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892 VOLUME 60J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S 2003 American Meteorological Society
 

Summary: 892 VOLUME 60J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S
2003 American Meteorological Society
Episodic Mixing and Buoyancy-Sorting Representations of Shallow Convection:
A Diagnostic Study
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 9 May 2002, in final form 24 September 2002)
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 nonprecipitating
clouds requires assumptions about the rate at which undilute subcloud air is eroded into the environment, an
algorithm to calculate the eventual detrainment level of cloud­environment mixtures, and the probability dis-
tribution 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 the Barbados Oceanographic Meteorological Experiment (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

  

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

 

Collections: Geosciences