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3.b. Circulation Structure of Convection
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Figure 4 shows the circulation fields of
thermal convection for the dust-free case (see
also Appendix D for the results of a
shorter output time interval). It is revealed that the observed
thermal convection is in the scale of kilometers; the maximum vertical
and horizontal scales of convective cells are 10 km and several km,
respectively. The aspect ratio of a convective cell estimated by the
depth of the 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 several hundreds of meters, and
nearly reach 1 km in the afternoon when convection is fully developed.
In the stratosphere, periodic patterns of potential temperature
deviation are observed. They are caused by internal gravity waves that
generate due to the penetration of convective plumes into the
stratosphere. The stratospheric turbulent diffusion coefficient shows
patterns similar to those of the potential temperature deviation. This
suggests that gravity wave breaking caused by unstable stratification
occurs in the stratosphere.
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 m sec-1. Positive
potential temperature deviation in an updraft can be seen within a
small area around the center of ascending motion. Positive potential
temperature deviation in a downdraft is an indication of a previous
plume of positive potential temperature deviation that once ascended
to the stratosphere, and is now pushed aside and forced to descend by
successive convective plumes from the surface. Some fragments of the
compulsorily descending plumes are accompanied with a vortex
circulation structure. Due to these plume motions, the convection
layer is efficiently mixed.
Magnitude of the wind velocity associated with convection is of the
order of the amount that is evaluated from the free acceleration
caused by 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 the convection layer.
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