Model GISS (Miller): Elaborations

Participation

Spinup/Initialization

• The atmospheric model was integrated for a decade with AMIP ocean/sea ice boundary conditions.

• The computed surface winds then were used to force the ocean model, while restoring the SST and SSS to Levitus (1982) annual-mean values. The ocean model was integrated for 6000 x 365 ocean-tracer time steps (equivalent to 6000 years of the tracer field, with time step length of 1 day).

• Seasonally varying values of SST and SSS then were used to force the ocean model for an additional 500 years, with the tracer time step reduced from 1 day to 4 hours.

• At this point, the globally integrated surface energy flux (proportional to the difference between the Levitus temperature and the first model layer temperature) was < 0.001 W m-2, and the ocean and atmosphere models were coupled.

• The coupled model was integrated for 110 years without flux adjustment.

Land Surface Processes

• Soil temperature is computed by solving a heat diffusion equation in six layers. The thickness of the top layer varies, but is approximately 0.1 m. The thicknesses of deeper layers increase geometrically, with the bottom boundary of the soil column at a nominal bedrock depth of 3.444 m. The upper boundary condition is the balance of surface energy fluxes; at the bottom boundary, zero net heat flux is specified. The thermal conductivity and heat capacity of the ground vary with snow cover, as well as soil moisture amount and phase.

• Land-surface hydrology is treated after the physically based model of Abramopoulos et al. (1988). The scheme includes a vegetation canopy, a composite over each grid box from the vegetation types of Matthews (1983, 1984), that intercepts precipitation and dew. Evaporation from the wet canopy and from bare soil is treated, as well as soil-moisture loss from transpiration according to moisture availability and variable vegetation resistance and root density. Diffusion of moisture is predicted in the six soil layers, accounting for spatially variable composite conductivities and matric potentials that depend on soil type and moisture content. Infiltration of precipitation and snowmelt is explicitly calculated, with surface runoff occurring when the uppermost soil layer is saturated; underground runoff that depends on topographic slope also is included.

Sea Ice

• Sea ice is represented by the two-layer model of Hansen et al. (1983), where the top layer simulates accumulating snow up to a depth of 0.1 m.  When this threshold depth is exceeded, 0.01m of the snow is added to the lower layer as ice, thereby keeping the upper layer thin enough to simulate diurnal temperature changes. A fraction of an ocean grid square may be covered by ice at a thickness that is a function of the fractional coverage.

• The snow-free albedo of sea ice is 0.55 in the visible (wavelengths < 0.7micrometers) and 0.3 in the near-infrared, resulting in a spectrally weighted ice albedo of 0.45.

• Sea ice dynamics and rheology are neglected.

Chief Differences from Closest AMIP Model

Horizontal Representation

A linear upstream scheme is used for the advection of tracers, rather than the AMIP model's more accurate quadratic advection scheme.

Chemistry

The model lacks the improved aerosol climatology used in the AMIP model.

Radiation

Rather than calculating radiative fluxes at every horizontal grid point as in the AMIP model, these fluxes are computed only at every other horizontal grid point, using interpolation to obtain values at intermediate grid points.

Land Surface Processes

The land surface hydrology scheme is somewhat less sophisticated than that of the AMIP model.

 
References

Hansen, J., G. Russell, D. Rind, P. Stone, A, Lacis, S. Lebedeff, R. Ruedy, and L. Travis, 1983: Efficient three-dimensional global models for climate studies: Models I and II. Mon. Wea. Rev., 111, 609-662.

Matthews, E., 1983: Global vegetation and land use: New high-resolution data bases for climate studies. J. Clim. Appl. Meteor., 22, 474-487. (Abstract)

Matthews, E., 1984: Vegetation, land-use, and seasonal albedo data sets: Documentation of archived data tape. NASA Tech. Memo. 86107, National Aeronautics and Space Administration, Washington, D.C., 20 pp.

Miller, R.L., and X. Jiang, 1996: Surface energy fluxes and coupled variability in the Tropics of a coupled General Circulation Model.  J. Climate, 9, 1599-1620
 
 

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CMIP Documentation Directory

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

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