1. Introduction index previous next
1. Introduction

It is 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. Vertical temperature profiles provided by spacecrafts are more frequently observed to be at a stable lapse rate than at a dry adiabatic lapse rate (e.g., Lindal et al., 1979). The stable profiles are considered to be caused by radiative heating associated with dust, which 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 Mars' large-scale atmospheric circulation, which is suggested by a comparison between simulation results of dusty conditions and clear sky (i.e., dust-free) conditions obtained by general circulation models (GCMs) (e.g., Pollack et al., 1990).

However, GCMs have not succeeded in the prognostic calculation of the amount of dust, an important element of the Martian atmosphere. Global dust storms, which are one of the most striking phenomena of the Martian atmosphere (e.g., Briggs et al., 1979), have yet to be self consistently simulated in current GCMs. With a dusty atmosphere as an initial condition, the GCMs produce intense large-scale wind to inject dust into the atmosphere in order to maintain the amount of dust in the atmosphere. However, when using dust-free or small amounts of dust as an initial condition for the 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 layer. Current GCMs are unable to describe the spontaneous transition from a dust-free Mars to a dusty Mars (Joshi et al., 1997; Wilson and Hamilton, 1996). It is suggested by Wilson and Hamilton (1996) that the amount of surface stress that is necessary to inject dust 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.

Vertical convection that is driven by radiative forcing and sensible heat from the surface is one "small-scale wind fluctuation" that is not represented in GCMs. In particular, it was determined that data obtained at the Viking Lander 1 site provide evidence of the existence of vertical convection (Hess et al., 1977; Ryan and Lucich, 1983). According to studies that utilized 1D radiative convective models, the depth of the convection layer is estimated to be approximately 9 to 10 km during dust-free conditions (Flasar and Goody, 1976; Pollack et al., 1979) and about 3 to 4 km during dusty conditions 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 have yet to be intensely examined, the fundamental characteristics, such as cell patterns and wind intensity associated with vertical convection, are not well known.

In general, vertical convection in the Martian atmosphere can be regarded as dry convection. Since the amount of water vapor is so quite small that condensation heating of water vapor can be neglected when compared to the amount of radiative heating in the Martian atmosphere. (e.g., Zurek et al., 1992). Excluding the polar regions, condensation of CO2, a major component of the Martian atmosphere, can also be neglected. Dry convection also occurs in the terrestrial atmosphere in the planetary boundary layer near the surface. However, dry convection occurs throughout the entire Martian troposphere. The characteristics of such "deep" dry convection have not been intensively examined.

In this study, we constructed a numerical model that can explicitly represent convective fluid motion, and investigated the possible characteristics of vertical convection in the Martian atmosphere. We performed the following two cases of numerical simulations:

  • Simulation of vertical convection without dust (dust-free case). Focus is placed on the circulation patterns and intensity of wind, and the amount of surface friction due to 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 dust-free conditions.
  • Simulation of vertical convection with dust (dusty case). Assuming that 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 a 2D space is the ability to employ a computational domain and a sufficiently high spatial resolution that resolves convective plumes explicitly. Moreover, it is expected that some characteristic features of convection, if any, will be more easily recognized by a 2D model than a three-dimensional (3D) model.


A numerical simulation of thermal convection in the Martian lower atmosphere with a two-dimensional anelastic model
Odaka, Nakajima, Ishiwatari, Hayashi,   Nagare Multimedia 2001
index previous next