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 by the
use of a two-dimensional anelastic model. The numerical model has the
resolution high enough to represent explicitly the lower tropospheric
convective motion and can reveal the characteristics of
Martian atmospheric convection which has not been well recognized so
far. Two cases of numerical simulations are performed; convection
without dust and convection allowing dust injection from the surface
by convective wind.
The results of the simulations reveal that the thermal convection in
the Martian lower atmosphere is km-size convection;
the maximum vertical and horizontal scales of convective
cells are 10 km and 5 km, respectively.
In the case of dust-free condition, the values of both
horizontal and vertical wind velocity often exceed 20 m/sec. The
instantaneous maximum value of the surface stress associated with the
km-size thermal convection reaches 0.04 Pa, which is equal to the
threshold value to raise dust from the surface obtained
experimentally. This result indicates that,
the Martian general circulation models (GCMs), which
have not been able to inject dust into the atmosphere,
are now expected to simulate dust injection and
the occurrence of global dust storm self-consistently
by parameterizing the surface stress contribution associated with the km-size
thermal convection.
When dust is allowed to be injected into the atmosphere, dust spreads
into the convective layer promptly and is well mixed within a few hours.
After dust reaches the stratosphere, the depth of the convection layer becomes
shallower and the intensity of convetive wind becomes smaller than those of the
dust-free case.
This is because the stratospheric temperature
increases due to the absorption of solar radiation by dust.
Horizontal contrast of radiative heating associated with the dust
distribution has negligible effect on the morphology of convection.
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