Model GFDL_R15_a: Elaborations

Participation

Spinup/Initialization

• Starting from the initial condition of an isothermal and dry atmosphere at rest, the atmospheric model was integrated for 22 years with seasonally and geographically varying observed SSTs and sea ice extents and thicknesses (the last being estimated from satellite observations of sea ice concentration by Parkinson et al. (1987) and Zwally et al. (1983). For purposes of calculating flux adjustments, the seasonal and geographic distributions of the atmospheric model's surface heat and freshwater (P-E) fluxes were averaged over the last 20 years of the integration.  In addition, the 20-year mean surface momentum fluxes were computed for use in the spinup of the ocean model described below.

• The ocean model was integrated for 3000 years with application of acceleration techniques to the deep ocean  extending the effective integration to 30,000 years (cf. Bryan et al. 1975 and Bryan 1984).  The ocean was forced at its upper boundary by the 20-year monthly mean surface fluxes obtained from the atmospheric model integration, with the SST,  SSS, and sea ice being relaxed, at a time scale of 50 days,  toward observed seasonal values. Flux adjustments in heat and freshwater were computed from the differences between the average fields needed to maintain realistic SST, SSS, and sea ice in the last 500 years of this ocean-only integration.

• The atmosphere and ocean were coupled using the conditions from the end of the atmosphere-only and ocean-only integrations.  The coupled model was integrated for 1000 years with application of flux adjustments in heat and freshwater that were computed in the ocean-only integration.

Land Surface Processes

• Ground temperature is determined from a surface energy balance without provision for soil heat storage.

• Soil moisture is represented by the single-layer "bucket" model of Manabe (1969), with field capacity everywhere 0.15 m. Soil moisture is increased by precipitation and snowmelt; it is depleted by surface evaporation, which is determined from a product of the evapotranspiration efficiency beta and the potential evaporation from a surface saturated at the local surface temperature and pressure. Over land, beta is given by the ratio of local soil moisture to a critical value that is 75 percent of field capacity, and is set to unity if soil moisture exceeds this value. Runoff, which occurs implicitly if soil moisture exceeds the field capacity, contributes to the freshwater flux of the model ocean. A simple routing scheme instantaneously transports the runoff from each land grid box to an ocean coastal grid box, where the direction of the flow is toward the steepest descent of the local continental topography (Manabe et al. 1991).

Sea Ice

• Sea ice forms at the freezing point of salt water (271.2 K).  The ice thickness is directly augmented by snowfall and by local freezing proportional to the difference between the heat lost to the atmosphere and that supplied by the underlying ocean.  Rainfall on the ice is assumed to penetrate directly to the ocean. The heat capacity and salt content of the ice, and the insulating effects of new snow are neglected.

 
• The ice albedo is calculated following  Manabe et al. (1991).  For ice at least 1 m thick, the surface albedo is 0.80 if the surface temperature is below -10 deg C and 0.55 at 0 deg C, with intermediate values determined by linear interpolation.  If the ice thickness is less than 1 m, the albedo decreases as the square root of the thickness between bounds given by the thick-ice albedo and that of  the underlying water surface.

 
• The temperature of the ice is predicted after Holloway and Manabe (1971) from a net balance of the surface energy fluxes and a conduction flux from the underlying ocean (i.e., heat exchange through ice leads is neglected). The conduction flux is proportional to the difference between the surface temperature of the ice and that of the ocean (assumed to be 271.2 K), and is inversely proportional to the ice thickness. The constant heat conductivity is that of pure ice.  The temperature of the upper surface of the ice is constrained to be less than the freshwater melt temperature (273.1 K).

• Dynamics follow the simple free-drift model of Bryan (1969) which neglects the internal pressure of the sea ice. The ice is advected with the surface ocean currents (50 m deep surface box for this model), except when the ice is thicker than 4 m. In this case, only divergent (not convergent) motion is allowed.  The ice also is mixed laterally by turbulent diffusion.

References

Bryan, K., 1984: Accelerating the convergence to equilibrium of ocean-climate models. J. Phys. Oceanogr., 14, 666-673.

Bryan, K., S. Manabe, and R.C. Pacanowski, 1975: A global ocean-atmosphere climate model. II: The oceanic circulation. J. Phys. Oceanogr., 5, 30-46.

Holloway, J.L., Jr., and S. Manabe, 1971: Simulation of climate by a general circulation model. I. Hydrological cycle and heat balance. Mon. Wea. Rev., 99, 335-370.

Manabe, S., 1969: Climate and ocean circulation. I. The atmospheric circulation and the hydrology of the earth's surface. Mon. Wea. Rev., 97, 739-774.

Manabe, S., R.J. Stouffer, M.J. Spelman, and K. Bryan, 1991:  Transient response of a coupled ocean-atmosphere model to gradual changes of atmospheric CO₂.  Part I: Annual mean response.  J. Climate, 4, 785-818

Parkinson, C.L., J.C. Comiso, H.J. Zwally, D.J. Cavalieri, P. Gloersen, and W.J. Campbell, 1987: Arctic sea ice, 1973-1976: Satellite passive-microwave observations. NASA SP-489, National Aeronautics and Space Administration, 296 pp.

Zwally, H.J., C. Comiso, C.L. Parkinson, W.J. Campbell, F.D. Carsey, and P. Gloersen, 1983: Antarctic sea ice, 1973-1976: Satellite passive-microwave observations. NASA SP-459, National Aeronautics and Space Administration, 206 pp.
 

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Last update 15 May, 2002.  This page is maintained by Tom Phillips (phillips@pcmdi.llnl.gov).
 

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