PMIP Documentation for LMD5
Laboratoire de Météorologie Dynamique: Model LMCE LMD5.3 (sin(lat)x5.6 L11) 1994
Dr. Pascale Braconnot, Laboratoire des Sciences du Climat et de l´Environnement, Bat 709 CEA-DSM, Orme des Merisiers, F 91191 Gif-sur-Yvette cedex, France, Phone: 33 (1) 69 08 77 11; Fax 33 (1) 69 08 77 16;
Dr. Gilles Ramstein Laboratoire des Sciences du Climat et de l´Environnement,
Bat 709 CEA-DSM, Orme des Merisiers, F 91191 Gif-sur-Yvette cedex, France,
Phone: 33 (1) 69 08 77 11; Fax 33 (1) 69 08 77 16;
Number of days in each month: 30 30 30 30 30 30 30 30 30 30 30 30
Sadourny R., Laval K., 1984, January and July performance of the LMD general circulation model. In New Perspectives in Climate Modelling, A. Berger and C. nicolis (eds), Developments in Atmospheric Science, 16,
Elsevier, pp 173-198
The model has a regular grid in longitude and in SINE OF THE LATITUDE
For a surface pressure of 1000 hPa, 3 levels are below 800 hPa and 2
levels are above 200 hPa.
Second-order vertical diffusion of momentum, heat, and moisture is applied
only within the planetary boundary layer (PBL). The diffusion coefficient
depends on a diagnostic estimate of the turbulence kinetic energy (TKE)
and on the mixing length (which decreases up to the prescribed PBL top)
that is estimated after Smagorinsky et al. (1965) . Estimation of TKE involves
calculation of a countergradient term after Deardorff (1966) and comparison
of the bulk Richardson number with a critical value. Cf. Sadourny and Laval
(1984) for further details. See also Planetary Boundary Layer and Surface
Longwave radiation is modeled in six spectral intervals between wavenumbers
0 and 2.82 x 105 m-1 after the method of Morcrette
(1990, 1991 ). Absorption by water vapor (in two intervals), by the water
vapor continuum (in two intervals in the atmospheric window, following
Clough et al. 1980) , by the carbon dioxide and the rotational part of
the water vapor spectrum (in one interval), and by ozone (in one interval)
is treated. The temperature and pressure dependence of longwave absorption
by gases is included. Clouds are treated as graybodies in the longwave,
with emissivity depending on cloud liquid water path after Stephens (1978)
. Longwave scattering by cloud droplets is neglected, and droplet absorption
is modeled by an emissivity formulation from the cloud liquid water path.
For purposes of the radiation calculations, all clouds are assumed to overlap
randomly in the vertical. See also Cloud Formation.
If the temperature lapse rate is conditionally unstable but the air is unsaturated, condensation also occurs following the Kuo (1965) cumulus convection scheme, provided there is large-scale moisture convergence. In this case, the lifting condensation level is assumed to be at the top of the PBL, and the height of the cumulus cloud is given by the highest level for which the moist static energy is less than that at the PBL top (see Planetary Boundary Layer). It is assumed that all the humidity entering each cloudy layer since the last call of the convective scheme (30 minutes prior) is pumped into this cloud. The environmental humidity is reduced accordingly, while the environmental temperature is taken as the grid-scale value; the cloud temperature and humidity profiles are defined to be those of a moist adiabat. The fractional area of the convective cloud is obtained from a suitably normalized, mass-weighted vertical integral (from cloud bottom to top) of differences between the humidities and temperatures of the cloud vs those of the environment. As a result of mixing, the environmental (grid-scale) temperature and humidity profiles evolve to the moist adiabatic values in proportion to this fractional cloud area, while the excess of moisture precipitates (see Precipitation). Mixing of momentum also occurs.
There is no explicit simulation of shallow convection, but the moist
convective adjustment produces similar effects in the moisture field (cf.
Le Treut and Li 1991) . See also Cloud Formation.
Cloud water content and cloud fraction are interactively used in the radiative code through the calculation of cloud optical thickness and cloud emissivity (Le Treut 1994).
The fraction of convective cloud in a grid box is unity if moist convective adjustment is invoked; otherwise, it is given by the surface fraction of the active cumulus cloud obtained from the Kuo (1965) scheme (see Convection). Cloud forms in those layers where there is a decrease in water vapor from one call of the convective scheme to the next (every 30 minutes), and the cloud LWC is redistributed in these layers proportional to this decrease.
The fraction of stratiform cloud in any layer is determined from the
probability that the total cloud water (liquid plus vapor) is above the
saturated value. (A uniform probability distribution is assumed with a
prescribed standard deviation--cloud typically begins to form when the
relative humidity exceeds 83 percent of saturation.) This stochastic approach
also crudely simulates the effects of evaporation of cloud droplets. Cf.
Le Treut and Li (1991) for further details. See also Precipitation.
For LGM, we add the anomaly of Peltier (Peltier(21k) -Peltier(0k)) to
the control run.
For 6 fix: SSTs and sea-ice prescribed at their present day value, as in the control run.
For 21 fix: SSTs and sea-ice: 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, was used. When points remain ocean, to obtain seasonally varying SSTs and sea ice edge from data for February and August, a simple sinusoidal variations, with extrems in February and August, is used. When points are ocean in summer and ice in winter (for Nothern Hemisphere), a "trapezoidal" function is used with an ad-hoc duration of the sea-ice deduced from present-day temperatures equivalent.
For computed SSTs experiments, the AGCM was coupled to a mixed layer
ocean (50m) (Le Treut et al. 1994). The ocean heat transport is daily prescribed
using the present-day diagnosed ocean heat transport accounting for the
sea level drop (105m.) as described in the PMIP Newsletter (Webb et al.
For 21fix, sea ice edge of CLIMAP data is used for February and August.There is not any percent of sea ice for each grid box. When the CLIMAP 21k sea-ice extent is interpolated over the model grid 48x36 if the result is higher than 50%, we consider that the grid box is sea ice. When the result is lower than 50%, the whole box is free of ice.
For computed SSTs experiments, the sea ice appears when the temperature
is lower than -2°C and disappear when it is higher.
The surface roughness lengths over the continents are prescribed as a function of orography and vegetation from data of Baumgartner et al. (1977) , and their seasonal modulation is inferred following Dorman and Sellers (1989) . Roughness lengths over ice surfaces are a uniform 1 x 10-2 m. Over ocean roughness length using coefficients (see Surface Fluxes) are determined using Charnock?s relationship.
Surface albedos for oceans and snow-free sea ice are prescribed from monthly data of Bartman (1980) , and for snow-free continents from monthly data of Dorman and Sellers (1989) . When there is snow cover, the surface albedo is modified according to the parameterization of Chalita and Le Treut (1994) , which takes account of snow age, the eight designated land surface types, and spectral range (in visible and near-infrared subintervals).
The longwave emissivity is prescribed as 0.96 for all surfaces.
In the lowest atmospheric layer, surface turbulent eddy fluxes of momentum, heat, and moisture are expressed as bulk formulae involving drag/transfer coefficients that are functions of wind speed, stability, and roughness length (see Surface Characteristics). The transfer coefficient for the surface moisture flux also depends on the vertical humidity gradient. Over the oceans, the neutral surface drag/transfer is corrected according to the local condition of surface winds. At the surface, the ECMWF (1992) parametrization of momentum, heat and moisture transfer is included in the model.
Above the surface layer, but only within the PBL, turbulent eddy fluxes
are represented as diffusive processes (see Diffusion and Planetary Boundary
As in the baseline model, the bare soil and vegetation in each grid box are treated as a single medium for calculations of the surface radiative budget and the sensible heat flux. The parameterization of evaporation from the oceans is also unchanged, but the evaporation from land surfaces is determined by the SECHIBA model rather than by the "bucket" scheme.
In SECHIBA, the evaporative flux is calculated separately for each of the 8 coexisting surface types (bare ground plus 7 vegetation classes with fractional areas specified according to grid box). The total evaporative flux in each grid box then is computed as an area-weighted average of the individual fluxes. The total flux includes sublimation from snow, evaporation from bare soil and from moisture intercepted by the canopy of each vegetation class, and transpiration from the dry foliage of each class. Sublimation and evaporation from intercepted canopy moisture occur at the potential rate, while canopy transpiration and evaporation from bare soil depend on the surface relative humidity which is parameterized in terms of soil moisture. Evaporation from sub-canopy soil is neglected.
In SECHIBA, the surface moisture flux is computed by a bulk method that
depends on the moisture gradient between the surface and the overlying
air and on resistances of different kinds (aerodynamic, soil, architectural,
and canopy) that vary according to surface type and/or the nature of the
moisture flux (sublimation, evaporation, transpiration). Cf. Ducoudré et
al. (1993) for further details. See also Surface Characteristics and Land
Soil hydrology is simulated using the land-surface scheme SECHIBA (Schématisation des Echanges Hydriques à l' Interface entre la Biosphère et l'Atmosphère) of Ducoudré et al. 1993. 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 layer being the most reactive: when precipitation exceeds evapotranspiration, the upper layer fills first; when the reverse is true, it empties first. Runoff occurs whenever the soil column is completely saturated (water depth 0.15 m). The remaining prescribed parameters for bare soil are a constant evaporative resistance and an empirical constant used to compute surface relative humidity for calculation of evaporation.
In SECHIBA, each of the 7 prescribed vegetation classes interact individually with the soil hydrology and contribute individually to the surface moisture flux. All the vegetation is assumed to have a single-story canopy that transpires or intercepts precipitation, but the canopy moisture capacity varies with the leaf area index, which is prescribed differently for each vegetation class. Different architectural and canopy resistances for evaporation/transpiration also are prescribed for each vegetation class. Cf Ducoudré et al. 1993 for further details. See also Surface Characteristics and Surface Fluxes.