Manabe, S., J. Smagorinsky, J. L. Holloway, Jr., and H. M. Stone, 1970: Simulated climatology of a general circulation model with a hydrologic cycle. III.
Effects of increased horizontal computational resolution. Monthly Weather Review, 98 (3), 175-212.
Abstract: The results of a numerical time integration of a hemispheric general circulation
model of the atmosphere with moist processes and a uniform earth's surface
has already been published by Manabe, Smagorinsky, and Strickler. In this
study, the integration is repeated after halving the midlatitude grid size
from approximately 500 to 250 km.
This increase in the resolution of the horizontal finite differences markedly
improves the features of the model atmosphere. For example, the system of
fronts and the associated cyclone families in the high resolution atmosphere
is much more realistic than that in the low resolution atmosphere. Furthermore,
the general magnitude and the spectral distribution of eddy kinetic energy
are in better agreement with the actual atmosphere as a result of the improvement
in resolution.
In order to explain these improvements, an extensive analysis of the energetics
of both the low and high resolution atmospheric models is carried out. It
is shown that these improvements are due not only to the increase of the
accuracy of the finite differences but also to the shift in the scale of
dissipation by the nonlinear lateral viscosity toward a smaller scale resulting
from the decrease in grid size. In the low resolution atmospheric model,
the transfer of energy from eddy to zonal kinetic energy is missing because
of excessive subgrid scale dissipation at medium wave numbers, whereas it
has significant magnitude in the high resolution atmospheric model. It is
speculated that futher increase of resolution should improve the results
because it tends to separate the characteristic scale of dissipation from
that of the source of eddy kinetic energy.
The analysis of the energetics in wave number space clearly demonstrates
the differences between the energetics of the different parts of the atmosphere.
In middle latitudes there are essential differences between the energetics
of the model troposphere and that of the model stratosphere. In the model
troposphere, the eddy kinetic energy is produced by the conversion of eddy
potential energy in the range of wave numbers from 2 to 8. Part of the energy
thus produced is dissipated by the subgrid scale dissipation, and most of
the remainder is decascaded to zonal kinetic energy. In the model stratosphere,
where very long waves predominate, the eddy kinetic energy is generated
in the range of wave numbers from 2 to 3 by the energy supplied from the
troposphere. Most of this energy is then decascaded barotropically to zonal
kinetic energy.
In the Tropics, eddy kinetic energy is mainly produced by the release of
eddy available potential energy generated by the heat of condensation. Although
the rate of conversion is maximum at very low wave numbers, the conversion
spectrum extends to very high wave numbers.
A box diagram of the energetics of the high resolution moist model shows
that the eddy available potential energy is generated by the heat of condensation
as well as by energy transfer from the zonal available potential energy.
Furthermore, it is noteworthy that the zonal kinetic energy is maintained
not only by the barotropic exchange from the eddy kinetic energy but also
from the conversions of zonal potential energy. The intensification of the
direct tropical cell and the weakening of the indirect Ferrel cell in the
middle latitudes caused by the moist processes are responsible for ths positive
zonal conversion.
One of the highlights of the results from the integration of the high resolution
moist model is the successful simulation of the evolution of fronts and
the associated cyclone families. The influence of moist processes upon frontal
structure as well as other synoptic features is investigated by comparing
the moist model atmosphere with the dry model atmosphere without the effect
of the selective heating of condensation. It is found that the heat of condensation
significantly reduces the width fronts and the characteristic scale of cyclones
in the lower troposphere.