Model BMRC1: Elaborations
Model BMRC1 is an entry in the CMIP1 intercomparison only.
The spinup/initialization procedure for the
coupled model was as follows (cf. Power
al. 1993 for further details):
The atmospheric model was integrated for 3.5 years with specified
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
also were applied. Integration of the ocean model proceeded in three
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
step for the tracer equations was set much larger than that for the
dynamics and the time step was increased with depth. In the first
the surface tracer time step was 2 days and the dynamic time step was
seconds; in this phase, an acceleration factor of 8:1 was applied to
time step at ocean bottom vs surface, and the ocean model was run for
surface time steps to ensure quasi-equilibrium of state variables
globally averaged salinity < 1x10-3 ppt per century). In
the second stage, the dynamic and tracer time steps were synchronous
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
steps remained the same and no acceleration was employed, while the
was extended another 450 years.
The ocean and atmosphere then were coupled and integrated for a
105 years without application of flux adjustments.
Land Surface Processes
Soil temperature is computed from heat storage in two layers with a
temperature specified in a deeper layer. The upper boundary condition
the surface energy balance. The heat conductivity of soil is fixed
Prognostic soil moisture is represented by a single-layer "bucket"
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
of the ratio of soil moisture to the field capacity. Runoff occurs
if this ratio exceeds unity. For calculating the contribution of
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
mouth of the appropriate river (Power
Sea ice, treated as a simple uniform slab without leads, is based on
"zero-layer" thermodynamic model of Semtner
At the bottom of the slab, temperature is prescribed as 271.18 K,
the freezing point ("brine zero" temperature) for sea
At an ocean point, if the temperature of the uppermost layer falls
brine zero, ice forms at a thickness consistent with the surface heat
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
with the additional freshwater flux also calculated.
The ice albedo is prescribed similarly to that of snow, as a function
temperature, following Washington
and Meehl (1984). The temperature dependence of the ice
implicitly represents the effects of leads and melt pools. The
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
point of freshwater, with excess surface heat contributing to snow/ice
melting. The vertical temperature gradient is assumed to be
in the ice/snow interior, and to react instantaneously to surface
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
(1979), so that a 1 K temperature contrast results in a flux of
2 Wm-2 in the Arctic; it is set to zero in the
Ice formation or ablation results at the bottom surface of the ice,
on the sum of the bottom flux and that passing through the ice from
Chief Differences from Closest
Aside from a reduced horizontal resolution (from spectral rhomboidal 31
to 21), the atmospheric component of the BMRC1 coupled model differs
AMIP model BMRC
BMRC2.3 (R31 L9) 1990 mainly as follows:
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
accounts for midlevel and penetrative convection, and also includes
of cumulus-scale downdrafts. The closure assumption for
convection is that large-scale moisture convergence determines the bulk
cloud mass flux. Entrainment and detrainment of mass in convective
occurs both through turbulent exchange and organized inflow and
Cf. Colman and
and McAvaney et al. (1995)
details on the consequences of the new convective scheme. Shallow
however, is parameterized in the same fashion as in the AMIP
model, following Tiedtke (1983, 1988).
Bryan, K., 1984: Accelerating the convergence
to equilibrium of ocean-climate models. J. Phys. Oceanogr., 14,
Bryan, K., and L.J. Lewis, 1979:
A water mass model of the world ocean. J. Geophys. Res., 84(C5),
Colman, R.A., McAvaney, B.J.,
J.R., and R.R., Dahni, 1992: Mixed layer ocean and thermodynamic sea
models in the BMRC GCM. BMRC Res. Rep. No. 30.
Colman, R.A., and B.J.
1995: Sensitivity of the climate response of an atmospheric general
model to changes in convective parameterization and horizontal
J. Geophys. Res., 100, 3155-3172.
Hellerman, S., and M.
1983: Normal monthly wind stress over the world ocean with error
J. Phys. Oceanogr., 13, 1093-1104.
Kuo, H.L., 1974: Further studies of the
of the influence of cumulus convection on large-scale flow. J.
Sci., 31, 1232-1240.
Levitus, S., 1982: Climatological atlas
the world's oceans. NOAA Professional Paper 13, 173 pp.
Manabe, S., and J.L. Holloway,
The seasonal variation of the hydrologic cycle as simulated by a global
model of the atmosphere. J. Geophys. Res., 80,
McAvaney, B.J., R.R. Dahni,
Colman, J.R. Fraser, and S.B. Power, 1995: The dependence of the
sensitivity on convective parameterisation: Statistical evaluation. Glob.
Plan. Change, 10, 181-200.
Parkinson, C.L. and
Washington, 1979: A large-scale numerical model of sea ice. J. Geophys.
Res., 84, 311-337.
Power, S.B., R.A.
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.
Bureau of Meteorology Research Centre, Melbourne, Australia, 58 pp.
Semtner, A.J., 1976: A model for the
growth of sea ice in numerical investigations of climate. J.
Oceanogr., 6, 379-389
Tiedtke, M., 1983: The sensitivity of the
time-mean large-scale flow to cumulus convection in the ECMWF model.
of the ECMWF Workshop on Convection in Large-Scale Models, 28
December 1983, European Centre for Medium-Range Weather Forecasts,
Tiedtke, M., 1988: Parameterization of
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
scheme for cumulus parameterization in large-scale models. Mon.
Rev., 117, 1779-1800.
Washington, W.M., and G.A.,
Meehl, 1984: Seasonal cycle experiment on the climate sensitivity due
doubling of CO2 with an atmospheric GCM coupled to a simple mixed-layer
ocean. J. Geophys. Res., 89, 9475-9503
Last update 15 May, 2002. This page is maintained by Tom Phillips (firstname.lastname@example.org).