Mirin – AMWG 2006 – Slide 1 Coupled Finite-Volume Simulations at One-Degree Resolution Art Mirin and Govindasamy Bala Lawrence Livermore National Laboratory

Mirin – AMWG 2006 – Slide 2 Outline of presentation Introduction to finite-volume dynamical core Parallelization, unification and simulation efforts Diagnosis of seasonal ice buildup Acknowledgments: — Dani Bundy, Brian Eaton — Phil Rasch, Mariana Vertenstein Work performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. This is LLNL Report UCRL-PRES

Mirin – AMWG 2006 – Slide 3 Attributes of finite volume dynamical core Developed by S.J. Lin and R. Rood of NASA GSFC Terrain-following “floating” Lagrangian control-volume vertical coordinate Two-dimensional conservative semi-Lagrangian transport within a control-volume Monotonicity-preserving mass-, momentum-, and total energy-conserving mapping algorithm to Eulerian reference coordinate

Mirin – AMWG 2006 – Slide 4 Terrain-following vertical coordinate Lagrangian coordinate evolves according to vertical transport

Mirin – AMWG 2006 – Slide 5 Attributes of finite volume dycore, cont. Dynamics subcycled with respect to remapping Multiple, staggered horizontal Eulerian grids invoked Semi-Lagrangian transport conserves key physical quantities Semi-Lagrangian algorithm circumvents polar singularity Fast timescale contains geopotential calculation that couples vertical levels through indefinite integrals

Mirin – AMWG 2006 – Slide 6 The FV dycore uses a hybrid parallel model Multi-two-dimensional domain decomposition — latitude-vertical (yz) 2-D domain decomposition for most of dynamics — longitude-latitude (xy) 2-D domain decomposition for remapping and geopotential calculation Shared memory parallelism (OpenMP) largely in vertical, but also in latitude Decompositions connected by transposes using Pilgrim and Mod_comm libraries (NASA/GSFC) — MPI derived types — MPI-2 one-sided communication

Mirin – AMWG 2006 – Slide 7 FV unification effort FV originally implemented in NASA FVGCM (GEOS4) FV then implemented in CAM NASA GEOS5 contains FV as ESMF module FV on cubed sphere under development Unification — unification of CAM and GEOS5 (ESMF) versions nearly complete; outside world will see only lat-lon decomposition — S-J Lin’s FVGCM improvements will be merged into CAM/GEOS5 version — common repository at GFDL — eventual merge with cubed sphere version

Mirin – AMWG 2006 – Slide 8 Coupled FV usage and study at LLNL CCSM3.0 ported to LLNL thunder IA64 Linux cluster — platform-relevant configuration script files — perturbation growth test for CAM — T85 validation test Instituted support for 1x1.25_gx1v3 mesh — created land surface file — coupler mapping files — historical run — ~10 simulated years/day using 118 (4-processor) nodes Detection and Attribution of Regional Climate Change — targeting western United States — coupled FV at 1x1.25 – 1000 year spin-up Investigating known sea ice problem with coupled FV

Mirin – AMWG 2006 – Slide 9 The ice problem in FV 2x2.5 Excessive ice south of Greenland

Mirin – AMWG 2006 – Slide 10 Sensitivity tests at 2x2.5 to diagnose sea ice issue We have performed the following sensitivity tests: — albedo reduction — increase/decrease wind stress over ocean — increase/decrease wind drag on ice — eliminate ice dynamics — upwind ice advection algorithm Removing ice dynamics largely eliminates the ice problem Other variations have little effect

Mirin – AMWG 2006 – Slide 11 Sea ice – T85, FV 2x2.5, FV 2x2.5 without ice dynamics Eulerian – top left FV 2x2.5 – top right FV 2x2.5 without ice dynamics – bottom left

Mirin – AMWG 2006 – Slide 12 Discussion of FV 1x1.25 simulation Modified the following parameters based on CAM stand- alone tuning exercise at 1x1.25 (Bala in correspondence with Hack) — low cloud threshold (rhminl): 0.88 => 0.87 — cold ice autoconversion (icritc): 9.5e-6 => 18.0e-6 Initiated 21-year CCSM run that showed SST drift of -1.57° Increased rhminl from 0.87 to 0.91 and initiated new 20- year run — ice buildup was as bad as with 2x2.5 case

Mirin – AMWG 2006 – Slide 13 Discussion of FV 1x1.25 simulation, cont. Lowered ice and snow albedos — albicev=0.68 (vs 0.73) — albicei=0.30 (vs 0.33) — albsnowv=0.90 (vs 0.96) — albsnowi=0.62 (vs 0.68) Lower albedos improve ice in winter but worsen summer ice — seasonal cycle has larger amplitude with FV Note: all runs use improved ‘remap’ topography

Mirin – AMWG 2006 – Slide 14 Winter ice fraction – FV 1x1.25 vs T85 (left), 1x1.25 vs 2x2.5 (right)

Mirin – AMWG 2006 – Slide 15 Winter ice thickness – FV 1x1.25 vs T85 (left), 1x1.25 vs 2x2.5 (right)

Mirin – AMWG 2006 – Slide 16 Summer ice fraction – FV 1x1.25 vs T85 (left), 1x1.25 vs 2x2.5 (right) Missing plot

Mirin – AMWG 2006 – Slide 17 Summer ice thickness – FV 1x1.25 vs T85 (left), 1x1.25 vs 2x2.5 (right)

Mirin – AMWG 2006 – Slide 18 Seasonal cycle – FV 1x1.25 vs T85

Mirin – AMWG 2006 – Slide 19 Discussion of FV ice issue Ice buildup near Greenland is seasonal Decreasing ice/snow albedos improves winter ice at expense of summer ice; seasonal amplitude insensitive to albedos Ice issue with coupled FV is not necessarily the fault of the FV dycore — nonlinear coupling among components –some cite errors in surface wind stress –some cite errors in ocean surface current Case at 1x1.25 with original albedos being continued further Next steps???