Abstract-
Is Owen Phillip's Hoary Old Chestnut (Entrainment) a key to
rationalizing cloud feedbacks to perturbations of the climate system?
Simple bulk theories of stratocumulus-topped boundary layers suggest
it might be. We review the sensitivity of the climate system to the
representation of such boundary layers, with a particular emphasis on
the role of cloud-top entrainment. The stratocumulus-topped boundary
layer is an intriguing fluid-dynamical problem wherein the state of
the layer results from the delicate interplay of turbulence, radiative
and surface forcing, and cloud microphysical processes. Central to
its evolution is the rate of entrainment at cloud top. The
combination of diverse physical processes make this a difficult regime
to reproduce in the laboratory, and attempts to attack it in the
field, or by computation, have proven only slightly less challenging.
We present recent estimates of entrainment, and turbulent structure,
made as part of the DYCOMS-II field study. Our measurements are used
to benchmark large-eddy simulation of this complex regime. In so
doing they highlight the sensitivity of the simulations to ad hoc
truncations of sub-filter length-scales at the entrainment interface,
as well as constrain theoretical work which attempts to investigate
the role of stratocumulus-topped mixed layers in the climate system.
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