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GEOPHYSICAL RESEARCH LETTERS,
VOL. 26, NO. 8,
PAGES 1181–1184,
1999
Homogeneous Freezing of Evaporating Cloud Droplets
Raymond A. Shaw
Department of Meteorology, Penn State University, University Park, Pennsylvania
Dennis Lamb
Department of Meteorology, Penn State University, University Park, Pennsylvania
Abstract
Freezing of evaporating cloud droplets by homogeneous nucleation is an important mechanism for ice formation in the atmosphere,
but it has been studied very little. Extension of the standard theory of homogeneous freezing to evaporating conditions results
in an analytical expression for the freezing probability. A dimensionless freezing number Fh characterizes the process and is found to be proportional to the fifth power of the initial drop radius. For any finite value
of Fh , the derived expression predicts that only a fraction of any uniform drop population will freeze, regardless of the time
available. New laboratory measurements of individual droplets levitated electrodynamicaly confirm the essential features of
the expression and yield nucleation rates that are consistent with those determined by others.
Received 28
December
1998;
accepted 22
February
1999.
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Citation: Shaw, R. A., and D. Lamb
(1999),
Homogeneous Freezing of Evaporating Cloud Droplets,
Geophys. Res. Lett.,
26(8),
1181–1184.
Copyright 1999 by the American Geophysical Union.
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