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JOURNAL OF THE ATMOSPHERIC SCIENCES, 54, 2785-2798, 1997
Parameterizing grid-averaged longwave fluxes for inhomogeneous marine boundary layer clouds
H. W. Barker, and B. A. Wielicki
Abstract
This paper examines the relative impacts on grid-averaged longwave flux transmittance (emittance) for marine boundary layer
(MEL) cloud fields arising from horizontal variability of optical depth tau and cloud sides. First, using fields of
Landsat-inferred tau and a Monte Carlo photon transport algorithm, it is demonstrated that mean all-sky transmittances
for 3D variable MBL clouds can be computed accurately by the conventional method of linearly weighting clear and cloudy
transmittances by their respective sky fractions. Then, the approximations of decoupling cloud and radiative properties and
assuming independent columns are shown to be adequate for computation of mean flux transmittance.
Since real clouds have nonzero geometric thicknesses, cloud fractions (A) over cap(c) presented to isotropic beams usually
exceed the more familiar vertically projected cloud fractions A(c). It is shown, however, that when A(c) less than or similar
to 0.9, biases for all-sky transmittance stemming from use of A(c) as opposed to (A) over cap(c) are roughly 2-5 times smaller
than, and opposite in sign to, biases due to neglect of horizontal variability of tau. By neglecting variable tau, all-sky
transmittances are underestimated often by more than 0.1 for A(c) near 0.75 and this translates into relative errors that can
exceed 40% (corresponding errors for all-sky emittance are about 20% for most values of A(c)). Thus, priority should be given
to development of general circulation model (GCM) parameterizations that account for the effects of horizontal variations in
unresolved tau, effects of cloud sides are of secondary importance.
On this note, an efficient stochastic model for computing grid-averaged cloudy-sky flux transmittances is furnished that assumes
that distributions of tau, for regions comparable in size to GCM grid cells, can be described adequately by gamma distribution
functions. While the plane-parallel, homogeneous model underestimates cloud transmittance by about an order of magnitude when
3D variable cloud transmittances are less than or similar to 0.2 and by similar to 20% to 100% otherwise, the stochastic model
reduces these biases often by more than 80%.
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