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6. Summary

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.


A Numerical Simulation of Thermal Convection in the Martian Lower Atmosphere with a Two-Dimensional Anelastic Model
Odaka, Nakajima, Ishiwatari, Hayashi,   Nagare Multimedia 2001
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