The optimal state for gravity currents in shear

AMS Citation:
Bryan, G. H., and R. Rotunno, 2014: The optimal state for gravity currents in shear. Journal of the Atmospheric Sciences, 71, 448-468, doi:10.1175/JAS-D-13-0156.1.
Date:2014-01-01
Resource Type:article
Title:The optimal state for gravity currents in shear
Abstract: This study examines the lifting of sheared environmental air by gravity currents, focusing primarily on the theoretical "optimal state" in which near-surface flow is turned into a vertically oriented jet. Theoretical models are presented from multiple perspectives, including the vorticity perspective that was first presented by Rotunno, Klemp, and Weisman and a flow-force balance perspective based on conservation of mass and momentum. The latter approach reveals a constraint on the depth of the environmental shear layer relative to the depth of the cold pool. Based on these control-volume constraints, a numerical solution for steady, inviscid, isentropic flow is obtained that shows how the cold-pool interface has a slightly concave shape and is nearly (although not strictly) vertical. Then, by initializing a time-dependent numerical model with a stagnant cold pool in an environment with low-level shear, it is shown that a statistically steady flow can be maintained with all the important elements of the analytic solution. Most notably, the front-relative flow is negligible behind the surface gust front at all levels, the interface of the cold pool maintains a predominantly vertical structure, and the net generation of vorticity by buoyancy within a control volume closely matches the horizontal flux of environmental vorticity on the side of the control volume. Sensitivity simulations confirm that the constraints identified by the analytic study must be met for the optimal state to be realized and that lifting of near-surface environmental air is optimized when a vertically oriented jet is created and maintained.
Subject(s):Density currents, Wind shear, Cold pools
Peer Review:Refereed
Copyright Information:Copyright 2014 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 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Law (17 USC, as revised by P.L. 94-553) does not require the Society's permission. Republication, systematic reproduction, posting in electronic form on servers, or other uses of this material, except as exempted by the above statements, requires written permission or license from the AMS. Additional details are provided in the AMS Copyright Policies, available from the AMS at 617-227-2425 or amspubs@ametsoc.org. Permission to place a copy of this work on this server has been provided by the AMS. The AMS does not guarantee that the copy provided here is an accurate copy of the published work.
OpenSky citable URL: ark:/85065/d76w9c0x
Publisher's Version: 10.1175/JAS-D-13-0156.1
Author(s):
  • George Bryan - NCAR/UCAR
  • Richard Rotunno - NCAR/UCAR
  • Random Profile

    CASUAL - ASSOC SCI III/IV

    Recent & Upcoming Visitors