Model GFDL_R15_a: Elaborations
Model GDDL_R15_a is an entry in both the CMIP1 and the CMIP2 intercomparisons.
The procedure for spinup/initialization to the simulation starting point
of the GFDL_R15_a coupled model was as follows, based on Manabe
et al. 1991 and personal communication from R. Stouffer concerning
amendments of this procedure that followed a change in computing platform
from a CDC 205 computer to a Cray Y/MP:
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
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
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
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 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.
Bryan, K., 1969: Climate and the ocean circulation.
III: The ocean model. Mon. Wea. Rev., 97, 806-827.
Bryan, K., 1984: Accelerating the convergence
to equilibrium of ocean-climate models. J. Phys. Oceanogr., 14,
Bryan, K., S. Manabe, and R.C. Pacanowski,
1975: A global ocean-atmosphere climate model. II: The oceanic circulation.
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.
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.
CMIP Documentation Directory
Last update 15 May, 2002. This page is maintained by Tom Phillips