Ice sheet model validation and glacier model construction by pptfiles

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									Greenland Ice Sheet model
simulations and validation
    Jeremy Fyke, Bill Lipscomb
  Los Alamos National Laboratory
• Simulated Greenland surface mass balance in CESM
• Greenland Ice Sheet model optimization within
  CESM framework
• Ongoing development
• The Glimmer Community Ice Sheet Model (Glimmer-
  CISM) has been coupled to version 1.0 of the Community
  Earth System Model (CESM 1.0).
   – Shallow-ice approximation; Greenland only
   – Higher-order ice sheet model (CISM 2.0) to be included in
     CESM 1.1 (aiming for Nov. 2012 release)
• The surface mass balance (SMB) of ice sheets is
  computed in the Community Land Model (CLM) and
  passed to Glimmer-CISM.
   – Multiple (~10) glacier elevation classes on CLM’s coarse
   – Downscaled and interpolated in z to CISM’s fine grid
                      Model details
• Fully coupled CESM 1.0 with 0.9 ox 1.25o FV atm/land, 1o ocean
• Focusing on the surface mass balance (accumulation minus
  ablation) of the Greenland ice sheet
   – SMB(ice+snow) = incoming snow + incoming rain – runoff – sublimation
   – Positive ice SMB when snow exceeds max depth (1 m water equivalent)
     and turns to ice
   – Negative ice SMB when snow depth is zero and bare ice melts
   – The SMB of ice (not snow) is passed to the ice sheet model
• Snow and ice physics:
   – Liquid water can percolate and refreeze in the snow, but not on bare ice
   – Snow albedo follows SNICAR model (depends on snow grain size, solar
     angle, etc.)
   – Bare ice albedo is prescribed (0.60 visible, 0.40 near IR)
      CMIP5 simulations with glacier elevation classes, SMB evolution
   Name                      Length          Initialization

   Pre-industrial            Years 1-100     100-yr IG run (snowpack) + BG1850CN

   20th century              1850-2005       from year 100 of Pre-industrial
   21st century (RCP8.5) 2005-2100           from year 2005 of 20th century

            Pre-industrial            1850         1940       2000             2100

                                                                                  400 Gt/yr
400 Gt/yr

                                                                                  SMB = 0

SMB = 0

            • Lower SMB in the 1940s than in the 1990s and 2000s
            • Negative SMB in several years after 2060
               Greenland SMB, downscaled to 5 km
              Pre-industrial (80-99)    20th-century (1980-1999)   RCP8.5 (2080-2099)
SMB (Gt/yr)         452 ± 91                   421 ± 107                61 ± 142

Red = net

Blue = net

                                 kg m-2 yr-1
    • 1980-99 ablation rates are higher than pre-industrial in N & NE
    • The equilibrium line rises by ~500 m by end of 21st century
        • It reaches almost 2000 m in the NE and southern half of E margin
        • High snowfall rates help to keep equilibrium line low in NW and mid-W margins
   SMB, comparison with RACMO (at 5 km res)
                1958-2007 (plot 1958-2005)            RACMO
SMB (Gt/yr)             409±106                       469±41

• Good match in ablation zones
• Accumulation rates are overestimated in the interior and underestimated in
  the SE (smoother orography in CESM)
• Snowfall local maxima along W coast and impact on melt (via albedo) are
  well captured
           Temperature and SMB: 1850-2005
 JJA mean temperature over ice sheet


                                       Runoff SMB
  1850                          2005
                                       1850                     2005

         • Warm period during 1930s and 1940s, with high melt
         • Precipitation rates are higher in the 1990s
         • High SMB following Pinatubo (Pi) eruption in 1991
Temperature anomalies: 2080-99 minus 1980-99
 annual                                   JJA

  • MOC reduction reduces warming SE of Greenland
  • JJA increase is highest
      • In ice-free regions to N & E, in part due to stronger sea ice losses
        (>40%) along the coast
      • In the interior of the ice sheet, which remains below melting point
            SMB (Gt/yr): 1980-2100

                                                          Blue = Precip
                                                          Red = Melting
                                                          Green = Runoff
                                                          Black = net SMB

SMB = 0

          1980                                     2100

• Precipitation increases with time
• Melt and runoff increase by a larger amount
• SMB is negative for the first time around 2030
       Summary: Greenland SMB
• The SMB scheme works well. Greenland’s simulated 20th
  century surface mass balance and trends are in good
  agreement with RACMO, a state-of-the-art regional model
  (with differences due to smoother CESM topography).
• During the 21st century simulation, the SMB decreases from
  ~400 Gt/yr to near zero.
• Greenland average warming in the 21st century is roughly
  equal to global average warming. There is more warming in
  the North and East (less summer sea ice) than in the
  Southeast (reduced MOC).
              Ice sheets in RASM
• Coupling to CISM is included in the current version of the
  CESM coupler; should not be hard to include in RASM.
• The coupler requires the ice-sheet surface mass balance
  in multiple elevation classes from the land model. Next
  step is to implement a similar scheme in VIC.
• How much code can be reused from CLM?
Greenland Ice Sheet (GIS) optimization
• Will be necessary for GIS in RASM
• Carried out in support of SeaRise: model
  intercomparison project to assess range of
  modelled ice sheet responses to idealized climate
  perturbations (Δclimate, Δdynamics)
• Initial state of ice sheet should reflect observed
  ice sheet: exercise in rapid (1 month turnaround)
  model optimization
• Tool: Latin Hypercube Sampling of uncertain
  parameter space
             Optimization approach
• Generate 100 GIS realizations with LHS-determined random
  combinations of:
   – Ice sheet enhancement factors
   – Basal sliding coefficients
   – Geothermal heat fluxes
• Compare equilibrium state (after 9 kyr simulation) to
  observed GIS state for:
   –   Ice volume error
   –   Ice area error
   –   RMSE of ice surface elevation
   –   Maximum ice elevation error
   –   Summit horizontal offset error
• Rank models by ‘worst diagnostic ranking’ to get best all-
  around GIS realization
Optimization approach

                         SeaRise ns

   9000 years           future
Optimization results: volume evolution
 Optimization results: example GIS
model-observed elevation differences
Optimization results: rankings for all
Optimization results: dependence of
  diagnostics on LHS parameters
Optimization results: top-performing
    ice sheet model realizations
         Ice sheet spinup issues
• Spinup/optimization issues to work on:
  – Thermal timescale of ice sheet (thus, ice viscosity)
    is 105 years – analogous to spinning up the deep
    ocean (but worse!)
  – How to spin up a GIS model, using forcing that is
    continuous between past and future, that
    captures transient thermal and geometric state of
    ice sheet?
  – LHS ensemble limited to sampling internal ice
    sheet parameters
• LHS sampling provides a fast way to determine
  optimal initial state for GIS models within a
  climate model framework
• Flow factor exerts major control on ice sheet
  optimization in CISM
• Similar optimization technique will be necessary
  to optimize the GIS under RASM forcing
• RASM surface mass balance field (reflected in
  long-term GIS spinup geometry) will be sensitive
  indicator of regional atmospheric model biases
             Ongoing development
• New ice-sheet dynamical cores
  1. Payne-Price: 3D higher-order, finite difference, structured
     grid, Trilinos solvers
  2. BISICLES: Vertically integrated higher-order, finite volume,
     Chombo adaptive mesh refinement software
  3. FELIX: Full-Stokes/higher-order, finite element,
     unstructured variable-resolution mesh (MPAS framework),
     Trilinos solvers
• BISICLES and FELIX will be further developed under a
  new 5-year DOE SciDAC project, Predicting Ice Sheet
  and Climate Evolution at Extreme Scales (PISCEES).
             Ongoing development
• Improved physics parameterizations
  – Subglacial hydrology and basal sliding (S. Price, M. Hoffman)
  – Calving (based on Potsdam-PIK)
• Two-way coupling with land model
  – Requires dynamic landunits (glaciers  vegetation)
  – May not be important on decadal time scales
• Coupling with ocean model
  – POP2X simulates ocean circulation beneath ice shelves (X.
    Asay-Davis); will be applied to Antarctica
  – May not be practical for RASM in near term; Greenland fjords
    require very high resolution (~1 km)
Extra slides
SMB trend 1958-2005 (kg m-2 yr-2)
               • Negative trend in ablation zones
               • Positive trend in the Southeast,
                 due to increasing precipitation
               • Consistent with RACMO results
                 and altimetry measurements
                           Terms of SMB
Units: Gt per year   RACMO 1958-2007   CLM 1980-1999      Diff CLM-RACMO
SMB (net)            469               403 ± 106          -66
MB (snow)                              -5
SNOW                 697               742 ± 82           +45
RAIN                 46                135 ± 23           +89
PRECIP               743               877 ± 98           +134
RUNOFF               248               425                +177
SUBLIMATION          26                54 ± 3             +28

Units: Gt per year   RACMO 1958-2007   1980-1999
MELT (only snow)                       430 ± 67
MELT (snow + ice)    404               530 ± 109          +126
MELT+RAIN            450               665 ± 117          +215
REFREEZING           202 (45% of       240 ± 27           +38
                     ME+RAIN)          (36% of ME+RAIN)
           Terms of SMB: 1980-1999

         SMB                 Melt               Runoff                  Rain

•   Runoff = Melt + Rain - Refreezing > 0 in the interior of the ice sheet, where
    all available liquid water should refreeze
•   In CLM, rain is overestimated in ice sheet interior (and rain cannot
    refreeze if snow thickness = 1 m w.e.)
21st century temperature increase (ref: 1980-1990)

                                                   Greenland + ocean

Greenland ice sheet



                      Temperature anomalies for 2080-2099
     region                    Annual (st. dev.)      Summer (st. dev.)
     Global                    3.6 (0.3)
     Greenland ice sheet       3.8 (0.6)              3.5 (0.8)
     Greenland region          3.5 (0.5)
    Terms of SMB: RCP8.5
Units: Gt per year   1980-1999          2080-2099
SMB-net              403 ± 106          12 ± 148
MB (snow)            -5                 -1
SNOW                 742 ± 82           807 ± 74
RAIN                 135 ± 23           279 ± 45
PRECIP               877 ± 98           1086 ± 105
RUNOFF               425                1018 ± 167
SUBLIMATION          54 ± 3             57 ± 5

Units: Gt per year   1980-1999          2080-2099
MELT (only snow)     430 ± 67           624 ± 65
MELT (snow + ice)    530 ± 109          1040 ± 160
MELT+RAIN            665 ± 118          1320 ± 187
REFREEZING           240 ± 27           301 ± 27
                     (36% of ME+RAIN)   (23% of ME+RAIN)
                Seasonal cycle of melt

                                                         Solid black line =
                                                         Ice melt, 1980-1999
                                                         Solid red line =
                                                         Ice melt, 2080-2099

                                                         Dotted black line =
                                                         Snow melt, 1980-1999
                                                         Dotted red line =
                                                         Snow melt, 2080-2099
                  J F M A M J        J A S O N D

• Length of snow melt season does not change (melt season begins in April)
• Ice begins to melt ~15 days earlier and melts for ~15 days more in late

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