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Unified treatment of thermodynamic and optical
variability in a simple model of unresolved low clouds
C. A. Jeffery and P. H. Austin
Journal of the Atmospheric Sciences , 2003, 60, 1621-1631.
Abstract
Comparative studies of global climate models have long shown a marked
sensitivity to the parameterization of cloud properties. Early attempts to
quantify this sensitivity were hampered by diagnostic schemes which were in-
herently biased towards the contemporary climate. Recently, prognostic cloud
schemes based on an assumed statistical distribution of subgrid variability
have replaced the older diagnostic schemes in some models. Although the
relationship between unresolved variability and mean cloud amount is known
in principle, a corresponding relationship between ice-free low cloud thermo-
dynamic and optical properties is lacking. We present a simple, analytically
tractable statistical optical depth parameterization for boundary layer clouds
that links mean reflectivity and emissivity to the underlying distribution of
unresolved
fluctuations in model thermodynamic variables. To characterize
possible impacts of this parameterization on the radiative budget of a large
scale model, we apply it to a zonally averaged climatology, illustrating the
importance of a coupled treatment of sub-grid scale condensation and optical
variability. We derive analytic expressions for two response functions that
characterize two potential low cloud feedback scenarios in a warming climate.
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