1 Chapter 9: Evaluation of Climate Models

Zero Order Draft Chapter 9 IPCC WGI Fifth Assessment Report 1 2 Chapter 9: Evaluation of Climate Models 3 4 Coordinating Lead Authors: Gregory Flato (Canada), Jochem Marotzke (Germany) 5 6 Lead Authors: Babatunde Abiodun (South Africa), Pascale Braconnot (France), Sin Chan Chou (Brazil), 7 William Collins (USA), Peter Cox (UK), Fatima Driouech (Morocco), Seita Emori (Japan), Veronika Eyring 8 (Germany), Chris Forest (USA), Peter Gleckler (USA), Eric Guilyardi (France), Christian Jakob (Australia), 9 Vladimir Kattsov (Russia), Chris Reason (South Africa), Markku Rummukainen (Sweden) 10 11 Contributing Authors: Johanna Baehr, Alejandro Bodas-Salcedo, Bode Gbobaniyi, Stephen Griffies, 12 Elizabeth Hunke, Tatiana Ilyina, Stephen A. Klein, Reto Knutti, Felix Landerer, Florian Rauser, Mark 13 Rodwell, Adam A. Scaife, John Scinocca, Hideo Shiogama, Ken Sperber, Bjorn Stevens, Keith Williams, 14 Tim Woollings 15 16 Review Editors: Isaac Held (USA), Andy Pitman (Australia), Serge Planton (France), Zong-Ci Zhao 17 (China) 18 19 Date of Draft: 15 April 2011 20 21 Notes: TSU Compiled Version 22 23 24 Table of Contents 25 26 Executive Summary..........................................................................................................................................3 27 9.1 Climate Models and their Characteristics..............................................................................................4 28 9.1.1 Introduction.....................................................................................................................................4 29 9.1.2 Overview of Models to be Evaluated ..............................................................................................4 30 9.1.3 The Path to Model Improvement.....................................................................................................5 31 Box 9.1: Potential Box on Model Tuning........................................................................................................6 32 9.2 Techniques for Assessing Model Performance ....................................................................................18 33 9.2.1 Objectives and Limitations............................................................................................................18 34 9.2.2 New Developments in Model Evaluation Aapproaches................................................................18 35 9.2.3 The Role of Model Intercomparisons............................................................................................23 36 9.2.4 Overall Summary of Approach that Will be Taken in this Chapter ..............................................24 37 9.3 Simulation of Recent and Longer-Term Records in Global Models .................................................25 38 9.3.1 Introduction – Basic Characterization of Climate Model Experiments .......................................25 39 9.3.2 Atmosphere....................................................................................................................................26 40 9.3.3 Simulation of Recent and Longer Term Trends – Ocean..............................................................33 41 9.3.4 Sea Ice...........................................................................................................................................39 42 9.3.5 Land Surface, Fluxes, and Hydrology ..........................................................................................40 43 9.3.6 Carbon cycle .................................................................................................................................42 44 9.3.7 Sulfur Cycle...................................................................................................................................44 45 9.4 Simulation of Variability and Extremes...............................................................................................45 46 9.4.1 Introduction...................................................................................................................................45 47 9.4.2 Diurnal and Seasonal Cycles........................................................................................................45 48 9.4.3 Simulation of Variability Around the Mean State.........................................................................48 49 9.4.4 Extreme Events..............................................................................................................................55 50 9.5 Downscaling and Simulation of Regional Scale Climate ....................................................................56 51 9.5.1 Introduction...................................................................................................................................56 52 9.5.2 Fidelity of Downscaling Methods and Value Added.....................................................................57 53 9.5.3 Transferability Experiments..........................................................................................................59 54 9.6 Sources of Model Errors and Uncertainty ...........................................................................................60 55 9.6.1 Introduction...................................................................................................................................60 56 9.6.2 Process Oriented Evaluation ........................................................................................................60 57 9.6.3 Targeted experiments....................................................................................................................62 Do Not Cite, Quote or Distribute 9-1 Total pages: 137 Zero Order Draft Chapter 9 IPCC WGI Fifth Assessment Report 1 9.6.4 Climate Sensitivity.........................................................................................................................66 2 9.6.5 Discussion of Results in Context of Section 9.3 and Section 9.4 ..................................................69 3 9.7 Relating Model Performance to Credibility of Projections ................................................................69 4 FAQ 9.1: Are Climate Models Getting Better, and How Would We Know? ...........................................71 5 References........................................................................................................................................................73 6 Tables...............................................................................................................................................................98 7 Figures ...........................................................................................................................................................101 8 Do Not Cite, Quote or Distribute 9-2 Total pages: 137 Zero Order Draft Chapter 9 IPCC WGI Fifth Assessment Report 1 Executive Summary 2 3 Climate models play an important role in climate research, enhancing our ability to understand past climate 4 change and providing quantitative information about the future. Confidence in using climate models is based 5 on careful evaluation of model performance, making use of increasingly comprehensive observationally- 6 based data sets and well-designed model intercomparison activities. This chapter provides an assessment of 7 climate model evaluation, focusing particularly on developments since the IPCC Fourth Assessment Report 8 (AR4). A range of models are considered, including: 9 10 • coupled Atmosphere-Ocean General Circulation Models (AOGCMs) used in both long-term climate 11 projection and shorter-term (seasonal to decadal) climate prediction; 12 13 • their extension to ‘Earth System’ Models (ESMs), in which representation of climatically important 14 biogeochemical cycles are included; 15 16 • higher resolution, limited area Regional Climate Models (RCMs) used extensively in downscaling global 17 climate results to particular regions; 18 19 • Earth System Models of Intermediate Complexity (EMICs) used to undertake very long (e.g., millennial) 20 climate simulations, or to provide large ensembles exploring parameter uncertainty. 21 22 The evaluation of climate models depends directly on the availability of high-quality observational data sets 23 whose uncertainty is understood and quantified. These observational data have been described in earlier 24 chapters. A particular advance since the AR4 has been in the area of model ‘metrics’ – that is, numerical 25 measures of model performance reflecting the difference between a model and a corresponding observational 26 estimate. These metrics allow more systematic evaluation of models and more concise presentation of 27 evaluation results. This chapter will make extensive use of such metrics, with more traditional presentation 28 of ‘error maps’ included in the supplementary material. 29 30 Another advance since the AR4 is the extensive use of ‘satellite simulators’ in climate models. This involves 31 on-line calculations which provide output more directly comparable to remote sensing observations from 32 satellites. This approach is particularly valuable in evaluating the representation of clouds in climate models. 33 34 The availability of carefully constructed multi-model experiments, notably the Coupled Model 35 Intercomparison Projects (CMIP3 and CMIP5) and the Coordinated Regional Downscaling Experiment 36 (CORDEX), allow for increasingly in-depth analysis of model results. The multi-model ensemble allows for 37 some assessment of uncertainty

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