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Update on CCSM3.5 Carbon Cycle Simulations

Update on CCSM3.5 Carbon Cycle Simulations. K. Lindsay, NCAR/CGD. Background. CCSM3.5 physics was finalized mid 2007 This is not a public release for CCSM. Single resolution: FV1.9x2.5-gx1v5 Model to be used for BGC development purposes, has changed considerably from CCSM3.0

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Update on CCSM3.5 Carbon Cycle Simulations

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  1. Update on CCSM3.5 Carbon Cycle Simulations K. Lindsay, NCAR/CGD Biogeochemistry Working Group June 2008

  2. Background • CCSM3.5 physics was finalized mid 2007 • This is not a public release for CCSM. • Single resolution: FV1.9x2.5-gx1v5 • Model to be used for BGC development purposes, has changed considerably from CCSM3.0 • Step 1: spinup/stabilize physics (done) • Step 2: spinup/equilibriate BGC Biogeochemistry Working Group June 2008

  3. Spinning Up BGC Cycles • Objective: Generate distribution of BGC pools that is in equilibrium with CCSM model climate and has atmospheric CO2 close to a preindustrial level. Biogeochemistry Working Group June 2008

  4. Spinning Up the Carbon Cycle • Incrementally couple BGC, leading to fully prognostic carbon cycle, to stable 1870’s configuration without BGC. • 1) Peform short run of base coupled model to generate surface forcing for ocean BGC spinup. • 2) Spinup ocean BGC with forcing from 1). Cycle POP physics to avoid drift away from model of Step 1). Fixed CO2. This step is a big hurdle. How spunup do we need to be? Biogeochemistry Working Group June 2008

  5. Step 2), Initializing Ocean Tracers with Observations Biogeochemistry Working Group June 2008

  6. Potential Ideas to Help 2),each with big caveats • Devise better BGC IC from model state. • Separate short timescale ecosystem from long timescale BGC. Spin up ecosystem and then spin up BGC using forcings from spun up ecosystem. • Interpolate circulation to coarser resolution and spin up there. • Green’s Function/Impulse Response techniques. • Extrapolation of trends. Biogeochemistry Working Group June 2008

  7. Merge Spunup BGC from CCSM3.1 with Physical State of CCSM3.5 Biogeochemistry Working Group June 2008

  8. Potential Ideas to Help 2),each with big caveats ---------------------------------------------------- • Devise better BGC IC from model state. • Separate short timescale ecosystem from long timescale BGC. Spin up ecosystem and then spin up BGC using forcings from spun up ecosystem. • Interpolate circulation to coarser resolution and spin up there. • Green’s Function/Impulse Response techniques. • Extrapolation of trends. • Newton-Krylov Solvers. Biogeochemistry Working Group June 2008

  9. Newton-Krylov Solvers • Model Map: u(t) = Φ(u(0),t) • Solve: Φ(u0,T)=u0 for u0. • Rewrite: F(u)≡Φ(u,T)-u=0 • Newton’s Method: uk+1 = uk – (∂F/∂u)-1 * F(uk) • Use Krylov iterative method (GMRES) to solve: (∂F/∂u)(δuk)=-F(uk) • Each iteration evaluates (∂F/∂u)(δu) • Finite Difference Approximation (∂F/∂u)(δu) ≈ (F(u+σδu)-F(u))/σ note this is a forward model run. Biogeochemistry Working Group June 2008

  10. Initial Efforts w/ Newton-Krylov • No preconditioner for linear system solve • Spotty/Poor convergence • not surprising, consistent with literature • Negative Chl values Biogeochemistry Working Group June 2008

  11. Current Efforts on Newton-Krylov • Develop preconditioner based on simplified representation of circulation • Offline transport is side benefit • quickly generate better IC for N-K • Do not explicitly include short timescale variables in solver • separate out long timescale tracers • use longer time span in iterations to allow short timescale variables to adjust Biogeochemistry Working Group June 2008

  12. Is there a missing source of C? Biogeochemistry Working Group June 2008

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