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21ka Experimental Design

Summary table

The changes listed in this table are with respect to the PI configuration.

PMIP3	Minimum solution
Orbital parameters	[ ecc = 0.018994 ] - [ obl = 22.949° ] - [ peri-180° = 114.42° ]
Date of vernal equinox	March 21 at Noon
Trace gases	[ CO2 = 185 ppm ] - [ CH4 = 350 ppb ] - [ N2O = 200 ppb ] [ CFC = 0 ] - [ O3 = same as in PI ]
Aerosols	Same as in CMIP5 PI (see Dust forcing note below)
Solar constant	Same as in CMIP5 PI
Vegetation	Same as in CMIP5 PI
Ice sheets	Ice sheet extent and related changes in topography provided
Land surface elevation and coastlines	Land-sea mask and elevation changes provided	Minimum changes (see below)
Ocean bathymetry	Up to groups, depending on the flexibility of their ocean model (see note below)
River outflow	Modified according to a river pathway map (provided, see below)	Same as in CMIP5 PI
Ice sheet mass balance	Add excess LGM freshwater to ocean (see below)	Same as in CMIP5 PI
Mean ocean salinity	+1 PSU everywhere (to be added once at the beginning of the simulation)
Mean atmospheric surface pressure	Global average equal to PI value (see below)

Boundary conditions

Insolation

Note that insolation should follows PMIP requirements. Please check it carefuly using the following tables (LGM insolation tables).

Check carefully the date of the Vernal equinox, because it has implications when we compare the paleo and PI seasonal cycles.

Ice-sheets and related changes

The ice sheet provided for PMIP3/CMIP5 LGM experiments is a blended product obtained by averaging three different ice sheets reconstructions: ICE-6G v2.0 provided by Dick Peltier, MOCA provided by Lev Tarasov and ANU provided by Kurt Lambeck. More information about this is available in the LGM ice-sheets description page.

Data files are available on the Files for PMIP3 experiments page.

Implementing the LGM ice-sheets implies changing the land-sea mask, the land surface elevation and the ocean bathymetry as described below. Groups having doubts about their setup should contact Masa Kageyama and Jean-Yves Peterschmitt.

Land-sea mask

This mask should be prescribed using the data provided in file.nc where the sftlf variable is 0 (%) over the ocean and 100 (%) over land.

If this proves difficult for a modelling group, the minimum implementation is to:

fill in the Hudson Bay and the Barents sea, which were covered by the main northern hemisphere ice-sheets.

close the Bering Strait (by changing the land-sea mask locally or imposing zero flux on water and sea-ice across the Bering Strait).

make sure that the ocean model configuration keeps the following straits open:

Denmark Strait (between Greenland and Iceland).

Gibraltar Strait.

Indonesia Strait (check exact name with Olivier Marti?).

Land surface elevation

The change in surface elevation specified by the orog_diff variable provided in file.nc should be added to the present-day surface elevation, so that all the participating models will be perturbed in the same way.

Note: this specified change assumes that the surface elevation over present-day oceans is zero (m).

Ice-sheet extent

The ice-sheet extent should be prescribed using the data provided in file.nc, where the sftgif variable is 0 (%) where there is no ice-sheet, or 100 (%).
This mask includes ice shelves, which should be included in models that allow this feature.

The ice shelves can be defined from the sftgif and sftlf variables as areas where:
sftgif == 100 and sftlf == 0

Groups implementing the provided ice shelves should keep the Greenland-Iceland ridge open in the ocean configuration.

Ocean bathymetry

The mean sea-level change corresponding to the ice-sheet reconstruction to be used in the PMIP3 LGM experiment is approximatively NNN m. A more complete description of the change in bathymetry can be computed from the Peltier ICE-6G v2.0 reconstruction.

The ocean bathymetry will be adapted by each group, depending on the flexibility of their ocean model.

Modelling groups should pay special attention to:

The bathymetry should be zero under the grounded ice-sheets.

... (see also the other ocean model configuration checks in this section).

River outflow

The river pathways and basins should be at least adjusted so that fresh water is conserved at the Earth's surface: care should be taken that rivers reach the ocean (due to the lower sea level at LGM, some river mouths have to be displaced towards the coast).

You can use the river routing provided by Lev Tarasov to change the routing scheme in your code, so that the river pathways is consistent with the presence of the ice-sheet. Please note that to our knowledge, so far, no group has attempted to introduce these changes in river runoff in their model configuration. We would appreciate some feedback from groups who who choose to use this data.

Data files are available on the Files for PMIP3 experiments page.

Ice-sheet mass balance

It is advised to ensure a closed fresh water balance at the Earth's surface: snow accumulating on the ice-sheets should be redistributed to the oceans, either globally or in the adjacent oceans.

Vegetation

The vegetation should be treated as in the CMIP5 PI experiment. The reason is that in CMIP5 we test the version of the model used for future climate projections. Since OA and ESM models will be considered, depending on the model used the vegetation will be

prescribed to PI (which means both vegetation types and LAI are prescribed )

Prescribed to PI with interactive LAI (models with interactive carbon cycle, but no vegetation dynamics)

Computed by the model (models with dynamic vegetation)

For Earth System Models with interactive carbon cycle

The simulations should be forced by the prescribed LGM CO2 concentrations. Please use the same protocol as in CMIP5 to store the diagnostic carbon fluxes and the variables needed for PCMIP (see PCMIP Project and CMIP5)

Dust forcing

Some ESM have interactive aerosols. In that case compute dust and associated forcing online, as in PI. If this is not the case then the recommendation is to keep dust and aerosols as in PI (i.e. no change for 21 ka).

Initial conditions

The surface pressure field must be adjusted to the change in surface elevation over the continents. This can be done:

either by gradually changing the surface elevation in order to avoid generating gravity waves,

or by adjusting the initial pressure field to the LGM surface elevation.
In this case, you must be careful to conserve atmospheric mass.

Mean ocean salinity: add +1 PSU everywhere once, at the beginning of the simulation.

The spin up procedure is up to each group, following CMIP5 approach or from a previous cold state.

Note several groups share the same ocean models for which an initial state can be provided from PMIP2 experiments.
If you need an initial state from another group, you can get in touch with Olivier Marti and Jean-Yves Peterschmitt.

Groups with high resolution model for which it is too difficult to run long simulations should contact Olivier Marti and Jean-Yves Peterschmitt to find the best alternative solution.

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