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Title: Thermal chains and entrainment in cumulus updrafts, Part 1: Theoretical description

Abstract

Recent studies have shown that cumulus updrafts often consist of a succession of discrete rising thermals with spherical vortex-like circulations. Herein, a theory is developed for why this “thermal chain” structure occurs. Theoretical expressions are obtained for a passive tracer, buoyancy, and vertical velocity in axisymmetric moist updrafts. Analysis of these expressions suggests that the thermal chain structure arises from enhanced lateral mixing associated with intrusions of dry environmental air below an updraft’s vertical velocity maximum. This dry air entrainment reduces buoyancy locally. Consequently, the updraft flow above levels of locally reduced buoyancy separates from below, leading to a breakdown of the updraft into successive discrete thermals. The range of conditions in which thermal chains exist is also analyzed from the theoretical expressions. A transition in updraft structure from isolated rising thermal, to thermal chain, to starting plume occurs with increases in updraft width, environmental relative humidity, and/or convective available potential energy. Corresponding expressions for the bulk fractional entrainment rate ε are also obtained. These expressions indicate rather complicated entrainment behavior of ascending updrafts, with local enhancement of ε up to a factor of ~2 associated with the aforementioned environmental air intrusions, consistent with recent large eddy simulation (LES) studies.more » These locally large entrainment rates contribute significantly to overall updraft dilution in thermal chain-like updrafts, while other regions within the updraft can remain relatively undilute. Part 2 of this study compares results from the theoretical expressions to idealized numerical simulations and LES.« less

Authors:
 [1];  [2];  [3];  [4];  [5]
  1. National Center for Atmospheric Research, Boulder, CO (United States)
  2. Naval Postgraduate School, Monterey, CA (United States)
  3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  4. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  5. Brookhaven National Lab. (BNL), Upton, NY (United States)
Publication Date:
Research Org.:
University Corporation for Atmospheric Research, Boulder, CO (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Brookhaven National Lab. (BNL), Upton, NY (United States); National Center for Atmospheric Research, Boulder, CO (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Biological and Environmental Research (BER); National Center of Meteorology; National Science Foundation (NSF)
OSTI Identifier:
1657537
Alternate Identifier(s):
OSTI ID: 1706674; OSTI ID: 1735703; OSTI ID: 1786983
Report Number(s):
PNNL-SA-148681; BNL-220684-2020-JAAM
Journal ID: ISSN 0022-4928
Grant/Contract Number:  
SC0020104; SC0016476; SC0000246356; AGS-1841674; AC05-76RLO1830; AC52-07NA27344; SC0012704
Resource Type:
Accepted Manuscript
Journal Name:
Journal of the Atmospheric Sciences
Additional Journal Information:
Journal Volume: 7; Journal Issue: 11; Journal ID: ISSN 0022-4928
Publisher:
American Meteorological Society
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES

Citation Formats

Morrison, Hugh, Peters, John M., Varble, Adam C., Hannah, Walter M., and Giangrande, Scott E.. Thermal chains and entrainment in cumulus updrafts, Part 1: Theoretical description. United States: N. p., 2020. Web. https://doi.org/10.1175/jas-d-19-0243.1.
Morrison, Hugh, Peters, John M., Varble, Adam C., Hannah, Walter M., & Giangrande, Scott E.. Thermal chains and entrainment in cumulus updrafts, Part 1: Theoretical description. United States. https://doi.org/10.1175/jas-d-19-0243.1
Morrison, Hugh, Peters, John M., Varble, Adam C., Hannah, Walter M., and Giangrande, Scott E.. Thu . "Thermal chains and entrainment in cumulus updrafts, Part 1: Theoretical description". United States. https://doi.org/10.1175/jas-d-19-0243.1.
@article{osti_1657537,
title = {Thermal chains and entrainment in cumulus updrafts, Part 1: Theoretical description},
author = {Morrison, Hugh and Peters, John M. and Varble, Adam C. and Hannah, Walter M. and Giangrande, Scott E.},
abstractNote = {Recent studies have shown that cumulus updrafts often consist of a succession of discrete rising thermals with spherical vortex-like circulations. Herein, a theory is developed for why this “thermal chain” structure occurs. Theoretical expressions are obtained for a passive tracer, buoyancy, and vertical velocity in axisymmetric moist updrafts. Analysis of these expressions suggests that the thermal chain structure arises from enhanced lateral mixing associated with intrusions of dry environmental air below an updraft’s vertical velocity maximum. This dry air entrainment reduces buoyancy locally. Consequently, the updraft flow above levels of locally reduced buoyancy separates from below, leading to a breakdown of the updraft into successive discrete thermals. The range of conditions in which thermal chains exist is also analyzed from the theoretical expressions. A transition in updraft structure from isolated rising thermal, to thermal chain, to starting plume occurs with increases in updraft width, environmental relative humidity, and/or convective available potential energy. Corresponding expressions for the bulk fractional entrainment rate ε are also obtained. These expressions indicate rather complicated entrainment behavior of ascending updrafts, with local enhancement of ε up to a factor of ~2 associated with the aforementioned environmental air intrusions, consistent with recent large eddy simulation (LES) studies. These locally large entrainment rates contribute significantly to overall updraft dilution in thermal chain-like updrafts, while other regions within the updraft can remain relatively undilute. Part 2 of this study compares results from the theoretical expressions to idealized numerical simulations and LES.},
doi = {10.1175/jas-d-19-0243.1},
journal = {Journal of the Atmospheric Sciences},
number = 11,
volume = 7,
place = {United States},
year = {2020},
month = {10}
}

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