The primary topic of today's discussion is outflow boundaries from
trade wind cumulus clouds. Shown in Fig. 1 are three (labeled here A
through C) very distinct cloudy areas embedded in the northeastern
Atlantic trade winds. As these areas convect they send out very
distinct outflow boundaries which travel along in the flow as shown
in the loop below. In the tropics, these boundaries are often
associated with very dry conditions aloft. As convection proceeds
upward it encounters the very dry middle levels, evaporates and
becomes much colder than its environment. This negatively-buoyant
air, now part of the downdraft, accelerates downward, spreading out
when it encounters the ground. This is also the case in this situation,
as there is a very distinct trade wind inversion present as shown in
the Barbados 12Z sounding (Fig.2). The tops (determined from Channel
4 using the MCIDAS command MINMAX) of these convective systems have
temperatures near 10 degrees Celsius, which according to the Barbados'
sounding is approximately at the lowest portion of the inversion.
In addition to the above discussion, there is evidence of the
propagation of gravity waves in this set of imagery. These waves are
most easily seen in cloudy area C when the loop is run very rapidly.
These waves cause the fluctuation of cloudiness as they propagate from
northwest to southeast through the imagery. From the sounding
temperature profile, the cloud top temperature of these cloudy areas,
and fluctuation of cloudiness associated with these waves, it can be
concluded that the gravity waves are propagating along the trade wind
inversion.
Click
here
for a visible loop.
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