Thursday, 6 June 2002
Drop Equilibrium in the Presence of Solar Absorption
Discussions of the radiative influences on the growth of drops
within stratus clouds have primarily focused on the effects of longwave
radiation. The impacts of solar absorption, though potentially
important, have been largely
ignored. In particular, solar absorption modifies the growth rate
of cloud drops by effectively raising the supersaturation needed
for drops to grow from the vapor. The magnitude and sign of this
modification is dependent upon the strength of the infrared emission
and solar absorption and how these quantities vary with drop size.
Hence, in this work we examine the strength of the competing influences
of infrared and solar radiative fluxes in the vicinity of cloud top.
Simple, 1-D stratus clouds are created and used in conjunction
with a radiative transfer model that accounts for cloud optical
properties. As one might expect, solar absorption reduces the radiative
cooling that small drops experience. However, we show that drops
with radii as small as 40 microns can experience a net heating effect.
This raises the supersaturation needed for a drop to experience
a zero net vapor growth rate. In some cases, the supersaturation
necessary becomes as larger as a few percent. The potential consequences of
this result are also explored.
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