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The thermal and circulation structure of Martian atmosphere are
affected by atmospheric dust.
The vertical gradient of atmospheric temperature observed by
spacecraft is frequently smaller than adiabatic lapse rate (c.f. Lindal et al. 1979);
Numerical studeies with one-dimensonal (1D) radiative convective
model show that it is caused by radiative heating associated with
dust. (Gierasch and Goody, 1972;
Pollack et al., 1979).
Numerical simualtions of General Circulation Model (GCM) under dusty
condithon show that the magnitude of zonal mean meridional
circulation is significantly larger than that under clear sky (ie.,
dust-free) condition (c.f. Pollack
et al., 1990)
However, the global dust storm which is most striking phenomenon in
the Martian atmosphere can not be simulated by use of current GCM
self consistently.
In the case of dust-free or small amount of dust, the surface stress
calculated by large scale wind in GCM is not sufficient to raise
dust from the surface (Joshi et
al., 1997; Wilson and
Hamilton, 1996).
Therefore, the transition from dust-free Mars to dusty Mars can not be
simulated by use of GCM naturally.
Wilson and Hamilton (1996) suggest
the surface stress could be supplemented by small-scale wind
fluctuations which are not represented in GCM contribute to dust
injection into the atmosphere, but neither the nature nor the
origin of small-scale wind motions have been investigated yet.
Thermal convection driven by radiatve forcing and sensible heat from
the surface is one of the small-scale motions which are not
represented in GCM.
The wind fluctuation associated with thermal convection is observed
at the site of Viking Lander 1 (Hess et al., 1977; Ryan and Lucich, 1983).
The depth of convection layer has been calculated by 1D model;
it is about 8 km under dust-free condition (Flasar and Goody, 1976; Pollack et al., 1979) and
is about from 3 to 4 km under dusty condition where visible optical
depth of dust is 0.3 (Savijärvi,
1991b; Haberle et al.,
1993).
However, the circulation structure of convection and the magunitude of
convective wind are not revealed, since the thermal convection in
Martian lower atmosphere has no been examined well.
The Martian atmospheric convection is generally dry convection.
The condensation of CO2 which is a major component of the
Martian atmosphere does not occur except in the poler region.
The magnitude of condensation heating of water vapor is much smaller
than that of atmospheric radiative heating in the Martian
atmosphere (Zurel et al., 1992)
, then the effect of condensation of water vapor on the
atmospheric circulation is negligible.
Dry convection also occurs in the planetary boundary layer near the
surface of the Earth.
However, the depth of dry convection in the Martian atmosphere is
larger than that of the terrestrial atmosphere, since dry
convection occurs in the whole region of Martian troposphere.
The characteristics of deep dry convection has not been examined.
In this study, we perform a numerical simulation of Martian
atmospheric convection driven by radiative forcing with a
two-dimensional (2D) anelastic model. Two cases of numerical
simulations are performed;
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Dust free case: we investigate the circulation structure
of thermal convection, the magnitudes of convective wind
velocity and surface stress, and whether the calculated surface
stress is sufficient to raise dust from the surface. It is
difficult to simulate by using GCM.
-
Dusty case: we assume that the convective wind injects dust into
the atmosphere. We investigate a feature of dust mixing by thermal
convection and effects of dust on the circulation structure of
thermal convection.
Advantages of using a 2D model are that we can obtain both large
computational domain and sufficient spatial resolution which can
resolve the convective plume explicitly.
Moreover, we will be able to reveal characteristics of the convective
structure easier than by using a three-dimensional (3D) model.
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