About Our Model

Atmospheric General Circulation Model (AGCM)

Atmospheric part of the model is identical to that used for the Atmospheric Model Intercomparison Project (AMIP) and its performance is described in Kitoh et al. (1995). Horizontal resolution of the AGCM is 5 degree by 4 degree in longitudinal and latitudinal directions, respectively. There are 15 vertical levels with the top at 1 hPa. Shortwave radiation calculation is based on Lacis and Hansen (1974) and longwave radiation calculation on Shibata and Aoki (1989). Five types of clouds are diagnostically determined: i.e., penetrative cumulus cloud, middle level convective cloud, planetary boundary layer stratus cloud, large-scale condensation cloud and cirrus anvil cloud. Partial cloudiness is allowed for the convective clouds. Effect of sub-grid-scale topography on the grid scale flow is included as gravity-wave drag following Palmer et al. (1986) with quantitative adjustment described by Yagai and Yamazaki (1988). Thermodynamic and hydrological processes of land surface are based on multi-layer soil model. There are four layers with the bottom at 10 m depth. Ground temperature, soil moisture and frozen soil moisture are predicted at each level.

Oceanic General Circulation Model (OGCM)

Oceanic part of the model (OGCM) is a global ocean general circulation model developed at the Meteorological Research Institute (MRI)(see Nagai et al., 1992). The model is now extended to include a realistic ocean bottom topography and a variable resolution in the meridional direction, ranging from 0.5 degree at the equator to 2.0 degree at 12 degree latitude and further poleward. The resolution in the longitudinal direction is 2.5 degree and there are 21 vertical layers, 11 of which are located in the first 300 m. The Mellor-Yamada l evel 2 turbulence closure scheme is included to simulate oceanic mixed layer. The lateral eddy viscosity and diffusivity are set to 2.0x10**9 cm**2/s and 5.0x10**7 cm**2/s, respectively, between 78 degree N and 78 degree S. The vertical eddy viscosity nd diffusivity are calculated following Mellor and Yamada (1974, 1982) andA HREF="model.html#mel1">Mellor and Durbin (1975).

Sea Ice Model

Sea ice model is included. The model predicts compactness (areal coverage ratio of sea ice) as well as thickness and surface temperature of sea ice. The movement of sea ice is predicted by considering stress from ceanic current at the first level of the ocean. The ice dynamics is parameterized following Mellor and Kantha(1989). Energy exchanges through the surface are calculated both in the sea ice area and "leads" separately and re weighted with the respective coverage ratio. The model can simulate seasonal variation of sea ice compactness and thickness realistically not only in the Arctic but also in the circum-Antarctic area. The performances of the sea ice model will be described fully in a separate paper. The current climate model has different horizontal resolution between he AGCM and the OGCM. Furthermore,we predict areal coverage ratio of sea ice in each ocean grid, as explained above. In calculating surface energy fluxes, e linearly add energy fluxes calculated between atmosphere and a part of surface area (which is either land , open ocean or sea ice) of one atmospheric grid with the areal ratio of the part to the atmospheric grid. We allow partial coverage of land at both coastal and island grids.

About Experiment - What we did

The ocean was spun up with an acceleration method for 1500 years from an isothermal motionless state with a uniform salinity. Then a time integration was made for 30 years by coupling the AGCM with the OGCM, to obtain flux corrections in the surface energy and water fluxes. In these runs, SST and sea surface salinity (SSS) were relaxed to observed climatological fields (Levitus, 1982). Two runs were performed, i.e., a run with a fixed atmospheric CO2 concentration (C run) at 345 ppmv and a run with a gradual increase of CO2 at a compound rate of 1%/yr (G run). This increasing rate of CO2 roughly corresponds to the actual increase of radiative forcing due to the increase of several greenhouse gases and has been used in other studies (Stouffer et al., 1989; Manabe et al.,1991; Murphy, 1992). In both C and G runs the flux corrections in the surface energy and water fluxes were included to predict realistic SST and surface salinity. Runs were continued up to the year 70 in both C and G runs.


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