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.

Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell*

“…The development of rotation at ground level, 1 on the other hand, cannot be achieved by mere upward tilting of initially horizontal vortex lines, but a downdraft is required, as discussed by Davies-Jones (1982), DaviesJones and Brooks (1993), Walko (1993), Markowski and Richardson (2010), and Davies-Jones and Markowski (2013). The reason is that an initially horizontal nearground vortex line is lifted away from the surface as it is reoriented into the vertical at the updraft edge.…”

“…One way that a downdraft facilitates the development of vertical vorticity at the surface is by baroclinic production of horizontal vorticity at the downdraft periphery, followed by tilting and downward advection, as suggested by full-physics simulations (Rotunno and Klemp 1985;Davies-Jones and Brooks 1993) and observations (Markowski et al , 2012. However, in idealized, axisymmetric simulations, near-ground rotation is achieved ''barotropically'' (i.e., by reorienting preexisting vortex lines at the downdraft edge; DaviesJones et al 2001;Markowski et al 2003b;Davies-Jones 2008;Parker 2012).…”

“…However, in idealized, axisymmetric simulations, near-ground rotation is achieved ''barotropically'' (i.e., by reorienting preexisting vortex lines at the downdraft edge; DaviesJones et al 2001;Markowski et al 2003b;Davies-Jones 2008;Parker 2012). This barotropic vorticity corresponds to vorticity that is imported into the storm from the environment.…”

“…2 However, even in cases where the importance of baroclinity is evident, one might still ask whether the ambient, preexisting horizontal vorticity plays a direct role in near-ground rotation of supercells (irrespective of whether additional vorticity is produced baroclinically). One motivation for this question is that the ambient vertical wind shear at low levels is an important predictor for tornadoes (Markowski et al 2003a). Moreover, whenever there are horizontal vortex lines that interact with a downdraft, one would expect the vortex lines to be depressed, potentially contributing to vertical vorticity at the surface.…”

“…Two exceptions areDavies-Jones and Brooks (1993), who discuss qualitatively the role of baroclinic and barotropic processes in full-physics simulations, andMarkowski et al (2012), who consider barotropic and baroclinic effects via the nonconservation of the circulation of a material circuit.3 Herein, nonbarotropic vorticity refers to the part of vorticity that is not due to the rearrangement of preexisting vortex lines.4 These three contributions will be referred to as ''rearrangement'' of vortex lines.…”