Ice Sheet for PMIP3/CMIP5 simulations

This page synthesises the Ongoing work for the definition of a new ice-sheet reconstruction to be used in LGM simulations for PMIP3/CMIP5.

Introduction

Several PMIP participants are in favour of using a new ice-sheet reconstruction that better matches the geomorphologic and glaciological constraints than ICE-5G used in PMIP2. We plan thus to have - a reference ice-sheet for the core simulations - alternatives for groups interested to run sensitivity experiment in order to discuss the sensitivity of the model results to the uncertainties in boundary conditions.

We are now considering the possibility to use a new ice-sheet reconstruction proposed by Lev Tarasov and/or Dick Peltier (see text below). The final decision on the ice-sheet will be taken in September. It will depend on what is available at that time, quality checking and climate model constraints. We will involve several PMIP participants in the process, and you are all invited to comment on the proposition in the discussion panel (at the bottom of this page). It is still possible that we have to find an alternative and less rigorous option by the end of September, which is the deadline to finalize the boundary condition for CMIP5.

From what is ready today, you can find below

a description of what is going on for the new reconstruction using both ICE-5G constraints and a glaciological model (the Lev-MOCA proposition discussed for PMIP3 deglaciation sub-project)

a new reconstruction using a new version of Dick's model (ICE-6G)

Please have a look at these files and make all the comments that can help us to finalise the CMIP5 boundary conditions rapidly.

Details on the proposed LGM and deglacial ice-reconstructions

Lev-MOCA

Northern Hemispheric components (GLAC-1)

The Greenland reconstruction is the GrB model described in Tarasov and Peltier( GJI 2002 and JGR 2003). It's a hand tuning of a 3D thermo-mechanically coupled glaciological shallow ice-sheet model and visco-elastic bedrock response model (using the same VM2 rheology used for ICE-5G) to fit the Relative Sea-Level (RSL) observations, the GRIP borehole temperature and age profiles, and approximate basal temperatures at CampCentury and Dye3 along with secondary constraints described in the references.

The North American and Eurasian reconstructions are objective Bayesian calibrations of the MUN/UofT glacial systems model (above 3D model plus asynchronously coupled down-slope surface drainage/lake depth solver, and various other components such as a thermodynamic lake ice, sub-glacial till-deformation, ice-shelf represention,…, some of which are described in Tarasov and Peltier, QSR 2004, and Nature 2005, and a more complete description is currently being written up). The calibration involves approximately 30 ensemble parameters to capture uncertainties in deglacial uncertainties in deglacial climate and ice dynamics. Model runs cover a full glacial cycle. The climate forcing for the calibration is based on interpolation between a glacial state (based on a combination of PMIPI for North America and PMIP II for Eurasia) LGM model output temperature and precipitation climatologies along with further regional precipitation controls) and a present-day climatology. The interpolation between glacial and present-day states is weighted to the regional temperature chronology from the tuned Greenland reconstruction. Calibration targets include a large set of RSL observations, geologically-inferred deglacial ice-margin chronologies, and geodetic constraints. For the case of North America, the calibrated ensemble is further scored with respect to strand-lines (paleo lake level indicators) and Marine Limit (maximum level of marine inundation) observations.

The data sets for North America and Eurasia will include surface elevation, ice mask, lake depth, surface drainage routing pointer fields and ice flux components of the surface drainage.

Southern hemisphere

Glaciological constraints have shown that the ice5G reconstruction was too high in East Antarctica. Several tests are now made using a revised version of ICE-5G (VM2=ICE-6G) to correct this aspect while keeping the global on see level and other geomorphological constraints. Dick Peltier’s group is working now on smoothing the refine model and taking the extension of the loading of the shelf out to a distance from the present coast consistent with a specific bathymetry contour. The final version should be ready by September.

ICE-6G

Using new calibrations and a revised version of his model, Dick Peltier proposes a revised version of the ice-sheet that should better match the different paleo data. The following figures are based on data supplied by Dick Peltier and Rosemarie Drummond (Sept 4th 2009 version). They are provided for evaluation and test, and do not correspond to the “official” boundary condition to be used for LGM simulations in CMIP5.

Here are also two submitted papers where you can find information about the new ice-sheet that are provided by Dick.

Notes:

Click on a figure to get the full size version

You can compare the figures with the ICE-5G v1.1 (PMIP2) and ICE-5G v1.2 versions avalaible on the PMIP2 web site

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Conclusion

In September we should be able to compare the results of the revised version of ICE-45G with those of the Lev-Dick collaboration and take a decision based on comparisons with data and on the advice of several specialists

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