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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.
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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).
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