Model BMRC1: Elaborations
Participation
Model BMRC1 is an entry in the CMIP1 intercomparison only.
Spinup/Initialization
The spinup/initialization procedure for the
BMRC1
coupled model was as follows (cf. Power
et
al. 1993 for further details):
-
The atmospheric model was integrated for 3.5 years with specified
observational
SSTs prior to coupling.
-
The ocean model was forced by restoring the upper-level temperature and
salinity to observational estimates of SST and SSS after Levitus
(1982) on a time scale of 20 days. Hellerman
and Rosenstein (1983) observational estimates of surface wind
stress
also were applied. Integration of the ocean model proceeded in three
stages,
with annually averaged forcings applied in the first stage and seasonal
forcings in subsequent stages. Intermediate quasi-equilibrium solutions
were obtained using the acceleration techniques of Bryan
and Lewis (1979) and Bryan (1984): the
time
step for the tracer equations was set much larger than that for the
barotropic/baroclinic
dynamics and the time step was increased with depth. In the first
phase,
the surface tracer time step was 2 days and the dynamic time step was
1200
seconds; in this phase, an acceleration factor of 8:1 was applied to
the
time step at ocean bottom vs surface, and the ocean model was run for
200,000
surface time steps to ensure quasi-equilibrium of state variables
(e.g.,
globally averaged salinity < 1x10-3 ppt per century). In
the second stage, the dynamic and tracer time steps were synchronous
and
of length 5000 seconds, while the acceleration factor was 4:1; in this
phase, the simulation length was 250 years. In the third stage, the
time
steps remained the same and no acceleration was employed, while the
simulation
was extended another 450 years.
-
The ocean and atmosphere then were coupled and integrated for a
simulated
105 years without application of flux adjustments.
Land Surface Processes
-
Soil temperature is computed from heat storage in two layers with a
climatological
temperature specified in a deeper layer. The upper boundary condition
is
the surface energy balance. The heat conductivity of soil is fixed
under
all conditions.
-
Prognostic soil moisture is represented by a single-layer "bucket"
model
with uniform field capacity of 0.15 m after Manabe
and Holloway (1975). Both precipitation and snowmelt contribute to
soil moisture. The evapotranspiration efficiency beta is a
function
of the ratio of soil moisture to the field capacity. Runoff occurs
implicitly
if this ratio exceeds unity. For calculating the contribution of
freshwater
from continental runoff to the ocean, the continents are broken up into
about 20 areas, each associated with a major river system; runoff from
the 0.15 m buckets within each region then is dumped without delay at
the
mouth of the appropriate river (Power
et al.
1993).
Sea Ice
-
Sea ice, treated as a simple uniform slab without leads, is based on
the
"zero-layer" thermodynamic model of Semtner
(1976).
At the bottom of the slab, temperature is prescribed as 271.18 K,
the freezing point ("brine zero" temperature) for sea
water.
At an ocean point, if the temperature of the uppermost layer falls
below
brine zero, ice forms at a thickness consistent with the surface heat
flux.
Sea ice melt, and snow melt if it exists, is converted to an equivalent
fresh water flux to the ocean model. In the rare case of sea ice
melting with snow still remaining, the extra snow is assumed to melt
instantly,
with the additional freshwater flux also calculated.
-
The ice albedo is prescribed similarly to that of snow, as a function
of
temperature, following Washington
and Meehl (1984). The temperature dependence of the ice
albedo
implicitly represents the effects of leads and melt pools. The
temperature
of the top surface of the ice (or that of its snow cover) is determined
by the atmospheric model; it is constrained to be no greater than the
freezing
point of freshwater, with excess surface heat contributing to snow/ice
melting. The vertical temperature gradient is assumed to be
uniform
in the ice/snow interior, and to react instantaneously to surface
temperature
changes. Snow/ice heat conduction coefficients and heats of fusion are
given by Colman et al. (1992).
-
A bottom flux flowing from the underlying ocean is proportional to the
difference between the temperature of the uppermost layer of the ocean
and the brine zero temperature. The constant of proportionality is
specified
after Parkinson
and Washington
(1979), so that a 1 K temperature contrast results in a flux of
about
2 Wm-2 in the Arctic; it is set to zero in the
Antarctic.
Ice formation or ablation results at the bottom surface of the ice,
depending
on the sum of the bottom flux and that passing through the ice from
above.
Chief Differences from Closest
AMIP
Model
Aside from a reduced horizontal resolution (from spectral rhomboidal 31
to 21), the atmospheric component of the BMRC1 coupled model differs
from
AMIP model BMRC
BMRC2.3 (R31 L9) 1990 mainly as follows:
Convection
The AMIP
model's representation of deep convection after Kuo
(1974) is replaced by the mass-flux scheme of Tiedtke
(1989), but without inclusion of momentum effects. The Tiedtke
scheme
accounts for midlevel and penetrative convection, and also includes
effects
of cumulus-scale downdrafts. The closure assumption for
midlevel/penetrative
convection is that large-scale moisture convergence determines the bulk
cloud mass flux. Entrainment and detrainment of mass in convective
plumes
occurs both through turbulent exchange and organized inflow and
outflow.
Cf. Colman and
McAvaney (1995)
and McAvaney et al. (1995)
for further
details on the consequences of the new convective scheme. Shallow
convection,
however, is parameterized in the same fashion as in the AMIP
model, following Tiedtke (1983, 1988).
References
Bryan, K., 1984: Accelerating the convergence
to equilibrium of ocean-climate models. J. Phys. Oceanogr., 14,
666-673.
Bryan, K., and L.J. Lewis, 1979:
A water mass model of the world ocean. J. Geophys. Res., 84(C5),
2503-2517.
Colman, R.A., McAvaney, B.J.,
Fraser,
J.R., and R.R., Dahni, 1992: Mixed layer ocean and thermodynamic sea
ice
models in the BMRC GCM. BMRC Res. Rep. No. 30.
Colman, R.A., and B.J.
McAvaney,
1995: Sensitivity of the climate response of an atmospheric general
circulation
model to changes in convective parameterization and horizontal
resolution.
J. Geophys. Res., 100, 3155-3172.
Hellerman, S., and M.
Rosenstein,
1983: Normal monthly wind stress over the world ocean with error
estimates.
J. Phys. Oceanogr., 13, 1093-1104.
Kuo, H.L., 1974: Further studies of the
parameterization
of the influence of cumulus convection on large-scale flow. J.
Atmos.
Sci., 31, 1232-1240.
Levitus, S., 1982: Climatological atlas
of
the world's oceans. NOAA Professional Paper 13, 173 pp.
Manabe, S., and J.L. Holloway,
1975:
The seasonal variation of the hydrologic cycle as simulated by a global
model of the atmosphere. J. Geophys. Res., 80,
1617-1649.
McAvaney, B.J., R.R. Dahni,
R.A.
Colman, J.R. Fraser, and S.B. Power, 1995: The dependence of the
climate
sensitivity on convective parameterisation: Statistical evaluation. Glob.
Plan. Change, 10, 181-200.
Parkinson, C.L. and
W.M.
Washington, 1979: A large-scale numerical model of sea ice. J. Geophys.
Res., 84, 311-337.
Power, S.B., R.A.
Colman,
B.J. McAvaney, R.R. Dahni, A.M. Moore, and N.R. Smith, 1993: The
BMRC Coupled atmosphere/ocean/sea-ice model. BMRC Research Report No.
37,
Bureau of Meteorology Research Centre, Melbourne, Australia, 58 pp.
Semtner, A.J., 1976: A model for the
thermodynamic
growth of sea ice in numerical investigations of climate. J.
Phys.
Oceanogr., 6, 379-389
Tiedtke, M., 1983: The sensitivity of the
time-mean large-scale flow to cumulus convection in the ECMWF model.
Proceedings
of the ECMWF Workshop on Convection in Large-Scale Models, 28
November-1
December 1983, European Centre for Medium-Range Weather Forecasts,
Reading,
England, 297-316.
Tiedtke, M., 1988: Parameterization of
cumulus
convection in large-scale models. In Physically-Based Modelling and
Simulation of Climate and Climatic Change, Part 1. M.E. Schlesinger
(ed.), Kluwer Academic Publishers, Dordrecht, 375-431.
Tiedtke, M., 1989: A comprehensive mass
flux
scheme for cumulus parameterization in large-scale models. Mon.
Wea.
Rev., 117, 1779-1800.
Washington, W.M., and G.A.,
Meehl, 1984: Seasonal cycle experiment on the climate sensitivity due
to
doubling of CO2 with an atmospheric GCM coupled to a simple mixed-layer
ocean. J. Geophys. Res., 89, 9475-9503
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Last update 15 May, 2002. This page is maintained by Tom Phillips (phillips@pcmdi.llnl.gov).
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