|
5.a. Comparison with observation resluts
|
The VL-1 and VL-2 observed the horizontal wind velocity and
atmospheric temperature at 1.6 m hight on each landing site.
The lowest level of our 2D numerical model is about 1.5 m hight and it
is nearly equal to that of meteorological observation point at the
site of Viking Lander.
Therefore, we can directly compare the horizontal wind velocity and
temperature at the lowest level of our 2D numerical model with
those observed at the site of Viking Lander which is associated
with thermal convection (Hess et
al., 1977).
Figure 15a shows the time series of the horizontal
wind speed, wind direction and surface temperature observed at the
site of VL-1 in the late afternoon of sol 22. (Ls
˜ 110°).
At that time, the atmospheric visible dust opacity is about 0.4 (Pollack et al., 1979), which
is intermediate value between that in the dust-free case and the
dusty case of our numerical simulation.
In the time series of wind velocity, the two components of wind
fluctuations which have different amplitude and period each other
can be recognized.
One is the low freqency component whose amplitude and time period is
about 5 msec-1 and from several to
ten-several minutes respectively, and the other is hight frequency
component whose amplitude and time period is about 3
msec-1 and 1 ˜ 2 minutes.
The two components of temperature fluctuation whose time periods are
same as those observed in the time series of wind veocity are also
observed in Figure 15a.
The amplitude of temperature fluctuation is about 3 K.
The low frequency wind and temperature fluctuation in Figure 15a are similar to those simulated
by our 2D numerical model under clear sky condition (Figure 15b).
Therefore, the low frequency fluctuations observed in Figure 15a can be consider as those
associated with the km-size thermal convection.
On the other hands, the hight frequency component in Figure 15a may be associated with the
subgrid scale thermal or forced turbulence in conduction or
transition layer which is parameterized in our 2D numerical model,
because the hight frequency fluctuations can no be represented in
Figure 15b.
These results suggest that realistic wind velocity and temperature
fluctuation associated with the thermal convection in the Martian
lower atmosphere driven by radiative forcing can be simulated by
our 2D numerical model except for the small scale turbulence.
In dusty case of our numerical simulation, niether low and high
frequency fluctuation which are similer to those observed in Figure 15a is represented (Figure 15c).
This is because the difference of atmospheric dust opacity.
The value of visible dust opacity in dusty case of our numerical
simulation is larger than that observed on sol 22 at the site of
VL-1.
The thermal convection associated with the wind and temperature
fluctuation observed in Figure 15a may
be surpressed by relatively large atmospheric stability owing to
radiative heating associated with dust in our numerical simulation.
In reality, the activity of km-size thermal convection in dusty case
simulation almost terminates at around LT = 16:00 (Figure 13).
|
|
Figure 15a:
Time series of horizontal wind velocity, wind direction and
atmospheric temperature in the late afternoon unstable convective case
from VL-1 sol 22 (Hess et
al., 1977, Figure 9).
|
|
|
Figure 15b:
Time series of horizontal wind velocity and atmospheric temperature
at the lowest level of 2D numerical model from LT = 16:00 to 17:00
in dust-free case. The sampling time interval is 30 seconds.
|
|
|
Figure 15c:
The same as Figure 15b but for 6th days in
dusty case.
Note that the range of vertical axis for temperature is different from
that in Figure 15a and Figure 15b.
|
|
