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It is recognized that radiative heating, caused by atmospheric dust,
is one of the most important factors in determining the thermal and
circulation structure of the Martian atmosphere. Vertical temperature
profiles provided by spacecrafts are more frequently observed to be at
a stable lapse rate than at a dry adiabatic lapse rate
(e.g., Lindal et al.,
1979). The stable profiles are considered to be caused by
radiative heating associated with dust, which is demonstrated by
one-dimensional (1D) radiative convective models
(Gierasch and Goody,
1972, Pollack et al.,
1979). Dust is also considered to intensify the magnitude of Mars'
large-scale atmospheric circulation, which is suggested by a
comparison between simulation results of dusty conditions and clear
sky (i.e., dust-free) conditions obtained by general circulation
models (GCMs) (e.g., Pollack et
al., 1990).
However, GCMs have not succeeded in the prognostic calculation of the
amount of dust, an important element of the Martian atmosphere. Global
dust storms, which are one of the most striking phenomena of the
Martian atmosphere (e.g., Briggs et
al., 1979), have yet to be self consistently simulated in
current GCMs. With a dusty atmosphere as an initial condition, the
GCMs produce intense large-scale wind to inject dust into the
atmosphere in order to maintain the amount of dust in the
atmosphere. However, when using dust-free or small amounts of dust as
an initial condition for the atmosphere, the intensity of the
large-scale winds calculated in the GCMs is so weak that the model
surface stress is not large enough to raise dust from the surface
layer. Current GCMs are unable to describe the spontaneous transition
from a dust-free Mars to a dusty Mars
(Joshi et al., 1997;
Wilson and Hamilton, 1996).
It is suggested by Wilson and Hamilton (1996)
that the amount of surface stress that is necessary to
inject dust into the atmosphere could be supplemented by small-scale
wind fluctuations, which are not resolved in GCMs. However, they
presented neither the nature nor the origin of small-scale wind
motions.
Vertical convection that is driven by radiative forcing and sensible
heat from the surface is one "small-scale wind fluctuation"
that is not represented in GCMs. In particular, it was determined that
data obtained at the Viking Lander 1 site provide evidence of the
existence of vertical convection
(Hess et al.,
1977; Ryan and Lucich, 1983).
According to studies that utilized 1D radiative convective
models, the depth of the convection layer is estimated to be
approximately 9 to 10 km during dust-free conditions
(Flasar and Goody, 1976;
Pollack et al., 1979)
and about 3 to 4 km during dusty conditions when the visible
optical depth of dust is from 0.3 to 0.5
(Savijärvi, 1991b;
Haberle et al., 1993).
However, since the fluid dynamical aspects of vertical convection in
the Martian atmosphere have yet to be intensely examined, the
fundamental characteristics, such as cell patterns and wind intensity
associated with vertical convection, are not well known.
In general, vertical convection in the Martian atmosphere can be
regarded as dry convection.
Since the amount of water vapor is so quite small that condensation
heating of water vapor can be neglected when compared to the amount of
radiative heating in the Martian atmosphere.
(e.g., Zurek et al., 1992).
Excluding the polar regions, condensation of CO2, a major component of
the Martian atmosphere, can also be neglected. Dry convection also
occurs in the terrestrial atmosphere in the planetary boundary layer
near the surface. However, dry convection occurs throughout the entire
Martian troposphere. The characteristics of such "deep" dry convection
have not been intensively examined.
In this study, we constructed a numerical model that can explicitly
represent convective fluid motion, and investigated the possible
characteristics of vertical convection in the Martian atmosphere. We
performed the following two cases of numerical simulations:
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Simulation of vertical convection without dust (dust-free
case). Focus is placed on the circulation patterns and intensity of
wind, and the amount of surface friction due to wind. By using the
calculated values of surface friction, we discuss the possibility of
dust injection from the surface, which has not been realized in GCMs
under dust-free conditions.
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Simulation of vertical convection with dust (dusty case). Assuming
that convective wind injects dust into the atmosphere, we investigate
the characteristics of dust mixing by thermal convection and the
effects of radiative heating due to dust on the circulation structure
of vertical convection.
The numerical model utilized here is spatially two-dimensional
(2D). The advantage of restricting the computational domain to a 2D
space is the ability to employ a computational domain and a
sufficiently high spatial resolution that resolves convective plumes
explicitly. Moreover, it is expected that some characteristic features
of convection, if any, will be more easily recognized by a 2D model
than a three-dimensional (3D) model.
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