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In order to visualize the actual vertical convection in the lower
Martian atmosphere, a direct numerical simulation of thermal
convection in the Martian lower atmosphere was performed by using a 2D
anelastic model. The results of the simulations reveal that the
thermal convection in the Martian lower atmosphere is kilometer in
size.
In the dust-free case, convective cells have maximum vertical and
horizontal scales of 10 km and 5 km, respectively. The values of both
horizontal and vertical wind velocity are approximately 20 m
sec-1. These values can be estimated via the work done by
an ascending convective plume.
The instantaneous maximum value of surface stress associated with
kilometer-size thermal convection in the dust-free case is 0.03 to
0.04 Pa, which is equal to the threshold value (obtained
experimentally) to raise dust from the surface. This result indicates
that the Martian GCMs, which have not been able to inject dust into
the atmosphere, are now expected to consistently simulate dust
injection and the occurrence of global dust storms by parameterizing
the contribution of surface stress associated with kilometer-size
thermal convection.
When dust is injected into the atmosphere, dust promptly spreads into
the convective layer and is well mixed within a few hours. The
circulation structure of convection is similar to that of the
dust-free case. This suggests that the horizontal contrast of
radiative heating, which is due to the horizontal contrast of dust
distribution between the updraft and downdraft regions, seems to have
no effect on convection. Dust can be considered as a passive tracer
when dust injection occurs.
After dust is well mixed, depth of the convection layer
decreases. This is caused by an increase in stratospheric temperature
that is due to the absorption of solar radiation by dust. The aspect
ratio of a convective cell is similar to that observed in the
dust-free case. The intensity of convective wind becomes smaller than
that of the dust-free case. This is because the depth of the
convection layer and the potential temperature deviation of the
convective plume become smaller.
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