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Table of Contents
Go to ⇒ [ PI ] - [ 6ka ] - [ 21ka ] - [ LM ]
21ka Experimental Design
Boundary conditions
Summary of 21ka boundary conditions
| 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 sheet | Ice sheet provided (see below) | |
| Topography and coastlines | land-sea mask provided (see below) | minimum changes (see note below) |
| Ocean bathymetry | up to groups depending of the flexibilty of the model | |
| River outflow | Modified according to a river pathway map (provided, see below) | Same as in CMIP5 PI |
| Ice sheet ice stream | add excess LGM freshwater to ocean (see below) | Same as in CMIP5 PI |
| Mean ocean salinity | +1PSU everywhere |
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 to compare the paleo and PI seasonal cycles
Ice-sheet
The ice sheet provided for PMIP3/CMIP5 LGM experiments is a blended product obtained by averaging three different ice sheets:
- ICE6G provided by Dick Peltier
- MOCA provided by Lev Tarasov
- ANU provided by Kurt Lambeck
A short description and references for the different ice sheets is provided under the link below: LGM icesheet description
This solution was proposed by PMIP bureau in light of a community checking. Given the uncertainties that still exist on the reconstruction of the ice-sheet, resulting from uncertainties in datation for the data used for global or regional constraints, climate intput from ice core temperature reconstructions or climate model simulations, etc… it sounds reasonable to consider that the average is a best estimate of the LGM ice-sheet.
The ice sheet should be implemented following PMIP2 protocol by considering the CHANGE in surface elevation (over ice sheets and land due to sea level lowering) in order that all models will be perturbed (forced) in the same way. This change in elevation would be added by each modeling group to the topography used in their control run.
Data files are available on the Files for PMIP3 experiments page.
Land-sea mask
The land-sea mask is provided by the ICE6G model, except for Antarctica where you should deduce it from the averaged ice-sheet.
Data files are available on the Files for PMIP3 experiments page.
The minimum change is to close the Bering Strait by changing the land-sea mask or imposing rero flux on water and sea-ice across the Bering Strait
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.
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 (see PMIP2 recommendation mass balance recommendation).
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 interative 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.
If you choose the second option, you must be careful to conserve atmospheric mass.
The spin up procedure is up to 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 a other group, you can contact Olivier.marti@lsce.ipsl.fr
Groups with high resolution model for which it is too difficult to run long simulations should contact Olivier.marti@lsce.ipsl.fr to find the best alternative solution.
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