Model LMD/IPSL1: Elaborations
Model LMD/IPSL1 is an entry in the CMIP1 intercomparison only.
The procedure for spinup/initialization of the LMD/IPSL coupled model
as follows (reference: Laurent Fairhead, personal
The CMIP I simulation starting point was the tenth year of a
coupled model integration which was initialized as follows:
- The atmospheric model was started from 1 January 1980 of an
AMIP simulation which began on 1 January 1979.
- The ocean model was initialized from the Levitus
(1982) temperature and salinity fields for climatological January.
- The atmospheric and ocean models were coupled without any further
and without application of flux adjustment (i.e., a "cold start").
Land Surface Processes
- Soil thermodynamics is determined by a 7-layer heat transfer
7 layers are of uneven depths and are spaced between 0.02 m and 3.0 m
the surface, providing for resolution of thermal forcing at periods
0.5 hour to 2 years. A zero-flux condition is imposed at the model's
boundary, and the thermal insulation of snow is accounted for at its
boundary. Introduction of the 7-layer model impacts ground temperature,
snow mass/melt, and the seasonal change in prescribed vegetation that
tied to the soil temperature at a depth of 0.4 m. In turn, changes in
cover and vegetation affect the albedo and roughness length over land.
Cf. Polcher (1994) for further details.
- Soil hydrology is simulated using the
scheme SECHIBA (Schématisation des Echanges Hydriques à
Interface entre la Biosphère et l'Atmosphère). The total
depth of the soil column (corresponding to the vegetation root zone) is
a constant 1.0 m. Soil moisture is computed in two layers, the upper
being the most reactive: when precipitation exceeds evapotranspiration,
the upper layer fills first; when the reverse is true, it empties
Runoff occurs whenever the soil column is completely saturated (water
0.15 m). The remaining prescribed parameters for bare soil are a
evaporative resistance and an empirical constant used to compute
relative humidity for calculation of evaporation.
- In SECHIBA, each of the 7 prescribed vegetation classes interact
with the soil hydrology and contribute individually to the surface
flux. All the vegetation is assumed to have a single-story canopy that
transpires or intercepts precipitation, but the canopy moisture
varies with the leaf area index, which is prescribed differently for
vegetation class. Different architectural and canopy resistances for
also are prescribed for each vegetation class. Cf Ducoudre
et al. (1993) for further details. See also Surface
- Sea ice extents are diagnostically determined, as described by Braconnot
et. al (1997). Ice forms in an ocean grid box with surface
less than the freezing point for salt water. The underlying ocean is
to supply the ice with a a constant heat flux of 2 W m-2 in
the Arctic and 4 W m-2 in the Antarctic.
- The upper surface temperature of the ice is determined from the
heat flux and the heat conduction from the ocean below, where the
is inversely proportional to the ice thickness (assumed to be a
- Sea ice dynamics and rheology are neglected.
Chief Differences from Closest
The atmospheric model (designated as version LMD5.3) is as described by
Aside from its use of higher atmospheric horizontal and vertical
model LMD5.3 differs in the following respects from AMIP model LMD
LMD5 (3.6 x 5.6 L11):
The interaction of radiation and cloud has been
some tuning. As in the AMIP
model, cloud optical thickness and emissivity are determined from
cloud liquid water path W (in kg m-2),
the effective radius r of cloud droplets (in m), and the
coefficient k (in m2 kg-1); however, the
cloud liquid water content (LWC) (and therefore W) as well as r
and k are determined differently:
- The LWC changes because of the introduction
parameterization of precipitation. Also,
place of the AMIP
model's specification of r as a linear function of LWC, the
effective droplet radius in the CMIP I experiment is prescribed as 10
for warm (water) clouds (with cloud-top temperatures > 0 C) and
as 40 microns for cold (ice) clouds (with cloud-top
< -25 deg C). (In the CMIP II experiments, r
set to somewhat different values.)
- The absorption constant k is also
different for warm
and cold clouds: 130 m2 kg-1 and 50 m2
kg-1, respectively, rather than a constant 130 m2kg-1for
clouds, as in the AMIP
- In each grid box, warm clouds constitute a fraction X,
clouds a fraction 1-X of the total cloud cover, where X = 0
for temperatures below -25 deg C and X = 1 for temperatures
0 deg C; X is assumed to vary linearly for intermediate
(Somewhat different temperature bounds are specified in the CMIP II
Cf. Le Treut et al. (1994)
further details. See also Precipitation.
In place of the AMIP
model's sharp distinction between precipitation in warm vs cold
a smoother transition is specified in model LMD5.3:
- For warm (water) clouds, the precipitation rate is
parameterized after Sundqvist
(1981) as the product of a characteristic precipitation time scale T
(value = 5.5 x 10-4 s-1), the prognostic cloud
water mixing ratio m, and an exponential function of (m/C)2,
where C is a prescribed precipitation threshold value = 2 x 10-4
- For cold (ice) clouds, the precipitation rate is determined by
of m to a different timescale t = z/v,
is the depth of the vertical layer and v is the terminal
of the water droplets, which is determined as an empirical function of m
after Heymsfield and
- The proportion of liquid and ice water mass within the clouds
between 0 and -25 deg C.
As in the AMIP
model, evaporation of falling convective and large-scale
is not explicitly modeled, but evaporation of small stratiform cloud
is simulated stochastically.
The formulation of surface characteristics in model LMD5.3 is the same
as in the AMIP
model, except for the following features:
- Fractional sea ice and free ocean
be specified within a single grid box, with roughness lengths and surface
fluxes computed separately over ocean and ice. Roughness lengths
ice surfaces are a uniform 1.1 x 10-3 m. Over ocean,
lengths are evaluated as a function of wind speed according to Charnock
In its treatment of surface fluxes, model LMD5.3 differs in the
respects from the AMIP
- The surface drag coefficient for the momentum, sensible heat,
fluxes depends on surface roughness and stability, following the
of Louis (1979).
- In addition, over the continents, the surface evaporative
by the SECHIBA land surface scheme. The
flux is calculated separately for each of the 8 coexisting surface
(bare ground plus 7 vegetation classes with fractional areas specified
according to grid box) that are also present in the AMIP
model. The total evaporative flux in each grid box then is computed
as an area-weighted average of the individual fluxes. The total flux
sublimation from snow, evaporation from bare soil and from moisture
by the canopy of each vegetation class, and transpiration from the dry
foliage of each class. Sublimation and evaporation from intercepted
moisture occur at the potential rate, while canopy transpiration and
from bare soil depend on the surface relative humidity, which is
in terms of soil moisture. Evaporation from sub-canopy soil is
- In SECHIBA also, the surface evaporative flux is computed by a
that depends on the moisture gradient between the surface and the
air, and on resistances of different kinds (aerodynamic, soil,
and canopy) that vary according to surface type and/or the nature of
moisture flux (sublimation, evaporation, transpiration). Cf. Ducoudre
et al. (1993) for further details. See also Land
Braconnot, P., O. Marti, and S. Joussaume,
1997: Adjustment and feedbacks in a global coupled ocean-atmosphere
Dyn., 13, 507-519.
Charnock, H., 1955: Wind stress on a
surface. Quart. J. Royal Meteor. Soc., 81, 639.
Ducoudre, N., K. Laval, A.
1993: SECHIBA, a new set of parameterizations of the hydrologic
at the land/atmosphere interface within the LMD atmospheric general
model. J. Climate, 6, 248-273.
Harzallah, A., and R.
1995: Internal versus SST forced atmospheric variability as simulated
an atmospheric general circulation model. J. Climate, 8,
Heymsfield, A.J., and L.J.
Donner, 1990: A scheme for parameterizing ice-cloud water content in
circulation models. J. Atmos. Sci., 47, 1865-1877.
Larson, J.C., and B.R.
1977: Effects of realistic angular reflection laws for Earth's surface
upon calculations of the earth-atmosphere albedo. In Proceedings of the
Symposium on Radiation in the Atmosphere, Science Press, 451-453.
Le Treut, H., Z.-X. Li, and M.
1994: Sensitivity of the LMD general circulation model to greenhouse
associated with two different cloud water parameterizations. J.
Levitus, S., 1982: Climatological atlas
the world's oceans. NOAA Professional Paper 13, 173 pp.
Louis, J.F., 1979: A parametric model of
eddy fluxes in the atmosphere. Bound. Layer Meteor., 17,
Polcher, J., 1994: Etude de la
du climat tropical a la deforestation. Ph.D. dissertation,
Pierre et Marie Curie, Paris, France.
Sundqvist, H, 1981: Prediction of
clouds: Results of a 5-day forecast with a global model. Tellus,
CMIP Documentation Directory
Last update 15 May, 2002. This page is maintained by Tom