Model COLA 2: Elaborations

Participation

Spinup/Initialization

• The model atmosphere was integrated to equilibrium using as a boundary conditions either prescribed SSTs or the forcing provided by coupling to a mixed-layer ocean model.

• The model ocean was initialized, whole-volume, to the observations (e.g., Levitus 1982 data).

• The models then were coupled and integrated without flux adjustments for 190 years.  The first 80 years of the run were discarded, and time-mean variables were computed from the last 100 years.

Land Surface Processes

• Land surface processes are simulated following the Xue et al. (1991) modification of the SiB model of Sellers et al. (1986). Within the single-story vegetation canopy, evapotranspiration from dry leaves includes detailed modeling of stomatal and canopy resistances; direct evaporation from the wet canopy and from bare soil is also treated. Precipitation interception by the canopy is simulated, and its infiltration into the ground is limited to less than the hydraulic conductivity of the soil.

• Soil temperature is determined in two layers by the force-restore method of Deardorff (1978). Soil moisture, which is predicted from diffusion equations in three layers, is increased by infiltrated precipitation and snowmelt, and is depleted by evapotranspiration, direct evaporation, and drainage. Both surface runoff and deep runoff from gravitational drainage are simulated, but the contribution of runoff to the freshwater flux into the ocean model is not included.

Sea Ice

• The sea ice parameterization (cf. the appendix to Schneider and Zhu 1998) is a simple prognostic single-layer thermodynamic model. Given surface fluxes provided by the atmospheric model, the scheme calculates changes in ice thickness and surface temperature, and the modified fluxes of heat and fresh water supplied to the underlying ocean. The time step is semi-analytic to maintain stability.

• Sea ice forms over the whole of an ocean grid box when the temperature of the topmost ocean layer is predicted to fall below the saltwater freezing temperature.  Melting occurs at  the top or bottom of the ice if the respective temperatures of these surfaces are predicted to be above this freezing point.  Freshwater fluxes from the atmosphere are unmodified by the sea ice, but freezing or melting at the ice bottom causes appropriate changes in the freshwater flux to the ocean.

• The ice albedo is a constant, independent of surface temperature or ice melt.  Solar radiation does not penetrate the ice and there are no other internal heat sources. Overlying snow cover also does not affect the ice thermodynamics.  The temperature gradient within the ice is assumed to be linear, and the temperature at the bottom surface is prescribed to be the saltwater freezing point.  The heat flux into the ocean is determined by conduction down the ice temperature gradient, which is controlled by the ice thickness and the top surface temperature. Heat is exchanged between the ice and the topmost ocean layer, consistent with freezing/melting, to keep the ocean surface temperature at the saltwater freezing point.

• Sea ice dynamics and rheology are not represented.

Chief Differences from the Closest AMIP Model

Convection

The relaxed Arakawa-Schubert scheme of Moorthi and Suarez (1992) replaces the modified Kuo convective parameterization that is used in the AMIP model.

Cloud Formation

A cloud scheme similar to that of the NCAR CCM3 model (cf. Kiehl et al. 1996) replaces the Slingo scheme that is used in the AMIP model.
 

References

Deardorff, J.W., 1978: Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation. J. Geophys. Res., 83, 1889-1903.

Kiehl, J.T., G.B. Bonan, B.A. Boville, B.P. Briegleb, D.L. Williamson, and P.J. Rasch, 1996: Description of the NCAR Community Climate Model (CCM3).  NCAR Tech. Note, NCAR/TN-420+STR, 152 pp. [Available from National Center for Atmospheric Research, Boulder, CO 80307.]

Levitus, S., 1982: Climatological atlas of the world's oceans. NOAA Professional Paper 13, 173 pp.

Moorthi, S., and M.J. Suarez, 1992: Relaxed Arakawa-Schubert: A parameterization of moist convection for general circulation models. Mon. Wea. Rev., 120, 978-1002.

Schneider, E.K., Z. Zhu, B.S. Giese, B. Huang, B.P. Kirtman, J. Shukla, and J.A. Carton, 1997: Annual cycle and ENSO in a coupled ocean-atmosphere general circulation model. Mon. Wea. Rev., 125, 680-702.

Schneider, E. K., and Z. Zhu, 1998: Sensitivity of the simulated annual cycle of sea surface temperature in the equatorial Pacific to sunlight penetration. J. Climate, 11, 1932-1950.

Sellers, P.J., Y. Mintz, Y.C. Sud, and A. Dalcher, 1986: A simple biosphere model (SiB) for use within general circulation models. J. Atmos. Sci., 43, 505-531.

Xue, Y.-K., P.J. Sellers, J.L. Kinter II, and J. Shukla, 1991: A simplified biosphere model for global climate studies. J. Climate, 4, 345-364.
 

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