PMIP Newsletter 2
*********************************************
PALEOCLIMATE MODELING INTERCOMPARISON PROJECT
*********************************************
Newsletter N. 2
----------------
12 March 1993
Dear Participant,
The present newsletter describes the recommended boundary conditions
to be used for:
-A- the computed SST 21kyr BP experiment
-B- the optional fixed SST 21kyr BP experiment
--------
You will also find an Appendix in which insolation values are provided for
you to check your insolation calculations for 21kyr BP (Appendix 1).
We also include an appendix 2 giving comments written by Tony Broccoli concerning
the "Q-flux correction" techniques for the LGM.
******************************************
-A- THE 21kyr BP EXPERIMENT - COMPUTED SST
******************************************
The Last Glacial Maximum is usually set at 18kyr BP radiocarbon date.
In the light of recent U/Th calibration of the radiocarbon time scale by
Bard et al. (1990), 3000 years are added to the radiocarbon age of 18000 yr BP
to get a calendar date of 21000 for the Last Glacial Maximum (LGM).
The PMIP boundary conditions for the LGM experiment are:
*1* SSTs and sea-ice computed, using the same techniques as those used for
----------------
2xCO2 experiments.
Most models use a Q-flux correction technique to compute SSTs.
Two problems then arise when performing ice-age experiments due to:
-a- sea-ice expansion.
This question highly depends on model parameterizations, i.e. how
Q-fluxes are prescribed with sea-ice cover and how sea-ice is
parameterized. It can then be considered as part of the model itself.
-b- land expansion due to sea-level lowering.
A method is suggested by Tony Broccoli in Appendix 2.
In order to improve model-model comparisons, it would be better to agree
on the way to use "Q-flux" method for the LGM.
WE DO WAIT FOR YOUR COMMENTS.
*2* Land-surface characteristics.
----------------------------
#2.1# Ice sheets
..........
A workshop was convened by Bill Ruddiman at the Lamont-Doherty
Earth Observatory in early June, 1992, to update the CLIMAP/Denton
& Hughes reconstruction of the LGM for use by PMIP. The updated
contains significant changes in ice thickness and elevation
and relatively minor changes in areal extent of the different
ice sheets.
It takes into account the recent results of sea-level
lowering measurements (120m, Fairbanks 1989),new dating (Bard
et al., 1990),major improvements in ice sheets thickness
reconstruction based on local sea-level rebound data (Tushinghan
and Peltier, 1991) and glacial marine geological field work.
The updated ice sheet reconstruction is being prepared by
------------------------------------------------------
Dick Peltier and will be available at NGDC April 1st.
-----------------------------
This dataset will give:
- the ice sheet extent at 21kyr BP,
- the change in surface elevation at 21kyr BP, compared to
present day (due to ice sheet elevation and 120m sea-level
lowering).
A detailed description of the dataset will be given when available.
#2.2# Coastlines
..........
Due to the 120m sea-level lowering, the coastlines are modified.
An updated version of the LGM values will be provided.
#2.3# 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.
#2.4# Land-surface albedo
...................
No change in land-surface albedo is recommended for snow-free and
ice-free surfaces. The same values as for present day will thus be used.
Nevertheless, with the change in land/sea distribution, we need
to specify land-surface albedo values for the margins that have
emerged at 21kyr BP: for ice-free and snow-free areas, we recommend that
you use the zonal mean value, averaged over all the snow and ice-free
land-surface grid points located at that same latitude.
We do not recommend any specific values for snow and ice surfaces
since they can be considered a part of each model's parameterization.
Indeed, most models use their own interactive snow
parameterization.
*3* Length of simulations
---------------------
The length of the simulations will be highly dependent on the surface
ocean model you use. For model outputs, we recommend (at least) a
10-year average starting from the "quasi-equilibrium regime" of your
model.
*4* Greenhouse Gases
----------------
#4.1# Measurements from ice cores give the following concentrations
at 21kyr BP (Raynaud et al., 1993;
Leuenberger and Siegenthaler, 1992):
CO2 - CH4 - N20
.....................................................
200 ppm - 350 ppb - 190 ppb
N.B.: According to Leuenberger and Siegenthaler (1992), the Holocene
##### N2O concentration is 270ppb rather than 280ppb as given in the
newsletter 1. Nevertheless, according to the large range of
pre-industrial measurements (260 to 285 ppb) and the small change
in radiative forcing associated to a 10ppb change in N2O, it seems
reasonable to keep our recommended 280ppb value at 6kyr BP.
#4.2# 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 21kyr BP
as follows:
C(21kyr BP) = (200/345) * control run concentration
= 0.58 * control run concentration
345 ppm is the recommended value for CO2 concentration of
the simulated present-day climate
200 ppm is the value at 21kyr BP obtained from ice core measurements
Using the above formula, we will all get the same radiative forcing
according to the IPCC report (1990):
DF = 6.3 * ln(C/Co) = -3.4 W/m2
#4.3# For models including CO2 + other trace gases:
.............................................
In this case the problem is more complicated. We recommend that
you set all your concentrations in order to get the same total
**********
change in radiative forcing, that is -3.4W/m2. This value must
***************************
include the effects of all the trace gases.
Please see newsletter N. 1 for more informations on this topic.
*5* Insolation
----------
Insolation changes are weak at 21kyr BP.
Nevertheless we recommend that we all use the same forcing.
#5.1# The orbital parameters are given by Andre BERGER (JAS, 1978):
......................
- Eccentricity: 0.018994
- Obliquity: 22.949 degrees
- Longitude of perihelion (w), relative to the moving vernal equinox
minus 180 degrees, i.e. angle between autumnal equinox and
perihelion: 114.42 degrees
#5.2# The solar constant must be kept as in the control run.
..................
The recommended value is: 1365 W/m2
#5.3# The problem of calendar
.......................
.5.3.1. It is BETTER if we all use the same reference date
******
for the 21kyr BP experiment, as we did for the 21kyr BP
experiment. We then recommend that you set
the 21 of March at noon (e.g. 21.00; time reference =
greenwich meridian, i.e. UT) as the date of your
*******************
vernal equinox (for both 360-day or 365-day year).
Please see newsletter N. 1 for more informations.
.5.3.2. We recommend that you all keep DAILY VALUES
************
Please see newsletter N. 1 for more informations.
*6* Checking insolation changes
---------------------------
We still recommend that you check your computed insolation values (see
newsletter N. 1). We thus provide you with tables for the 21kyr BP
minus present day insolation changes (see Appendix 1).
************************************************
-B- THE OPTIONAL 21kyr BP EXPERIMENT - FIXED SST
************************************************
Some of you may not be able to perform a computed SST simulation but may want
to perform a fixed SST simulation of the LGM. In order to be able to compare
the results we would like to make a few recommendations.
*1* SSTs and sea-ice.
----------------
We recommend that you use the change in SSTs (LGM minus present-day) given
by CLIMAP (1981), available at NGDC rather than the LGM absolute values in
order to avoid differences due to differences in present day climatologies.
For sea-ice we recommend that you use the extent given by CLIMAP (1981)
for the LGM.
To go a step further we would like to recommend, in a future newsletter,
a procedure to get the seasonal variations of SSTs and sea-ice from the
CLIMAP datasets of February and August. Please, any comments are welcome.
*2* Land-surface characteristics
----------------------------
Please see part A.
*3* Length of the simulations
-------------------------
Carry out a 10-year simulation with full seasonal cycle to account for
interannual variability.
*4* Greenhouse gases
----------------
Please see part A.
*5* Insolation
----------
Please see part A.
*6* Checking insolation changes
---------------------------
Please see part A.
Sincerely yours,
Sylvie JOUSSAUME (LMCE, France)
& Karl TAYLOR (LLNL, USA)
& Robert WEBB (NGDC, USA)
& Tony BROCCOLI (GFDL, USA)
**********
REFERENCES
**********
* IPCC Report or Climate Change, Cambridge University Press, 354 pp, 1990.
* Berger A., "Long-term variations of daily insolation and Quaternary
climatic changes", JAS, 35, 2362-2367, 1978.
* Bard et al., 1990: "Calibration of 14C time-scale over the past 30000 years
using mass spectroscopie U-Th ages from Barbados corals" - Nature, vol. 345
(31/05/89), pp 409-410.
* Fairbanks, 1989: "A 17000-year glacio-eustatic sea-level record: influence of
glacial melting rates on the Younger Dryas event and deep ocean circulation"
- Nature, vol. 342 (07/12/89), pp 637-642.
* Tushinghan and Peltier, 1991: "Ice 36: a new global model of late Pleistocene
deglaciation based upon geophysical predictions of post-glacial relative sea-
level change" - Journal of Geophysical Research, vol. 96, N. B3, pp 4497-4523.
* Leuenberger and Siegenthaler 1992: "Ice-age atmospheric concentration of nitrous
oxide from an Antarctic ice core" - Nature, vol. 360, pp 449-451.
* Raynaud Domio., J. Jouzel, J.M. Barnola, J. Chappellaz, R.J. Delmas, C. Lorius,
1993: "The ice record of greenhouse gases" - Science, 259, pp 926-934.
*****************************************
APPENDIX 1: About Insolation Computations
*****************************************
In the following, we provide tables and information concerning
insolation in order to help you check your insolation code.
All the results we give have been obtained using :
- the orbital parameters given above in the present newsletter
- a solar constant value of 1365 W/m2
- a calendar based on the 21 of March at noon (21.00) for the
date of the vernal equinox.
All the values of insolation are given in W/m2. They are given at every 10
degree of latitude (no latitudinal band average is done!). All the computations
follow the method proposed by Berger (JAS, 1978) and are based on an
expansion accurate to order e**3 for the computation of the true longitude
(lambda, angle defining the Earth position relative to the Vernal Equinox).
1 - DATES of EQUINOXES and SOLSTICES
=====================================
21 kyr BP orbit :
---------------
- 365 day year :
date of vernal equinox = 21.00 march
date of summer solstice = 21.32 June
date of autumnal equinox = 23.52 Sept
date of winter solstice = 22.65 Dec
date of perihelion = 15.51 January
date of aphelion = 17.01 July
- 360 day year
date of vernal equinox = 21.00 march
date of summer solstice = 22.06 June
date of automnal equinox = 24.96 Sept
date of winter solstice = 23.86 Dec
date of perihelion = 17.39 January
date of aphelion = 17.39 July
2 - INSOLATION
==============
We give insolation values for :
- monthly means, which depend on the length of the year and on the
reference date used
- "mid-month" values which are daily mean insolation values
given for specific true longitude values
2.1 Monthly means
------------------
* 21 kyrBP minus present day difference, 365 day year, 21.00 march ref
----------------------------------------------------------------------
LAT JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
90. .00 .00 -.09 -1.46 -5.36 -10.50 -12.95 -9.44 -1.39 .00 .00 .00
80. .00 .07 .60 -1.13 -5.28 -10.34 -12.75 -9.24 -1.27 .88 .00 .00
70. .28 1.35 1.31 .02 -4.37 -9.86 -11.89 -7.35 -1.70 2.06 1.46 .00
60. 2.49 2.22 1.95 .76 -2.41 -6.58 -8.47 -6.50 -2.20 1.81 3.43 3.00
50. 3.32 2.85 2.52 1.37 -1.35 -4.82 -6.85 -5.98 -2.67 1.23 3.63 3.93
40. 3.75 3.33 3.01 1.91 -.47 -3.45 -5.53 -5.46 -3.07 .51 3.30 4.15
30. 3.93 3.68 3.40 2.37 .31 -2.21 -4.27 -4.88 -3.39 -.30 2.65 3.97
20. 3.89 3.90 3.69 2.74 1.01 -1.06 -3.04 -4.22 -3.60 -1.14 1.79 3.51
10. 3.67 3.98 3.87 3.02 1.62 .01 -1.84 -3.48 -3.71 -1.97 .76 2.81
0. 3.28 3.93 3.93 3.20 2.14 .98 -.68 -2.68 -3.71 -2.77 -.37 1.92
-10. 2.73 3.74 3.87 3.27 2.55 1.83 .41 -1.84 -3.61 -3.52 -1.59 .86
-20. 2.04 3.42 3.69 3.24 2.84 2.53 1.38 -.99 -3.39 -4.19 -2.84 -.32
-30. 1.23 2.99 3.40 3.10 2.99 3.03 2.20 -.16 -3.08 -4.76 -4.12 -1.63
-40. .28 2.44 3.00 2.84 2.97 3.30 2.80 .61 -2.68 -5.23 -5.40 -3.05
-50. -.81 1.79 2.51 2.49 2.76 3.22 3.06 1.26 -2.20 -5.61 -6.74 -4.65
-60. -2.19 1.02 1.94 2.02 2.23 2.51 2.74 1.68 -1.67 -5.92 -8.29 -6.70
-70. -4.79 .04 1.30 1.39 .67 .00 .28 1.53 -1.12 -6.33 -11.27 -10.52
-80. -5.32 -1.30 .59 .31 .00 .00 .00 .06 -.66 -7.91 -13.33 -11.03
-90. -5.40 -1.39 -.10 .00 .00 .00 .00 .00 -.77 -8.93 -13.54 -11.20
* 21 kyrBP minus present day, 360 day year, 21.00 march ref
----------------------------------------------------------
LAT JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
90. .00 .00 -.08 -1.41 -5.24 -10.37 -12.96 -9.77 -1.74 .00 .00 .00
80. .00 .06 .60 -1.06 -5.16 -10.21 -12.76 -9.59 -1.61 .95 .00 .00
70. .19 1.32 1.32 .07 -4.22 -9.74 -11.95 -7.62 -1.97 1.96 1.61 .00
60. 2.51 2.23 1.96 .80 -2.31 -6.48 -8.48 -6.67 -2.44 1.65 3.41 3.04
50. 3.37 2.87 2.53 1.41 -1.27 -4.74 -6.84 -6.09 -2.88 1.05 3.55 3.95
40. 3.80 3.35 3.02 1.94 -.40 -3.37 -5.49 -5.53 -3.24 .32 3.19 4.15
30. 3.95 3.69 3.41 2.40 .37 -2.15 -4.23 -4.91 -3.51 -.48 2.52 3.95
20. 3.89 3.90 3.70 2.77 1.06 -1.01 -2.99 -4.20 -3.68 -1.30 1.64 3.45
10. 3.63 3.97 3.88 3.05 1.66 .05 -1.78 -3.43 -3.75 -2.11 .61 2.72
0. 3.20 3.90 3.94 3.22 2.17 1.01 -.62 -2.60 -3.70 -2.88 -.52 1.81
-10. 2.62 3.70 3.88 3.29 2.57 1.85 .47 -1.73 -3.55 -3.58 -1.73 .73
-20. 1.89 3.37 3.69 3.25 2.85 2.54 1.44 -.86 -3.29 -4.21 -2.96 -.48
-30. 1.03 2.92 3.40 3.10 2.99 3.04 2.25 -.02 -2.94 -4.73 -4.21 -1.80
-40. .04 2.36 3.00 2.85 2.96 3.29 2.84 .75 -2.51 -5.15 -5.47 -3.24
-50. -1.10 1.69 2.51 2.49 2.74 3.21 3.09 1.39 -2.00 -5.47 -6.76 -4.85
-60. -2.54 .90 1.94 2.02 2.23 2.50 2.74 1.79 -1.46 -5.72 -8.25 -6.91
-70. -5.29 -.12 1.30 1.39 .70 .00 .23 1.56 -.90 -6.03 -11.05 -10.80
-80. -5.75 -1.45 .58 .32 .00 .00 .00 .03 -.48 -7.42 -13.26 -11.32
-90. -5.84 -1.54 -.10 .00 .00 .00 .00 .00 -.60 -8.50 -13.46 -11.50
2.2 "MID-MONTH" values
-----------------------
"Mid-month" values are obtained as daily mean insolation values
in W/m2 and are computed at fixed true longitudes with longitude increments
of 30 degrees, starting from the vernal equinox(Berger, JAS,1978) ... i.e.
around the 20th of each month.
Using this definition, we have :
longitude = 0 corresponds to the vernal equinox (VE)
longitude = 90 corresponds to the summer solstice (SS)
longitude = 180 corresponds to the autumnal equinox (AE)
longitude = 270 corresponds to the winter solstice (WS)
These tables of "mid-month"values:
1) allow direct comparisons of insolation at equinoxes and solstices
2) avoid any problem of calendar, either between 0 and 6 kyr BP
or between 360-day or 365-day years
* "mid-month" values for the 21 kyr BP minus present day difference :
--------------------------------------------------------------------
VE SS AE WS
LAT 0 30 60 90 120 150 180 210 240 270 300 330
90. .00 -3.59 -7.87 -11.43 -11.77 -7.51 .00 .00 .00 .00 .00 .00
80. .68 -3.53 -7.75 -11.26 -11.59 -7.40 -.64 .00 .00 .00 .00 .00
70. 1.33 -1.39 -7.18 -10.74 -10.84 -5.75 -1.27 1.44 .23 .00 .24 2.12
60. 1.94 -.39 -4.02 -7.33 -7.81 -5.47 -1.85 1.29 2.69 2.85 3.11 3.00
50. 2.50 .43 -2.58 -5.45 -6.54 -5.30 -2.38 .84 3.01 3.87 4.01 3.60
40. 2.98 1.16 -1.40 -3.99 -5.47 -5.07 -2.84 .25 2.84 4.19 4.46 4.02
30. 3.37 1.81 -.34 -2.67 -4.43 -4.74 -3.21 -.40 2.39 4.12 4.60 4.27
20. 3.65 2.38 .62 -1.44 -3.38 -4.31 -3.48 -1.08 1.75 3.76 4.51 4.37
10. 3.83 2.86 1.48 -.29 -2.33 -3.78 -3.65 -1.76 .95 3.16 4.19 4.31
0. 3.89 3.23 2.23 .76 -1.29 -3.17 -3.70 -2.41 .05 2.37 3.69 4.10
-10. 3.83 3.48 2.85 1.69 -.29 -2.48 -3.65 -3.02 -.93 1.40 3.00 3.75
-20. 3.65 3.61 3.31 2.47 .64 -1.74 -3.48 -3.55 -1.97 .29 2.17 3.26
-30. 3.37 3.61 3.60 3.04 1.46 -.99 -3.21 -4.01 -3.04 -.95 1.18 2.66
-40. 2.98 3.47 3.66 3.36 2.10 -.24 -2.84 -4.39 -4.14 -2.33 .06 1.95
-50. 2.50 3.18 3.45 3.31 2.48 .46 -2.38 -4.70 -5.32 -3.90 -1.23 1.13
-60. 1.94 2.71 2.80 2.57 2.39 1.03 -1.85 -4.97 -6.72 -5.94 -2.81 .21
-70. 1.33 1.98 .22 .00 .22 1.31 -1.27 -5.35 -9.98 -9.53 -6.19 -.91
-80. .68 .00 .00 .00 .00 .00 -.64 -7.12 -10.70 -9.99 -6.74 -3.18
-90. .00 .00 .00 .00 .00 .00 .00 -7.23 -10.87 -10.14 -6.84 -3.23
We hope all this information will help you ! Please let us know if it is unclear
or if you have any trouble!
*****************************
N.B.: ERRATUM NEWSLETTER N. 1
*****************************
In Appendix 1:
..............
360-day year, 21kyr BP orbit:
date of perihelion = 21.90 SEPTEMBER
---------
date of aphelion = 21.90 MARCH
-----
*************************************
APPENDIX 2: About the "Q-flux" method
*************************************
The purpose of this note is to share some ideas I have regarding the use of a
prescribed ocean heat flux (the so-called "Q-flux method") in atmosphere-mixed
layer ocean models when simulating the last glacial maximum. The problem in the
implementation of this technique arises due to differences in the land-sea
distribution due to lower sea level of glacial times. As a result, some model
gridpoints that are ocean in the control climate simulation are land in the
glacial climate simulation. Thus it is not a simple matter to decide how to
prescribe the same ocean heat flux in both the control and glacial simulations.
My proposal assumes that the goal is to prescribe the same flux of heat by the
ocean across each latitude circle. While there is no reason to believe that the
ocean heat flux at the last glacial maximum was the same as today, the assumption
of no change in heat flux is commonly made in simulations of greenhouse warming.
One of the purposes of performing LGM simulations with predicted sea surface
temperatures is to intercompare the sensitivity of various climate models. Thus
the use of prescribed ocean heat fluxes is essential to avoid the distortions
in sensitivity that can result from biases in the distribution of sea ice. So
while there are some uncertainties in implementing this technique in LGM
simulations, I believe it is preferable to the alternative of having no prescribed
ocean heat transport.
Ocean heat transport in atmosphere-mixed layer ocean models is mimicked by
prescribing a heating (or cooling) function at each ocean grid point. If there
were no change in the land-sea distribution, the same heating function would
be used at each grid point for both the control and perturbation experiments.
Since this is not possible for LGM simulation experiments, the objective is
to provide the same net heating (or cooling) integrated around each latitude
circle. Thus my suggestion is to take the net heating integrated over the points
along each latitude circle that are ocean in the control but land in the LGM
experiment, and uniformly redistribute that heating over the points that remain
ocean in both experiments.
Symbolically:
If ocean in both experiments, then:
qlgm(i,j) = qcontrol(i,j) + sum(qcontrol(i,j))/npoints(j)
If land in the LGM, then:
qlgm(i,j) = 0.0
where i is the longitude index,
j is the latitude index,
qcontrol(i,j) is the heating function at each grid point
from the control experiment,
qlgm(i,j) is the heating function at each grid point for
the LGM experiment,
sum() is a function that sums a quantity over the grid points
at each latitude circle that are ocean in the control
experiment but land in the LGM experiment,
npoints(j) is the number of grid points along each latitude
circle that are ocean in both the control and the LGM
experiments.
The above describes the computation for a latitude-longitude grid, or any other
grid where the grid boxes along each latitude circle are of equal area.
Reformulation will be necessary for other types of grids.
This describes the methodology I plan to employ in the PMIP simulation. I
confess that I have not yet tested this method in a climate model integration,
but I plan to do so in the near future in preparation for the PMIP experiment.
I will share my experiences with all of you.
Anthony J. Broccoli
----------------------------------------------------------------------------
Contact Address:
################
Laboratoire de Modelisation du Climat et de l´Environnement
D.S.M. / Orme des Merisiers / Bat. 709
C.E. Saclay
9119 Gif-sur-Yvette cedex
FRANCE
Tel.: (33) 1 69.08.77.11
Fax.: (33) 1 69.08.77.16
email: paleo (NEW! Please check the PMIP 'Contacts' web page)
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