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Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part II: Mixing Dynamics
 

Summary: Life Cycle of Numerically Simulated Shallow Cumulus Clouds.
Part II: Mixing Dynamics
MING ZHAO* AND PHILIP H. AUSTIN
Atmospheric Science Programme, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver,
British Columbia, Canada
(Manuscript received 2 June 2003, in final form 1 September 2004)
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 subcloud 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

  

Source: Austin, Philip H. - Department of Earth and Ocean Sciences, University of British Columbia

 

Collections: Geosciences