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Episodic Mixing and Buoyancy-Sorting Representations of Shallow Convection: A Diagnostic Study Austin, Philip H.; Zhao, Ming
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 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 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 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. Copyright 2003 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (http://www.ametsoc.org/) or from the AMS at 617-227-2425 or copyright@ametsoc.org.
Item Metadata
Title |
Episodic Mixing and Buoyancy-Sorting Representations of Shallow Convection: A Diagnostic Study
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Creator | |
Publisher |
American Meteorological Society
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Date Issued |
2003-04
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Description |
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 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 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 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. Copyright 2003 American Meteorological Society (AMS). Permission
to use figures, tables, and brief excerpts from this work in scientific and educational
works is hereby granted provided that the source is acknowledged. Any use of material in
this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act
or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17
USC §108, as revised by P.L. 94-553) does not require the AMS’s permission.
Republication, systematic reproduction, posting in electronic form, such as on a web site
or in a searchable database, or other uses of this material, except as exempted by the
above statement, requires written permission or a license from the AMS. Additional
details are provided in the AMS Copyright Policy, available on the AMS Web site
located at (http://www.ametsoc.org/) or from the AMS at 617-227-2425 or
copyright@ametsoc.org.
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Genre | |
Type | |
Language |
eng
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Date Available |
2016-11-07
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Provider |
Vancouver : University of British Columbia Library
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Rights |
Attribution-NonCommercial-NoDerivatives 4.0 International
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DOI |
10.14288/1.0041785
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URI | |
Affiliation | |
Citation |
Zhao, Ming, Austin, Philip H. 2003: Episodic Mixing and Buoyancy-Sorting Representations of Shallow Convection: A Diagnostic Study. Journal of the Atmospheric Sciences 60(7) 892–912.
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Publisher DOI |
10.1175/1520-0469(2003)060<0892:EMABSR>2.0.CO;2
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Peer Review Status |
Reviewed
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Scholarly Level |
Faculty
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Copyright Holder |
Austin Philip H.
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Rights URI | |
Aggregated Source Repository |
DSpace
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Item Citations and Data
Rights
Attribution-NonCommercial-NoDerivatives 4.0 International