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The Pretornadic Phase of The Goshen County, Wyoming, Supercell of 5 June 2009 Intercepted By Vortex2. Part Ii: Intensification of Low-level Rotation

Abstract

The dynamical processes responsible for the intensification of low-level rotation prior to tornadogenesis are investigated in the Goshen County, Wyoming, supercell of 5 June 2009 intercepted by the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The circulation of material circuits that converge upon the low-level mesocyclone is principally acquired along the southern periphery of the forward-flank precipitation region, which is a corridor characterized by a horizontal buoyancy gradient; thus, much of the circulation appears to have been baroclinically generated. The descending reflectivity core (DRC) documented in Part I of this paper has an important modulating influence on the circulation of the material circuits. A circuit that converges upon the low-level mesocyclone center prior to the DRC’s arrival at low levels (approximately the arrival of the 55-dBZ reflectivity isosurface in this case) loses some of its previously acquired circulation during the final few minutes of its approach. In contrast, a circuit that approaches the low-level mesocyclone center after the DRC arrives at low levels does not experience the same adversity. An analysis of the evolution of angular momentum within a circular control disk centered on the low-level mesocyclone reveals that the area-averaged angular momentum in the nearby surroundings of the low-level mesocyclone increases while the mesocyclone is occluding and warm-sector air is being displaced from the near surroundings. The occlusion process reduces the overall negative vertical flux of angular momentum into the control disk and enables the area-averaged angular momentum to continue increasing even though the positive radial influx of angular momentum is decreasing in time.

Article / Publication Data
Active/Online
YES
ISSN
Print 0027-0644/Online 1520-0493
Volume
140
Available Metadata
Accepted On
March 09, 2012
DOI ↗
Fiscal Year
Publication Name
Mon. Wea. Rev.
Published On
September 01, 2012
Final Online Publication On
September 01, 2012
Publisher Name
Amer Meteorological Soc
Print Volume
140
Print Number
9
Page Range
2916-2938
Submitted On
November 23, 2011
URL ↗

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