3. Results: dust-free case up previous next
3.b. Circulation structure of convection

Figure 4 shows the circulation fields of thermal convection of the dust-free case (See also Appendix D for the results with a shorter output time interval). It is revealed that the obtained thermal convection is km-size; the maximum vertical and horizontal scales of convective cells are 10 km and several km, respectively. The aspect ratio of convective cell estimated by the depth of convection layer and the horizontal interval of ascending convective plumes is about 2 to 1. The magnitude of potential temperature deviation associated with convective plumes is about 1 to 2 K in the morning, and 2 to 3 K in the afternoon. The average width of ascending convective plumes is about several hundreds meters; it reaches almost 1 km in the afternoon when convection is fully developed.

In the stratosphere, periodic patterns of potential temperature deviation is observed. They are caused by internal gravity waves which are generated by the penetration of convective plumes into the stratosphere. Turbulent diffusion coefficient in the stratosphere shows patterns similar to those of potential temperature deviation. This suggests that there occurs gravity wave breaking caused by unstable stratification.

The area of an updraft is of the same order of that of a downdraft, and their intensities are also similar. Both values of horizontal and vertical wind velocities often exceed 20 msec-1. Positive potential temperature deviation in the updraft can be seen within a small area around the center of the ascending motion. Positive potential temperature deviation in the downdraft is a mark of an ex-plume of positive potential temperature deviation which once ascended to the stratosphere, and is now pushed aside and forced to descend by the successive convective plumes from the surface. Some of the fragments of the compulsorily descending plumes are accompanied with a vortex circulation structure. Owing to those plume motions, the convection layer is efficiently mixed.

The magnitude of the wind velocity associated with convection is of the order of the amount which is evaluated from the free acceleration due to the buoyancy force acting on an ascending convective plume. It can be estimated as

(1)

Where is the estimated magnitude of wind velocity, 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 of the dust-free case. Every one hour data from LT = 10:00 to 18:00 is shown. (Upper left) Potential temperature deviation form the horizontal mean. (Lower left) Turbulent diffusion coefficient. Areas with values lager than 1.0×10-5 m2 sec-1 are colored. (Upper right) Vertical wind velocity. (Lower right) Horizontal wind velocity. Contour interval of the wind panels is 5 msec-1. In Appendix D, data produced with the output time interval of 2 minutes are also shown.


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