BOUNDARY CONDITIONS FOR PMIP SIMULATIONS
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Table 1: Summary of PMIP boundary conditions for PMIP
runs.
For 6 kyr BP, more information can be found in the PMIP newsletter 1.
For 21 kyr BP, more information can be found in the PMIP newsletters 2, 3 and 5.
All simulations should be at least 10-year long with full seasonal cycle
for control and 6k and 21k with fixed SSTs to account for interannual variability.
For 21k with computed SSTs: we recommend (at least) a 10-year average starting
from the "quasi-equilibrium regime" of the model.
Orbital parameters and insolation
The solar constant is 1365 W/m2 for control, 6k, 21k.
The orbital parameters are given by A. BERGER (1978) and A. BERGER et M.F. LOUTRE (1991) : 6k, 21k
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w: longitude of perihelion relative to the moving vernal equinox minus 180 degrees, i.e. angle between autumnal equinox and perihelion:102.04 degrees.
Insolation tables are given in appendix. It is very important that we
set the 21 of March at noon (e.g. 21.00; time reference = greenwich meridian,
i.e. UT) as the date of our vernal equinox for the 6kyr and 21kyr BP simulation
(for 360 as well as for 365-day year).
SSTs and sea-ice
Control:
PMIP datasets recommended for SSTs and sea-ice have been prepared at PCMDI and were calculated by averaging the 10-year AMIP datasets (1979-1988). They are available from NGDC.
6kyr BP:
SSTs and sea-ice prescribed at their present day value, as in the control run.
21kyr BP:
fixed SSTs
Fixed SSTs we use the CLIMAP (1981) dataset available at NGDC. In order to avoid differences due to differences in present day climatologies, we recommend that you use the change in SSTs produced by CLIMAP for February and August, then use a sine function to get daily LGM SSTs values, and add the daily change to your daily control run SSTs. (See Newsletter n°3 for details).
For sea-ice we recommend that you use the extent given by CLIMAP (1981) for the LGM. The sea ice seasonal cycle is more difficult to infer from February and August values :
- when sea ice occurs for both February and August, then assume a permanent sea ice cover for all months.
- when there is sea ice during the winter season but none during summer : we assume a sea ice during N months around the winter season, with N depending on latitude and inferred from the control run.
computed SSTs
Computed SSTs and sea-ice use the same techniques as those used for 2xCO2 experiments.
Most models use a Q-flux correction technique to compute SSTs. The treatment
of ocean heat flux under sea ice is an integral part of each model's mixed
layer ocean and has to be considered in that context. Therefore we do not
recommend any specific way to adjust the imposed ocean heat fluxes, there
is certainly no unique solution (See Newsletter n°2, appendix 2 for
details).
Greenhouse Gases
The recommended values for CO2 concentration are listed in table 1.
For models including only CO2: In order to be sure that we all get the same change in radiative forcing, we recommend setting the CO2 concentration for 6kyr BP as follows:
C(6kyr BP) = (280/345) * control run concentration
= 0.81 * control run concentration
C(21kyr BP) = (200/345) * control run concentration
= 0.58 * control run concentration
For models including CO2 + other trace gases: this case recommends that
you set all your concentrations in order to get the same total change in
radiative forcing, that is -1.3W/m2 for 6kyr BP and -3.4W/m2 for 21kyr
BP. These values must include the effects of all the trace gases. (See
Newsletters n°1 and n°2 for details).
Land-Surface
6kyr BP:
No change in the land-surface characteristics.
21kyr BP:
The change for land-surface characteristics are:
1 Ice sheets and coastlines
The both 21 kyr BP experiments use the ice sheet reconstructions (elevation and extent) provided by Peltier et al. (1994) (available at NGDC).
The topography at LGM is found by computing the difference between the 21 ka BP and present day elevation and then by adding this difference to the topography of your "control" (present-day) simulation.
Due to the 120m sea-level lowering, the coastlines are modified.
2 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
3 Surface properties
For snow-free and ice-free surfaces the surface properties (e.g., vegetation, soil, surface albedo) should be the same as for the control run (i.e., present day). However, with the change in land/sea distribution, we will need to specify land-surface properties for the continental margins that have emerged at 21 ka BP. We recommend that you use the zonal mean value, averaged over all the snow and ice-free land-surface grid points located at the same latitude. An alternative is to prescribe surface properties to be the same as properties of the grid cell's nearest neighbor. (See Newsletter n°3 for details).