|
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.
| 
