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UCLA Numerical Weather Prediction Model
Entry ID: UCLA_AGCM
Abstract: The UCLA atmospheric GCM is a state of the art grid point model of the global
atmosphere extending from the Earth's surface to a height of 50 km. The model
predicts the horizontal wind, potential temperature, water vapor mixing ratio,
cloud water and cloud ice mixing ratios, planetary boundary layer (PBL) depth
and the surface pressure, as well as the surface temperature and snow depth
over land. ... The horizontal finite differencing of the primitive equations is
done on a staggered Arakawa "C" grid and is based on a fourth order version of
the scheme of Arakawa and Lamb (1981) that conserves the potential enstrophy
and energy when applied to the shallow water equations (Takano and Wurtele
1982). The differencing of the thermodynamic energy and water vapor advection
equations is also based on a fourth-order scheme. The vertical coordinate
used is the modified sigma-coordinate of Suarez et al. (1983). In this
coordinate, the lowest model layer is the planetary boundary layer.
The vertical finite differencing is performed on a Lorenz-type grid following
Arakawa and Lamb (1977) above 100 mb and Arakawa and Suarez (1983) below. This
differencing is of second order accuracy and is designed to conserve the global
mass integrals of potential temperature and total energy for adiabatic,
For the integration in time of the momentum, thermodynamic energy and water
vapor and cloud water/ice advection equations, a leapfrog time-differencing
scheme is used with a Matsuno step regularly inserted. To avoid the use of the
extremely short timestep necessary to satisfy the CFL condition near the poles,
a longitudinal averaging (which takes the form of a Fourier filter) is
performed on selected terms in the prognostic equations to increase the
effective longitudinal grid size. The filter acts poleward of 45 degrees
latitude and its strength is gradually increased towards the pole by increasing
the number of affected zonal wavenumbers and the amount by which they are
damped (Arakawa and Lamb 1977). A more localized spatial filter is applied to
the predicted PBL depths (Suarez et al. 1983) everywhere. A nonlinear
horizontal diffusion of momentum is included following Smagorinsky (1963). The
coefficient used is one order of magnitude smaller than that used by
Smagorinsky. The diffusion is applied at each timestep, using a forward time
differencing scheme. In layers where an unstable stratification develops
(potential temperature decreasing with height), we assume that subgrid-scale
dry convection occurs and that the prognostic variables (horizontal momentum,
potential temperature and water vapor mixing ratio) in the layers involved are
Planetary boundary layer processes are parameterized using the mixed-layer
approach of Suarez et al. (1983). In this parameterization, surface fluxes are
calculated following the bulk formula proposed by Deardorff (1972). The
formulation of moist processes in the PBL and moisture exchange with the layer
above has been recently revised (Li et al. 1999, Li et al. 2002), resulting in
an improved simulation of the geographical distribution and optical properties
of PBL stratocumulus clouds. Parameterization of cumulus convection, including
its interaction with the PBL, follows the prognostic version of Arakawa and
Schubert (1974) presented by Pan and Randall (1998). The effects of convective
downdrafts and vertical momentum and rainwater budgets are included in the
cumulus parameterization (Cheng and Arakawa 1997). The current model version
also includes an implementation of the prediction scheme for cloud liquid water
and ice due to Kohler (1999). Parameterization of cumulus convection, including
its interaction with the PBL, follows Arakawa and Schubert (1974) and Lord et
al. (1982), with a relaxed adjustment time scale for the cloud work function as
described in Cheng and Arakawa (1994)> and Ma et al. (1994). The
parameterization of both long and shortwave radiative heating follows
Harshvardhan et al. (1987, 1989). The ozone mixing ratios used in the radiation
calculations are prescribed as a function of latitude, height and time based on
values from a monthly UGAMP climatology (Li and Shine 1995) as used by Kim et
al. (1998). The cloud optical properties are specified following Harshvardhan
et al. (1989). This prescription makes a distinction between stratiform clouds
and "cumulus anvil"-type clouds. "Cumulus anvil"-type clouds are assumed to
exist at each model layer above 400 mb where the cumulus mass flux is positive;
all other clouds are assumed to be stratiform-type clouds. The effects of
subgrid-scale orography are included via a gravity wave drag parameterization
and envelop orography ( Kim and Arakawa 1995, Kim 1996).
The geographical distribution of sea surface temperature is prescribed
using climatological or yearly varying values from the Reynolds (1998) dataset;
sea ice thickness and extents are prescribed following Alexander and Mobley
(1976). Surface albedo and roughness lengths are specified following Dorman and
Sellers (1989), in which roughness lengths over land vary according to the
vegetation type. Daily values of these surface conditions (as well as sea ice
thickness) are determined from the monthly mean values by linear interpolation.
The parallel version of the UCLA AGCM code was designed for distributed
memory multiple-instruction-multiple-data (MIMD) computing environments (Wehner
et al., 1995). It is based on a two dimensional (longitud-latitude) domain
decomposition, message-passing strategy. Subdomains consist of vertical columns
from the Earth's surface to the top of the atmosphere. The code is written in
standard FORTRAN, including machine-architecture independent directives that
are expanded to machine-architecture dependednt source code at pre-processing
time. The has been ported to and time in several machines including the
SGI/Origin 2000, IBM SP, CRAY T3E, Intel and Compaq Workstation clusters.
Summary provide by UCLA.]
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Arakawa, A., and W. H. Schubert, 1974: Interaction of a cumulus cloud ensemble
with the large-scale environment, Part I. J. Atmos. Sci., 31, 674-701.
Cheng, M.-D., and A. Arakawa, 1997: Inclusion of rainwater budget and
convective downdrafts in the Arakawa-Schubert cumulus parameterization. J.
Atmos. Sci. , 54, 1359-1378.
Farrara, J. D., C. R. Mechoso and A. W. Robertson, 2000: Ensembles of AGCM
two-tier predictions and simulations of the circulation anomalies during winter
1997-1998. Mon. Wea. Rev., 128, 3589-3604.
Harshvardhan, R. Davies, D. A. Randall and T. G. Corsetti, 1987: A fast
radiation parameterization for atmospheric circulation models. J. Geophys.,
Res., 92, 1009-1016.
Harshvardhan, D. A. Randall, T. G. Corsetti, and D. A. Dazlich, 1989: Earth
radiation budget and cloudiness simulations with a general circulation model.
J. Atmos. Sci., 46, 1922-1942.
Kim, Y. -J., 1996: Representation of subgrid-scale orographic effects in a
general circulation model: Part I. Impact on the dynamics of simulated January
climate. J. Climate, 9, 2698-2717.
Kim, Y.-J., J. D. Farrara and C. R. Mechoso, 1998: Sensitivity of AGCM
simulations to modifications in the ozone distribution and refinements in
selected physical parameterizations. J. Meteor. Soc. Japan, 76, No. 5, 695-709.
Kim, J., N. Miller, J. D. Farrara and S. Hong, 2000: A numerical study of
precipitation and streamflow in the western United States during the 1997-98
winter season. J. Hydrometeor., 1, 311-329.
Kohler, M., 1999: Explicit prediction of ice clouds in general circulation
models. Ph.D. Dissertation, Department of Atmospheric Sciences, University of
California, Los Angeles, 167 pp.
Li, J.-L., A. Arakawa and C. R. Mechoso, 1999: Improved simulation of PBL moist
processes with the UCLA GCM. Proceedings, Seventh Conference on Climate
Variations, 2-7 February 1999, Long Beach, CA, Amer. Meteor. Soc., 423-426.
Mechoso, C. R., A. Kitoh, S. Moorthi and A. Arakawa, 1987: Numerical
simulations of the atmospheric response to a sea surface temperature anomaly
over the equatorial eastern Pacific Ocean. Mon. Wea. Rev., 115, 2936-2956.
Mechoso, C. R., S. W. Lyons and J. A. Spahr, 1990: The impact of sea surface
temperature anomalies on the rainfall over northeast Brazil. J. Climate, 3,
Li, J.-L., M. Köhler, J. D. Farrara and C. R. Mechoso, 2002: The impact of
stratocumulus cloud radiative properties on surface heat fluxes simulated with
a general circulation model. Mon. Wea. Rev., 130, 1433-1441.
Mechoso, C. R., L. A. Drummond, J. D. Farrara and J. A. Spahr, 1998: The UCLA
AGCM in high performance computing environments. Proceedings of SuperComputing
98, November, 1998, Orlando, Florida, IEEE Society.
Mechoso, C. R., J.-Y. Yu and A. Arakawa, 2000: A coupled GCM pilgrimage: From
climate catastrophe to ENSO simulations. General circulation model development:
Past, present and future. Proceedings of a Symposium in Honor of Professor Akio
Arakawa, D. A. Randall, Ed., Academic Press, pp. 539-575.
Pan, D.-M., and D. A. Randall, 1998: A cumulus parameterization with a
prognostic closure. Quart. J. Roy. Meteor. Soc., 124, 949-981.
Suarez, M. J., A. Arakawa and D. A. Randall, 1983: The parameterization of the
planetary boundary layer in the UCLA general circulation model: Formulation and
results. Mon. Wea. Rev., 111, 2224-2243.
Creation and Review Dates