3. Results: dust-free case

3.b. Circulation structure of convection Figure 4 shows the circulation structure of thermal convection in dust-free case. It reveals 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 several km, respectively. The aspect ratio of convective cell is about 2 to 1 which is estimated by using the depth of convection layer and horizontal interval of ascending convective plumes. The potential temperature deviation of convective plume is 1 to 2 K in the morning and 2 to 3 K in the afternoon. The width of ascending convective plume is about several hundreds meter to 1 km. In the stratosphere, the periodic pattern of potential temperature deviation is observed. It is caused by internal gravity wave which is generated by penetration of convective plume into the stratosphere. The large value of turbulent diffusion coefficient in the stratosphere suggests that the gravity wave breaking which generates unstable stratification occurs. The intensity of updraft is almost equal to that of downdraft. The values of both horizontal and vertical wind velocity often exceed 20 msec-1. The width of asending convective plume is smaller than that of updraft region. The existence of downdraft with positive potential temperature deviation shows that the convective plume with positive potential temperature deviation which goes up to the stratosphere is pushed aside and forced to descend by the following convective plume from the surface. Some fragments of the compulsorily descending plume are accompanied with vortex circulation structure. It resluts in efficient mixing in the convection layer. The wind velocity () associated with the km-size convection can be estimated by work done by ascending convective plume as follows. Where is gravitational acceleration, is horizontal mean potential temperature, is potential temperature deviation from , and is depth of convection layer.

Figure 4: Time development of convective fields from LT = 10:00 to 18:00 every 1 hour in dust-free case: potential temperature deviation form horizontal mean value (upper left), turbulent diffusion coefficient (lower left), vertical wind (upper right) and horizontal wind (lower right). The areas where the value of turbulent diffusion coefficient is lager than 1.0×10^-5 m² sec^-1 are colored in the lower left panel. The contour intervals are 5 msec^-1 for wind velocity. The results of output time interval on every 2 minutes are shown in Appendix D.

A numerical simulation of thermal convection in the Martian lower atmosphere.