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Climate Modeling Basics

Climate Modeling Basics. Michael Winton 19 September 2007 NOAA/GFDL. Six Basic Questions. What is a global climate model? How do we use them? Why do we believe them? What do they agree on? Why do they disagree? How do we improve them?. What is a global climate model?.

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Climate Modeling Basics

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  1. Climate Modeling Basics Michael Winton 19 September 2007 NOAA/GFDL

  2. Six Basic Questions • What is a global climate model? • How do we use them? • Why do we believe them? • What do they agree on? • Why do they disagree? • How do we improve them?

  3. What is a global climate model? A GCM is a mathematical representation of the major climate system components and their interactions. The GCM equations operate on a global grid and are solved on a computer. Atmosphere Land Ocean Ice Physical CM Concentrations of radiatively active species Emissions of radiatively active species ESM* *Earth System Model

  4. GCM evolution

  5. GCM components: Atmosphere • Radiation • Winds • Water cycle (vapor, clouds, precipitation) • Chemistry Atmosphere Land Ocean Ice

  6. GCM components: Land • Surface characteristics • Snow cover • Soil water, rivers • Carbon components Atmosphere Land Ocean Ice

  7. GCM Components: Ocean • Currents and mixing • Biogeochemistry • Main climate system heat and carbon store Atmosphere Land Ocean Ice

  8. GCM components: Ice • Sea Ice – surface reflectivity, ocean freshwater forcing • Ice sheets and shelves – sea level, ocean freshwater forcing (currently offline) Atmosphere Land Ocean Ice

  9. Components cast on global grids

  10. Simulated vs. Parameterized • Simulated processes: larger than grid-scale, based on bedrock scientific principles (conservation of energy, mass and momentum). Example: storms. • Parameterized processes: smaller than grid scale, formulations guided by physical principles but also make use of observational data. Example: clouds.

  11. How do we use GCMs? • Diagnostic: current climate, last glacial maximum • Detection and Attribution: role of anthropogenic forcing in 20th century climate change • Prognostic: • 21st century scenarios • decadal prediction • seasonal/interannual

  12. How do we use GCMs?Role in the policy process Emissions Scenarios Impacts/Adaptations Analysis Climate Simulations Cost-Benefit Analysis Mitigation • Climate modeling is essential for optimal decision-making • Climate model disagreement is responsible for part of the uncertainty • Climate modeling informs but does not prescribe policy

  13. Why do we believe GCMs? • Based on well-founded physical principles. • Extensively checked by a large community of modelers and analysts • Accurate simulations of current and past large-scale climates • Accurate hindcast of 20th century climate change including ocean heat content

  14. What do the GCMs agree on? • Temperatures will rise • Precipitation will change • Sea level will rise • Arctic sea ice will decrease

  15. Why do GCMs disagree? • Forcings are different: e.g. aerosols • Feedbacks are different: e.g. clouds • Natural variability: but we can reduce this with ensemble of simulations Radiative Forcings (CO2, aerosols, …) Natural Varaibility Climate Feedbacks (water vapor, clouds, …)

  16. How do we improve GCMs? • Improve completeness: addition of new processes, more comprehensive forcing • Improve correctness: compare to observations, better theory, reformulate • Improve resolution: climate simulation advances as increased computing power allows better resolution of previously resolved processes and resolution of previously parameterized processes.

  17. GFDL’s Climate Modeling Effort • Past: first and foremost (Manabe era) • Present: one of a handful of leading centers worldwide, one of the best climate simulations • Diverse and deep pool of climate modeling expertise built up over many years • State-of-the art climate modeling at GFDL depends upon the availability of state-of-the art computational resources

  18. Where is world-class GCM work done?

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