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Episodic Mixing and Buoyancy-sorting Representations of
Shallow Convection: A Diagnostic Study
M. Zhao and P. H. Austin
Journal of the Atmospheric Sciences , 2003, 60, 892-912.
Abstract
Episodic mixing and buoyancy-sorting (EMBS) models have been proposed
as a physically more realistic alternative to entraining plume models
of cumulus convection. Applying these models to shallow
non-precipitating clouds requires assumptions about the rate at which
undilute sub-cloud air is eroded into the environment, an algorithm to
calculate the eventual detrainment level of cloud-environment
mixtures, and the probability distribution of mixing fraction. A
diagnostic approach is used to examine the sensitivity of an EMBS
model to these three closure assumptions, given equilibrium convection
with known large-scale forcings taken from phase III of BOMEX. The
undilute eroding rate (UER) is retrieved and found to decrease
exponentially with height above cloud-base, suggesting a strong
modulation by the cloud size distribution. The EMBS model is also used
to calculate convective transport by individual clouds of varying
thickness. No single cloud from this ensemble can balance the
large-scale BOMEX forcing; the observed equilibrium requires a
population of clouds with a cloud size distribution that is maximum
for small clouds and decreases monotonically with cloud size.
The EMBS model depends sensitively on the assumptions governing the
detrainment of positively buoyant mixtures. In particular, given the
requirement that positively buoyant mixtures detrain at their neutral
buoyancy level, there is no positive-definite undilute eroding rate
that is consistent with the BOMEX forcing. The model is less
sensitive to the assumed distribution of cloud-environment mixtures,
given a multiple-mixing treatment of positively buoyant parcels and
detrainment at the unsaturated neutral buoyancy level.
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