to provide the atmospheric models with CO2
fluxes and to simulate the evolution of the vegetation and thus the feedbacks between climatic variations
and the state of the biosphere. The second feature that will set this new generation apart will bethe larger importance given to the horizontal
complexity of the surface. Indeed, previous LSSs
were characterized by a very sophisticated description of the vertical processes while retaining simple
assumptions on the horizontal variance of surface
conditions. The final innovation to be included in
the next generation will be data assimilation methods
for either the analysis of the state of the surface or
the evaluation of surface schemes at the global
scale. The remotely sensed variables that will become available in the years to come will demand
May 2000from LSSs new methods for taking advantage ofthe new information. With this increase in complexity, new feedbacks between the atmosphere andthe surface will be simulated and a more systematic analysis of the sensitivity of the climate tosurface processes is needed.
Land surface scheme validation andintercomparison projects such as the Project forIntercomparison of Land-surface Parameterization
Schemes (PILPS) (Henderson-Sellers et al., 1995)
and the Global Soil Wetness Project (GSWP)(Dirmeyer et al., 1999) helped to assess the previous generation of LSSs. PILPS made major
contributions to their evaluation at the local scaleusing observed atmospheric forcing and high quality
measured fluxes for the validation of the simulated
processes. In GSWP on the other hand, the LSSs
were used in a mode much closer to their application in GCMs (i.e., on a global grid with a resolution
coarser than the typical spatial scales of surface
processes). Both of these projects proved very usefuland need to be continued with the next generation
for the validation of added processes. However,they also need to be supplemented by new experiments.
As land surface schemes are applied from theplot scale to the regional scale and in a forced orcoupled mode, the new experiments will specificallyhave to address the issues of sub-grid scale variability of the surface and the feedbacks existing
between surface processes and the atmosphere. The
complementarity of the various applications of LSSs
provides a richness of research and can be illustrated, for instance, with the case of simulated soil
moisture. In the PILPS experiments the soil moisture simulated with the atmospheric forcing observed
at a site could be compared to the measured values(Shao and Henderson-Sellers, 1996). In GSWP, only
the soil moisture anomalies at 1ox1o could be evaluated as the models use different water holding
capacities, which were all different from the onethat can be deduced from observed values (Entin etal., 1999). When an LSS is coupled to an atmospheric column model the relationship between thesoil moisture and the atmospheric conditions can beexplored at the local scale. This set-up was used
by Douville et al. (2000) to determine how theassimilation of near surface temperature and humidity could help improve the simulated soil moisture.
Finally, with coordinated GCM sensitivity experiments the impact of the different approaches to soil
moisture modelling on climate change could be evaluated (Crossley et al., 2000). Unfortunately, we are3not presently able to say how the results obtainedin PILPS can be related to the uncertainty in soil
moisture changes found for a climate with increased greenhouse gas concentrations. Thus, acoordination of projects is needed to take fulladvantage of the diversity of application areas forLSSs.
The aim of GLASS is to foster an evaluation of the next generation of LSSs and tocoordinate the evaluation of LSSs in their different applications. GLASS will also serve as
an interface between the land-surface community and other GEWEX projects. The proposed
structure of GLASS (see figure below) highlights
the spatial scales at which the schemes are applied and the degree of interaction allowed withthe atmosphere. This denotes four actions which
will coordinate intercomparisons in their field ofland-surface scheme applications. GLASS will also
include one transversal action [Assistance for Landsurface Modelling Activities (ALMA)] which will
provide an infrastructure and technical support forthese intercomparisons.
The action dedicated to local scales and offline simulations is the continuation of the PILPS
project. The next intercomparisons will be carried out over the Torne basin in Sweden (seearticle on page 8) and over a mature forest at
the Russian Valdai site. These experiments willallow an evaluation of cold-season processes inLSSs. It is expected that in the near future aPILPS experiment will be launched to evaluate
the newly gained ability of land-surface schemes
to simulate CO2 fluxes.

4The action concentrating on global off-line applications of LSSs is the continuation of GSWP.
Before the next phase of GSWP can be launchedwith the International Satellite Land Surface Climatology Project (ISLSCP) Initiative II data,
sensitivity experiments will be carried out to evaluate the impact of errors in the forcing data.
Experiments are also planned to evaluate our ability to take into account the spatial variability of soil
wetness at a 1ox1 o resolution. Later, with the 10years of ISLSCP Initiative II data the inter-annual
variability of LSSs will be evaluated.
The local coupled action will concentrate on
data assimilation issues. The first step towards thisgoal will be to set up a simplified coupled system
in which LSSs and their interaction with the atmosphere can be compared. With this tool in hand thesimulated interactions between sub-grid variability
and the planetary boundary layer will be compared.
This will also serve to evaluate data assimilation
methods for land-surface schemes.
In the global coupled action, the sensitivity of
the atmosphere to land-surface processes will be
studied using an ensemble of general circulation
models and LSSs. The two issues that are most in
need of model independent assessments are therole of climate variability and the sensitivity ofclimate to anthropogenic forcing. This action will
work in close collaboration with the two Atmospheric Model Intercomparison Project (AMIP)
diagnostic sub-projects which deal with the validation of surface processes in GCM simulations.
The infrastructure action, ALMA, will help thefour actions described above by providing data exchange standards and software to manage the data.

May 2000

ALMA will also help participating groups with theimplementation of the PILPS-4c coupler (Polcher etal., 1998) and ensure the exchange of software andexpertise with the aim of facilitating the participation in GLASS projects.
GLASS will be managed by a science panel
composed of the leaders of the five actions
and representatives of the GEWEX Hydrometeorology Panel (GHP), ISLSCP, IGBP/
Biospheric Aspects of the Hydrologic Cycle(BAHC) and the Working Group on Numerical
Experimentation (WGNE). Its role will be to coordinate the five actions and ensure that all application
areas of LSSs benefit from progress made in thevarious projects. The panel will hold its first meeting this summer and at that time the implementation
plan of GLASS will be finalized. The first local
scale off-line experiment (PILPS-2e) will be launchedthis month and the data exchange protocol to be
used has been developed in collaboration with ALMA.
Other actions have submitted projects for funding
and will start before the end of the year. Theevolution of GLASS can be followed at its Web
site: http://hydro.iis.u-tokyo.ac.jp/GLASS/.
References
Crossley, J., J. Polcher, P. Cox, N. Gedney, and S. Planton,
2000. Uncertainties linked to land-surface processes in climatechange simulations. Clim. Dyn., in press.
Dirmeyer, P. A., A. J. Dolman, and N. Sato, 1999. The global
soil wetness project: A pilot project for global land surface
modeling and validation. Bull. Amer. Meteor. Soc., 80:851-878.
Douville, H., P. Viterbo, J.-F. Mahfouf, and A. C. M. Beljaars,
2000. Evaluation of the optimum interpolation and nudging
techniques for soil moisture analysis using FIFE data. Mon.
Wea. Rev., 128:in press.
Entin, J. K., A. Robock, K. Y. Vinnikov, V. Zabelin, S. Liu, A.
Namkhai, and T. Adyasuren, 1999. Evaluation of global soil
wetness project soil moisture simulations. J. Meteor. Soc. Japan, 77:17-32.
Henderson-Sellers, A., A. J. Pitman, P. K. Love, P. Irannejad,and T. Chen, 1995. The project for intercomparison of landsurface parameterization schemes (PILPS): Phase 2 and 3. Bull.
Amer. Meteor. Soc., 76:489-503.
Polcher, J., B. McAvaney, P. Viterbo, M.-A. Gaertner, A.
Hahmann, J.-F. Mahfouf, J. Noilhan, T. Phillips, A. Pitman, C.
A. Schlosser, J.-P. Schulz, B. Timbal, D. Verseghy, and Y. X.,
1998. A proposal for a general interface between land-surface
schemes and general circulation models. Glob. and Plan. Change,
19:263-278.
Shao, Y. and A. Henderson-Sellers, 1996. Modelling soil moisture: a project for intercomparison of land surface parameterization
schemes phase 2(b). J. Geophys. Res., 101(D3):7227-7250.

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