Model NCAR (CSM): Elaborations
Participation
Model NCAR (CSM) is an entry in both the CMIP1 and CMIP2
intercomparisons,
and also is supplying an optional extended set of output data in
compliance
with the
CMIP2+ initiative.
Spinup/Initialization
The spinup/initialization procedure for the CMIP I
intercomparison
experiment was as follows (cf. Boville
and Gent 1998 and NCAR
Oceanography
Section 1996 for further details):
- The atmospheric model was spun up in a 10-year integration with
prescribed
monthly climatological SSTs (Shea et
al. 1990).
- The ocean model was spun up first in an uncoupled mode by
applying
fluxes
of momentum, heat, and freshwater. The momentum and turbulent heat
fluxes
and the net surface longwave flux were derived by bulk formulae from
surface
winds, air temperatures, and specific humidities obtained from the
6-hourly
National Center for Environmental Prediction (NCEP) reanalysis data for
1985-1988 (Kalnay et al. 1996).
The net
longwave radiation also was derived from a bulk formula that included
(in
addition to the surface temperatures and humidities) daily cloud
fraction
data of the International Satellite Cloud Climatology Project (ISCCP) (Rossow
and Schiffer 1991). The surface net shortwave radiation was derived
from daily ISCCP insolation data of Bishop
and Rossow (1991). The freshwater flux was derived from satellite
Microwave
Sounding Unit (MSU) estimates of monthly precipitation (Spencer
1993) minus the surface evaporation obtained directly from the
surface
latent heat flux plus a weak salinity restoring term. (Before their
application
to the ocean model, the net heat and freshwater fluxes were adjusted to
ensure that their global annual averages were near zero.) In addition,
in areas of sea ice, ocean temperature and salinity were strongly
restored
to the 50-meter average climatology of Levitus
(1982). Sea ice extents were diagnostically determined from the
monthly
climatological SSTs prescribed in the atmospheric spin up.
- To reduce the coupling shock, the ocean model was spun up for
another
60
years by recycling the fluxes obtained from the 10-year atmospheric
integration.
The model atmosphere and ocean then were coupled and integrated for 300
years without application of any flux corrections.
Land Surface
Processes
Land surface processes are simulated by the Land Surface Model (LSM) of
Bonan
(1996 , 1998). See also Web site http://www.cgd.ucar.edu/cms/lsm/
for
summary documentation and other information.
Sea Ice
- The simulated thermodynamics involve lateral ice growth in leads,
lateral
ice melt, and changes of state associated with the energy balance at
the
top and bottom of the ice. The representation of lateral growth/melt is
after Parkinson and
Washington
(1979), while that of the vertical energy balance follows Semtner
(1976) with some modifications. The calculation of the
ice/snow
thermodynamic state variables is based on the diffusion of heat through
the external and internal boundaries of a three-layer system. If the
ice
thickness is greater than 0.5 m, two layers of ice are maintained;
snow,
if present, constitutes a third layer. When the ice thickness is
between 0.25 m and 0.5 m, only a single layer is retained. If the
ice thickness falls below 0.25 m, the zero-layer model of Semtner
(1976) is employed. The entire net heat flux from the ocean is
applied
to lateral ice growth/melt; thus, when computing the vertical energy
balance,
the heat flux at the bottom of the ice is set to zero.
- Snow accumulation on sea ice is computed from the
freshwater flux
(precipitation
minus evaporation). The freshwater flux to the ocean due to ice
growth/melt
is parameterized by a method similar to Parkinson
(1979), with conservation of salt. Any snow melt is added to the
freshwater
flux into the ocean. Precipitation on sea ice at the freshwater melting
point (0 deg C) is assumed to fall directly into the ocean.
- A fraction (0.3, assuming cloudy conditions 75 percent of the
time) of
solar radiation is allowed to penetrate snow-free ice. The penetrating
radiation may be transmitted to the ocean or absorbed in the ice,
depending
on its thickness. The absorbed fraction contributes to the internal
melting
of ice in brine pockets, following Semtner
(1976)
(i.e., the absorbed flux is stored in a heat reservoir without
affecting
the overall ice thickness). The ice albedo depends on snow cover/depth
and surface temperature. (The albedo computation assumes a mix of
snow-covered and snow-free ice and uses a proxy ‘snow fraction’.)
The albedos of ice and snow depend on spectral interval, and on whether
these surfaces are dry or melting.
- Sea ice dynamics are based on the cavitating-fluid approximation
of Flato
and Hibler (1990): ice has a finite resistance to compression, but
diverges freely without resistance to shear stress. The ice momentum
equations
include the Coriolis force, wind stress, ocean current stress,
down-slope
acceleration from the ocean surface tilt, and the gradient of
accumulated
ice pressure due to compression. Ice velocity equations are
solved
at the corner points of an Arakawa B-grid in spherical coordinates,
following
a modified Pollard and
Thompson (1994)
advection scheme. Advected variables include ice concentration and
volume,
heat content of each ice layer, brine heat reservoir, and snow volume
and
heat content. For further details, cf. Bettge
et al. (1996).
Chief Differences from Closest AMIP Model
The NCAR (CSM) coupled model includes the NCAR
Community
Climate Model Version 3 (CCM3) as its atmospheric component (see also
Web
site http://www.cgd.ucar.edu/cms/ccm3/).
The
chief differences between CCM3 and AMIP model NCAR
CCM2 (T42 L18) 1992 include:
Radiation
In CCM3, effects of trace greenhouse gases on longwave radiation
are
added to the radiation scheme of the AMIP
model. Cloud radiative properties also are changed. Cf. Kiehl
et al. (1998a, b) for
further details.
For further details on the NCAR (CSM) model, see Web
site
http://www.cgd.ucar.edu/csm/.
References
Bettge, T.W., J.W. Weatherly, W.M.
Washington,
D. Pollard, B.P Briegleb, and W.G. Strand, 1996: The NCAR CSM Sea Ice
Model.
NCAR Tech. Note NCAR/TN-425+STR, 25 pp.
Bishop, J.K.B., and W.B.
Rossow,
1991: Spatial and temporal variability of global surface solar
irradiance.
J.
Geophys. Res., 96, 16,839-16,858.
Bonan, G.B., 1996: A Land
Surface
Model (LSM Version 1.0) for Ecological, Hydrological, and Atmospheric
Studies:
Technical Description and User's Guide. NCAR Technical Note
NCAR/Tn-417+STR,
National Center for Atmospheric Research, Boulder, Colorado, 150 pp.
Bonan, G.B., 1998: The land
surface
climatology of the NCAR land surface model (LSM 1.0) coupled to the
NCAR
Community Climate Model (CCM3). J. Climate, 11,
1307-1326.
Boville, B.A.,
and
P.R. Gent, 1998: The NCAR Climate System Model, Version One. J.
Climate,
11,1115-1130.
Flato, G.M., and W.D. Hibler,
1990:
On a simple sea-ice dynamics model for climate studies. Ann.
Glaciol.,
14, 72-77.
Hack, J. J.,
and
A.A.M. Holtslag, 1998: Climate simulation sensitivity to a revised
non-local
boundary layer parameterization. In preparation.
Hack, J.J., J.T.
Kiehl,
and J. Hurrell, 1998: The hydrologic and thermodynamic structure of the
NCAR CCM3. J. Climate, 11, 1179-1206.
Kalnay, E.C., M. Kanamitsu, R.
Kistler,
W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J.
Woollen,
Y. Zhu, M. Chelliah, W. Ebisuzaki, W. Higgins, J. Janowiak, K.C. Mo, C.
Ropelewski, A. Leetmaa, R. Reynolds, and R. Jenne, 1996: The NCEP/NCAR
reanalysis project. Bull. Am Meteor. Soc., 77, 437-471.
Kiehl, J.T., J.J. Hack, G. Bonan,
B.A.
Boville, D. Williamson, and P. Rasch, 1998a: The National Center for
Atmospheric
Research Community Climate Model: CCM3. J. Climate,
11,
1131-1149.
Kiehl, J.T., J.J. Hack, and J.
Hurrell,
1998b: The energy budget of the NCAR Community Climate Model: CCM3. J.
Climate, 11, 1151-1178.
Levitus, S., 1982: Climatological atlas
of
the world's oceans. NOAA Professional Paper 13, 173 pp.
NCAR Oceanography
Section
(NCAR OS), 1996: The NCAR CSM ocean model. NCAR Technical Note
NCAR/TN-423+STR,
National Center for Atmospheric Research, Boulder, Colorado.
Parkinson, C.L. and W.M.
Washington, 1979: A large-scale numerical model of sea ice. J. Geophys.
Res., 84, 311-337.
Pollard, D., and S.L.
Thompson,
1994: Sea-ice dynamics and CO2 sensitivity in a global climate model.
Atmosphere-Ocean
32, 449-467.
Rossow,W.B., and R.A.
Schiffer,
1991: ISCCP cloud data products. Bull. Am. Meteor. Soc., 72,
2-20.
Semtner, A.J., 1976: A model for the
thermodynamic
growth of sea ice in numerical investigations of climate. J.
Phys.
Oceanogr., 6, 379-389.
Shea, D.J., K.E. Trenberth, and R.W.
Reynolds, 1990: A global monthly sea surface temperature climatology.
NCAR
Technical Note NCAR/TN-345, 167 pp.
Spencer, R.W., 1993: Global oceanic
precipitation
from the MSU during 1979-91 and comparisons to other climatologies. J.
Climate, 6, 13021-1326.
Washington, W.M., and G.A.
Meehl,
1996: High-latitude climate change in a global coupled
ocean-atmosphere-sea
ice model with increased atmospheric CO2. J. Geophys. Res.,
101(D8), 12,795-12,801.
Zhang, G.J., and N.A.
McFarlane,
1995: Sensitivity of climate simulations to the parameterization of
cumulus
convection in the Canadian Climate Centre general circulation model. Atmos.
Ocean, 33, 407-446.
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CMIP Documentation Directory
Last update 15 May, 2002. For questions or comments, contact
Tom
Phillips (phillips@pcmdi.llnl.gov).
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