3. Results: dust-free case
 3.c. Surface stress (2)

Surface stress associated with the km-size thermal convection reaches the threshold value required to raise dust from the surface (Figure 5). It is expected that, if convection appears in the presence of a large scale background wind, superposition of convective and background winds enables the surface stress to exceed the threshold value more easily. In this section, we will examine a possible value of surface stress which may be achieved when the km-size thermal convection and a large scale background wind coexist. Since an existence of a background wind affects the convection field, a simple superposition of winds needs some care in the application to a reality. However, it may be useful in recognizing a plausible amount of possible surface stress.

An estimate of the large scale background wind velocity can be obtained from a result of a numerical simulation by using a GCM. In the GCM simulation of a dust-free Martian atmosphere by Joshi et al. (1997), the daytime horizontal wind at z= 250 m is about 26 msec-1, and the corresponding value of surface stress is 0.015 Pa. From these values, the large scale wind speed which is expected at the lowest level of our model, i.e., at the height of 1.5 m, can be estimated by the bulk formula. Assuming that the atmosphere is in a neutral stratification, the wind speed at the height of 1.5 m is given as

.

Here, k is the Karman constant and ρ is the atmospheric density. In the above estimation, we have adopted k = 0.35, and ρ = 1.5×10 -2kgm-3 which is calculated from the surface tepmperature in daytime.

Figure 6 (upper panel) shows the magnitude of surface stress that would be realized when 10 msec-1 background horizontal wind is superposed on the wind associated with the km-size convection in the direction parallel to it. It is shown that the value of surface stress frequently exceeds the threshold value required to raise dust. However, in the presence of a background wind, there is a tendency that the axis of convection role is located in the direction parallel to the background wind (Asai, 1970). It may be more plausible that the convective wind is in the direction perpendicular to the background wind. In this case, the magnitude of surface stress (Figure 6 (lower panel)) is reduced compared to the case of parallel superposition. However, there still appear the occasions when the value of surface stress exceeds the threshold value.

 Figure 6: Horizontal distributions of the magnitudes (absolute values) of surface stress from LT = 13:00 to 16:10. (Upper panel) a background wind is superposed in the direction parallel to the convective wind. (Lower panel) a background wind is superposed in the direction perpendicular to the convective wind. Green line indicates the superposed surface stress, while blue line indicates the results of model output only. Orange and red lines show the minimum and maximum values of the threshold surface stress required to raise dust from the surface (Greeley and Iversen, 1985).

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