1. Introduciton index previous next
1. Introduction

It has been recognized that radiative heating caused by atmospheric dust is one of the most important factors in determining the thermal and circulation structure of the Martian atmosphere. The vertical profiles of temperature observed by spacecrafts frequently give stable lapse rate compared to dry adiabat (e.g., Lindal et al., 1979). Those stable profiles are considered to be caused by radiative heating associated with dust as is demonstrated by one-dimensional (1D) radiative convective models (Gierasch and Goody, 1972; Pollack et al., 1979). Dust is also considered to intensify the magnitude of the Martian large scale atmospheric circulation, as is suggested by the comparison between the simulation results under dusty and clear sky (i.e., dust-free) conditions obtained with general circulation models (GCMs) (e.g., Pollack et al., 1990).

However, GCMs have not succeeded in the prognostic calculation of the amount of dust, the important element of the Martian atmosphere. The global dust storm, which is one of the most striking phenomena in the Martian atmosphere (e.g., Briggs et al., 1979), has not been self consistently simulated in the current GCMs. Starting from a dusty atmosphere as the initial condition, the GCMs produce intense large scale wind to inject dust into the atmosphere to maintain the dust amount in the atmosphere. However, starting from a dust-free or a small amount of dust atmosphere, the intensity of the large scale winds calculated in the GCMs is so weak that the model surface stress is not large enough to raise dust from the surface. Current GCMs can not describe a spontaneous transition from the dust-free Mars to the dusty Mars (Joshi et al., 1997; Wilson and Hamilton, 1996). It is argued by Wilson and Hamilton (1996) that the amount of surface stress which is necessary for dust injection into the atmosphere could be supplemented by small-scale wind fluctuations which are not resolved in GCMs. However, they presented neither the nature nor the origin of small-scale wind motions.

As a candidate of the small-scale motions which are not represented in GCM, one can think of vertical convection driven by radiative forcing and sensible heat from the surface. Actually, it has been pointed out that the data obtained at the site of Viking Lander 1 demonstrates the existence of vertical convection (Hess et al., 1977; Ryan and Lucich, 1983). According to the studies by the use of 1D radiative convective models, the depth of convection layer is estimated to be about from 9 to 10 km under the dust-free condition ( Flasar and Goody, 1976; Pollack et al., 1979), and is about from 3 to 4 km under the dusty condition when the visible optical depth of dust is from 0.3 to 0.5 (Savijärvi, 1991b; Haberle et al., 1993). However, since the fluid dynamical aspects of vertical convection in the Martian atmosphere has not been intensely examined so far, the fundamental characteristics such as cell patterns and wind intensity associated with vertical convection have not been well revealed.

Vertical convection in the Martian atmosphere can be regarded as dry convection in general. Since the amount of water vapor is quite small, condensation heating of water vapor can be neglected compared to radiative heating in the Martian atmosphere (e.g., Zurek et al., 1992). Condensation of CO2, the major component of the Martian atmosphere, does not have to be considered except in the poler region. Dry convection occurs also in the terrestrial atmosphere in the planetary boundary layer near the surface. However, dry convection in the Martian atmosphere covers the whole height of Martian troposphere. The characteristics of such "deep" dry convection has been little examined so far.

In this study, we construct a numerical model which can explicitly represent convective fluid motion, and investigate possible characteristics of the vertical convection in the Martian atmosphere. We perform the following two cases of numerical simulations;

  • Simulation of vertical convection without dust (the dust-free case): Focus is placed on the patterns of circulation and intensity of wind, and the amount of surface friction due to the wind. By using the calculated values of surface friction, we discuss the possibility of dust injection from the surface, which has not been realized in GCMs under the dust free condition.
  • Simulation of vertical convection without dust (the dusty case): Assuming that the convective wind injects dust into the atmosphere, we investigate the characteristics of dust mixing by thermal convection and the effects of radiative heating due to dust on the circulation structure of vertical convection.

The numerical model utilized here is spatially two-dimensional (2D). The advantage of restricting the computational domain to two-dimensional space is that we can employ both a large computational domain and a sufficiently high spatial resolution that resolves convective plumes explicitly. Moreover, by using a 2D model, it is expected to be easier to recognize some characteristic features of convection, if any, than by using a three-dimensional (3D) model.


A numerical simulation of thermal convection in the Martian lower atmosphere.
Odaka, Nakajima, Ishiwatari, Hayashi,   Nagare Multimedia 2001
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