A numerical simulation of thermal convection
in the Martian lower atmosphere with a two-dimensional anelastic model.
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Masatsugu Odaka
(Graduate School of Mathematical Sciences, University of Tokyo)
Kensuke Nakajima
(Department of Earth and Planetary Sciences, Kyushu University)
Masaki Ishiwatari
(Graduate School of Environmental Earth Science, Hokkaido University)
Yoshi-Yuki Hayashi
(Division of Earth and Planetary Sciences, Hokkaido University)
Abstract
A possible circulation feature of thermal convection in the Martian
lower atmosphere driven by radiative heating is investigated via a
two-dimensional anelastic model. This numerical model is high enough
in resolution to explicitly represent lower tropospheric convective
motion and reveal the characteristics of Martian atmospheric
convection, which have yet to be determined. Two numerical simulations
were performed, which included 1) convection without dust and 2)
convection with convective wind that initiated dust injection from the
surface.
The results of the simulations reveal that kilometer-size thermal
convection occurs in the Martian lower atmosphere; convective cells
had a maximum vertical and horizontal scale of 10 km and 5 km,
respectively. For the case with dust-free conditions, horizontal and
vertical wind velocity often exceeded 20 m sec-1. The instantaneous
maximum value of surface stress associated with the kilometer-size
thermal convection was 0.04 Pa, which is equivalent to the threshold
value that is experimentally used to raise dust from the surface. This
result indicates that the Martian general circulation models (GCMs),
which have not been able to inject dust into the atmosphere, can now
be expected to consistently simulate dust injection and the occurrence
of global dust storms by parameterizing the contribution of the
surface stress associated with the kilometer-size thermal convection.
When dust is injected into the atmosphere, it rapidly spreads into the
convective layer and becomes well mixed within a few hours. After dust
reaches the stratosphere, the depth of the convection layer becomes
shallower and the intensity of convective wind becomes smaller than
that of dust-free case. This is caused by an increase in stratospheric
temperature, which is due to the absorption of solar radiation by the
dust. Horizontal contrast of radiative heating associated with the
distribution of dust has a negligible effect on the morphology of
convection.
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