Roll circulations in the convective region of a simulated squall line

AMS Citation:
Bryan, G. H., R. Rotunno, and J. M. Fritsch, 2007: Roll circulations in the convective region of a simulated squall line. Journal of the Atmospheric Sciences, 64, 1249-1266, doi:10.1175/JAS3899.1.
Date:2007-04-01
Resource Type:article
Title:Roll circulations in the convective region of a simulated squall line
Abstract: In high-resolution numerical simulations (using horizontal grid spacing less than 1 km), the convective region of squall lines will sometimes overturn as quasi-horizontal convective rolls. The authors study one case in detail using output from a simulation with 125-m grid spacing. The rolls have an average spacing of 3 km and are aligned parallel to the vertical wind shear. Individual convective cells often have long-lived, undiluted cores that entrain primarily on the sides of the rolls (i.e., between the roll updraft and downdraft). The following set of conditions is proposed for obtaining roll overturning: the formation of a moist absolutely unstable layer (MAUL); vertical shear of the horizontal wind within the MAUL; an environment without large-amplitude perturbations; and quasi-horizontal flow over the squall line’s surface-based cold pool. Further insight is gained through a series of more idealized simulations wherein a specified MAUL is perturbed by analytic potential temperature perturbations. These simulations confirm classical studies based on linear analysis because the smallest perturbations grow fastest (with the exception of the very smallest scales that are affected by diffusion). The results also confirm that, with shear, updrafts oriented across the shear vector are inhibited by the shear. An explanation for the 3-km roll spacing also emerges from these simulations. The argument focuses on the perturbations that exist in the cold pool underneath the MAUL; they induce pressure fields that extend upward into the overlying MAUL. The perturbations with large horizontal scale have pressure fields that extend farther vertically and with a greater amplitude, and thus are more effective at initiating motions in the overlying MAUL. The convective scale that ultimately emerges within the MAUL is somewhere between two scales, whereby comparatively large scales are perturbed more strongly by perturbations in the cold pool, but the comparatively small scales grow faster.
Subject(s):Squall lines, Convective-scale processes, Wind shear
Peer Review:Refereed
Copyright Information:Copyright 2007 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/d7sf2wdn
Publisher's Version: 10.1175/JAS3899.1
Author(s):
  • George Bryan - NCAR/UCAR
  • Richard Rotunno - NCAR/UCAR
  • J. Fritsch
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