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Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part II: Mixing Dynamics
M. Zhao and P. H. Austin
Journal of the Atmospheric Sciences , 2005, 62, 1291-1310.
Abstract
This paper is the second in a two-part series in which life cycles of
six numerically simulated shallow cumulus clouds are systematically
examined. The six clouds, selected from a single realization of a
large eddy simulation, grow as a series of pulses/thermals detached
from the sub-cloud layer. All six clouds exhibit a coherent vortical
circulation and a low buoyancy, low velocity trailing wake. The
ascending cloud top (ACT), which contains this vortical circulation,
is associated with a dynamic perturbation pressure field with high
pressure located at the ascending frontal cap and low pressure below
and on the downshear side of the maximum updrafts. Examination of the
thermodynamic and kinematic structure, together with passive tracer
experiments, suggests that this vortical circulation is primarily
responsible for mixing between cloud and environment. As the cloud
ACTs rise through the sheared environment, the low pressure, vortical
circulation and mixing are all strongly enhanced on the downshear side
and weakened on the upshear side. Collapse of the ACT also occurs on
the downshear side, with subsequent thermals ascending on the
upshear side of their predecessors. The coherent core structure is
maintained throughout the ACT ascent; mixing begins to gradually
dilute the ACT core only in the upper half of the cloud's depth. The
characteristic kinematic and dynamic structure of these simulated
ACTs, together with their mixing behavior, corresponds closely to that
of shedding thermals. These shallow simulated clouds,
however, reach a maximum height of only about 4 ACT diameters, so that
ACT mixing differs from predictions of self-similar laboratory
thermals.
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