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Constraints on droplet growth in radiatively cooled
stratocumulus clouds
Philip H. Austin, Steven Siems and Yinong Wang
Journal of the Geophysical Research, 1 July 1995, 100, No. D7, pages 14231-14243.
Abstract
Radiative cooling near the top of a layer cloud plays a dominant role
in droplet condensation growth. The impact of this cooling on the
evolution of small droplets and the formation of precipitation-sized
drops is calculated using a microphysical model that includes
radiatively-driven condensation and coalescence. The cloud top
radiative environment used for these calculations is determined using
a mixed-layer model of a marine stratocumulus cloud with a subsiding,
radiatively cooled inversion.
Calculations of the radiatively-driven equilibrium supersaturation
show that net long-wave emission by cloud droplets produces
supersaturations below 0.04\% for typical nocturnal conditions.
While supersaturations as low as this will force evaporation for
droplets smaller than $\approx$ 5 $\mu m$, radiatively enhanced growth
for larger droplets can reduce the time required to produce
precipitation-sized particles by a factor of 2-4, compared with
droplets in a quiescent cloud without flux divergence. The impact of
this radiative enhancement on the acceleration of coalescence is
equivalent to that produced in updrafts of 0.1 - 0.5 $m\,s^{-1}$, and
varies linearly with the total emitted flux (the ``radiative
exchange'').
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