Michel M. VERSTRAETE1 and Martin BENISTON2

1Space Applications Institute, EC Joint Research Centre, Ispra, Italy 2Department of Geography, University of Fribourg, Switzerland

This volume contains the proceedings of the workshop entitled "Satellite Remote Sensing and Climate Simulations: Synergies and Limitations" that took place in Les Diablerets, Switzerland, September 20-24, 1999. This international scientific conference aimed at addressing the current and potential role of satellite remote sensing in climate modeling, with a particular focus on land surface processes and atmospheric aerosol characterization.

Global and regional circulation models incorporate our knowledge of the dynamics of the Earth's atmosphere. They are used to predict the evolution of the weather and climate. Mathematically, this system is represented by a set of partial differential equations whose solution requires initial and boundary conditions. Limitations in the accuracy and geographical distribution of these constraints, and intrinsic mathematical sensitivity to these conditions do not allow the identification of a unique solution (prediction). Additional observations on the climate system are thus used to constrain the forecasts of the mathematical model to remain close to the observed state of the system. Ultimately, these models are useful mainly to predict the future values of environmental variables or to estimate these variables wherever and whenever they are not observed directly. Current validation of global and regional climate models is based on comparison between model outputs of standard meteorological fields and meteorological observations. The main problem with traditional meteorological observations when used to validate models is their poor representation of the grid-point average simulated by a model. Now that comprehensive radiative measurements are available from space platforms, models should produce comparable fields as standard outputs to be confronted with these new observations. Remote sensing from space platforms thus provides a unique opportunity to yield reliable and accurate information in support of global and regional weather or climate models.

Remote Sensing and Climate Modeling: Synergies and Limitations, 1-3. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

Indeed, space-based platforms permit the systematic and repetitive observation of the planetary surface and the atmosphere, at spatial resolutions generally much higher than those used in modeling. Remote sensing data can be exploited either to provide the initial and boundary conditions required to run climate models, to force these models to remain close to the real atmospheric situation, or to evaluate the accuracy of the forecasts. A number of other scientific and technological issues arise at the interface between climate modeling and remote sensing observations. This conference provided a unique opportunity to review the state of the art in the integration of the information derived from satellite remote sensing technologies in global and regional climate models. Specifically, papers were solicited along the following lines:

- The analysis of satellite remote sensing data to derive environmental variables of direct relevance to specify the initial and boundary conditions of GCMs, including surface albedo, emissivity, temperature and roughness, as well as atmospheric composition, aerosols and cloudiness, among others.

The direct assimilation of radiative measurements made in space into GCMs, to improve the accuracy of forecasts.

The evaluation of the effectiveness, reliability and accuracy of the models by comparing their results with independent remote sensing observations.

The need to produce remote sensing "observations" as standard outputs from models to compare with real remote sensing observations in order to evaluate model performances.

Scaling issues, in particular the methodological problems posed by combining field observations acquired at the local scale, remote sensing observations relative to small but spatially averaged conditions, and model simulations valid for relatively large areas. Contributions on the interpretation and proper exploitation of related but different concepts, such as skin and bulk temperatures, and interpolation in space and in time to match the needs of models with the data offered by satellite systems were also welcome.

The development of soil-vegetation-atmosphere transfer schemes (SVATs), and the improvement of these models to take advantage of observations from space.

The design and implementation of observational strategies optimized to provide the information required by the global and regional climate models at the appropriate resolution and with the necessary accuracy. The technical and institutional challenges which hinder or prevent a more exhaustive exploitation of satellite remote sensing data in regional and global climate models, including combining large data streams of remote sensing data in computationally demanding models, locating and accessing appropriate data, and designing models so that they can effectively take advantage of such observations. Also of interest and direct relevance is a discussion of how to improve the methods of remote sensing data analysis so that their products are compatible with the requirements of models.

A new generation of satellite platforms is in the process of being launched (e.g., Spot-4, Landsat-7, Terra, ENVISAT, ADEOS-II). These platforms do or will embark high performance instruments with improved spatial resolutions, enhanced radiometric accuracy, additional spectral bands and observation directions, and many other new features. At the same time, significant improvements have been made in climate modeling techniques at different scales. Last but not least, computer processing speed and communications capabilities continue to increase dramatically. This convergence creates new opportunities to document the state and evolution of the climate system, at a time when concern about climatic change and impacts has reached new heights. This conference thus provided a timely forum to discuss some of the most critical issues arising at the interface between simulation and the observation of our Earth system.

The chapters that follow contain some of the most interesting papers that were presented at this conference. Clearly, these issues will continue to be relevant for the foreseeable future. The Editors of this volume hope that these manuscripts will contribute to the debates and lead to improvements in model performance and satellite data interpretation.

We would like to acknowledge the financial support of ENAMORS (European Network for the development of Advanced models to interpret Optical Remote Sensing data) and the Swiss National Science Foundation. This funding enabled the conference organizers to support a number of outstanding speakers.

Thanks also to Sylvie Bovel-Yerly who, as always, put this volume into shape in her efficient manner.

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