I would agree with most of your comments concerning descending air from
orographic effects. However, for convection, and especially deep convection,
much of air that ascends within the cloud descends over a much larger
volume, and mostly sinks rather slowly, if at all. Much of the cloud
material that was once the active cloud spreads out quasi-horizontally near
the cloud top, while a good deal is also mixed into the close environment of
where the cloud had been. Unless it is dynamically driven, it will tend to
soon spread out rather than descend very far, as it will mostly find itself
warmer than its environment due to adibatic heating, inhibiting descent.
Descent in and around deep convection is generated mainly by evaporative
cooling, as precipitation falls into dry air beneath, or where drier air is
entrained into the precipitation from the cloud's surroundings. This cooled
air leads to acceleration downwards, and can produce very strong surface
gusts when it is constrained to flow horizontally at the surface. It is
also the reason for the well documented 'downburst' phenomenon, that has led
to several aircraft accidents. It should also be noted that precipitation
itself drags air down as it falls, contributing to any acceleration due to
evaporation.
On occasions, air accelerated downwards by evaporative cooling can continue
to descend for a while after all the precipitation has evaporated, and
increasing temperature at the dry lapse rate may lead to it becoming warmer
than its environment. Then its arrival at the surface can lead to a rise in
temperature there, as the deceleration of the descending air takes time, the
restoring force not being very strong.




A very similar effect can occur near showers and storms. The air
spilling out near the tops of clouds heats at the dry adiabatic as it
descends. It kills the thermals (big clear holes on satpics) and can
on occasions manifest itself as a sudden rapid rise in surface
temperature. I have seen documented examples (Jersey rings a bell)
when the surface temperature rose several degrees in a matter of
minutes.

Jack












--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
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