1. Modelling Theme White Paper G. Flato, G. Meehl and C. Jakob

2. Introduction: Numerical models are fundamental tools in climate research and environmental prediction. They are used in weather forecasting, process studies, data assimilation and analysis, attribution, historical and paleo-climate simulation, seasonal to interannual climate prediction, future climate projections, regional downscaling, etc. Reducing errors and uncertainties requires a strategy that combines lessons learned from model applications, improved understanding and representation of processes, and comprehensive evaluation based on high-quality observations.

3. 1)Promoting the confrontation of models with observations and the results of detailed process studies 2)Promoting the collaboration among the various climate science communities that are integral parts of WCRP modelling 3)Promoting the application of models to problems of societal relevance, and the quantification of model strengths and weaknesses 4)Promoting the development of model improvements WCRP Modelling Theme should comprise four complementary activities:

4. Builds on productive coordinate experiments like CFMIP – comprehensive comparison of models to a broad suite of observations, as well as detection/attribution and other coordinated diagnostic studies. Involves development of increasingly sophisticated diagnostics and observation simulators, along with alternative ways of assessing model response on long time scales. Requires close collaboration with observational programs and process sudies. Builds upon exising panels and working groups such as GMPP, CCMVal, WGOMD, CFMIP, PMIP, WOAP, CRP, CEOP, etc. 1) Promoting the confrontation of models with observations and the results of detailed process studies

5. Initially involves interaction amongst numerical weather prediction, seasonal-to-interannual prediction and climate projection communities Growing interaction with biogeochemistry, air quality, terrestrial ecology, glaciology and other communities as we move toward more comprehensive ‘Earth System’ models Such interactions allow alternative insights into model errors (e.g. NWP applications focus on ‘fast’ processes whereas climate applications depend more on ‘slow’ processes). Examples include IGBP/AIMES collaboration with WGCM as well as collaborations with NWP community via WGNE and WGSIP 2) Promoting collaboration among various climate science communities involved in modelling

6. Feeds into the theme of climate applications and services, providing model-derived information to inform decision-making, adaptation planning and mitigation policy. Objective is to insure that model results are widely available and that their uncertainties are well quantified. Demands ongoing international coordination to provide an ensemble of model results that are subject to extensive scrutiny. Builds upon WGCM coupled model intercomparisons (and their direct feed into IPCC assessments), as well as emerging efforts at shorter time scales under the auspices of WGSIP and WGNE. New activity on coordinate climate downscaling (via TFRCD) is also directly relevant. 3) Promoting application of models to problems of societal relevance, and the quantification of strengths and weaknesses

7. Ultimate WCRP goal must be to improve model performance. Involves improving existing model formulations as well as development of novel approaches. WCRP is in a unique position to help reverse the decline in basic model development activity by: implementing a new strategy for model development and promoting the importance of this underpinning activity. 4) Promoting the development of model improvements

8. Modelling clearly cuts across all core projects and connects directly with other central themes. There are already well-established panels and working groups and these should be maintained. No need for an ‘overarching’ body, but should institute a WCRP Modelling Council made up of the co-chairs of existing groups and a small subset of JSC members charged with liaising with the various groups. Role would be: to insure coordination and connection amongst relevant activities organize one day session at each JSC meeting to review modelling activities, progress and connections to other themes Organize a semi-regular WCRP-wide modelling conference or workshop (organization to rotate among the three main modelling groups: WGNE, WGSIP and WGCM) – the WGNE workshops on systematic errors serve as an example Where appropriate, organize limited lifetime cross-cutting activities to promote attention to particular high-priority topics (these might resemble the CLIVAR Climate Process Teams) Proposed organizational changes

9. 3) Applications and intercomparisons – applying models for scientific and societal benefit, quantifying errors, multi-model ensembles, analysis and dissemination of results 2) Collaborating with others – ‘outward looking’ – insuring that WCRP modelling is informed by and complements activities in related fields. WCRP 1) Confronting models with obs – ‘inward looking’ – making the most of what is produced by core projects. Connects to observations and diagnostics. WCRP GMPP CCMVal WGOMDSNOMIP NWP Air-quality modelling Ecosystem modelling Integrated assessment modelling Numerical methods WGCMWGSIP TFRCD ACC Impact assessments Stakeholders IPCC Services WCRP Modelling Theme 4) Promoting improvement in models; new strategies; renewed effort and investment. Biogeochemistry WGNE CFMIPPMIP