5. Discussions up previous next
5.b. Treatment of Convection in a GCM

Our simulation results suggest that GCMs could simulate dust injection into the atmosphere from a dust-free initial condition if a small-scale wind fluctuation associated with kilometer-size convection is properly incorporated. Current GCMs for the Martian atmosphere employ a convective adjustment scheme to represent the effects of unresolvable thermal convection. The convective adjustment scheme adjusts a convectively unstable temperature gradient to a neutral state as the result of convective heat transport. This convective adjustment scheme, and any other parameterization scheme, does not properly represent wind fluctuations associated with kilometer-size convection and their contribution to surface stress.

Since the structures of vertical convection in the Martian atmosphere are not well investigated, we had almost no choice other than to adopt a convective adjustment scheme for the Martian atmospheric GCMs. Cumulus convection, which is a dominant process for thermal transport from the surface to the troposphere in Earth's atmosphere, is similar to the kilometer-size convection of the Martian atmosphere. As for terrestrial GCMs, various types of cumulus convective parameterization schemes have been developed in order to produce a proper representation of cumulus convection. A simple scheme is convective adjustment, and more complex schemes incorporate the distribution of vertical size, entrainment into cumulus clouds, and so on (e.g., Arakawa and Schubert, 1974). Cumulus convective parameterization is possible due to the accumulation of real images of cumulus convection and by breakthroughs of theories. However, what is emphasized in these cumulus parameterizations is not wind fluctuation but vertical thermal transport. A convective parameterization scheme that incorporates wind fluctuations has not been considered in the development of terrestrial GCMs.

The present study reveals several features of vertical convection in the Martian atmosphere; that is, kilometer-size convection. Ground surface stress in GCMs is calculated from the intensity of winds, which is explicitly represented in a model parameterized by a formulation called the bulk formula. In the numerical model used in the present study, ground surface stress is also calculated via the bulk formula. Nevertheless, the magnitude of surface stress calculated in this study is larger than that of GCMs. This is caused by an explicit calculation of kilometer-size convection.

As is observed in (Figure 5 and Figure 6), the horizontal mean surface stress calculated in the present study is not very large. However, the local values of surface stress are much larger than the horizontal mean values due to large wind fluctuations associated with the kilometer-size convection. The values of surface stress that are represented in the parameterizations of GCMs, even if the contribution of subgrid-scale turbulence and kilometer-size convection is incorporated, are averaged for a wide horizontal area of the GCM grid size. These values correspond to the horizontal mean stress obtained in the present study, and consequently, should be smaller than maximum values that appear locally.

Based on the features of the kilometer-size convection revealed by the present numerical study, a new convective parameterization scheme can be developed that can estimate wind fluctuation and the maximum value of surface stress associated with convection. The scheme would basically consist of a convective adjustment scheme modified with the addition of the kinetic energy calculations scheme. By using the potential temperature fluctuation and depth of the convective layer , convective kinetic energy is evaluated in the form of Equation (1). Assuming the turbulent diffusion coefficient near the surface, for instance, is the value of the lowest level of the GCM, or given simply as $K$ = 15 m2 sec-1, can be evaluated by heat flux Q which can be obtained from GCM output as in Section 3.d. The magnitude of wind fluctuations that are associated with kilometer-size convection can be estimated by such values. If wind fluctuation magnitude is estimated in this manner and incorporated into the calculation of surface stress in the surface flux parameterization, we expect dust will be spontaneously lifted from the surface of GCM simulations.


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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