Bow Echo Mesovortices. Part II: Their Genesis

Abstract

This two-part study examines the damaging potential and genesis of low-level, meso-g-scale mesovortices formed within bow echoes. This was accomplished by analyzing quasi-idealized simulations of the 10 June 2003 Saint Louis bow echo event observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). In Part II of this study, mesovortex genesis was investigated for vortices formed at different stages of convective system evolution. During the early ''cellular'' stage, cyclonic mesovortices were observed. The cyclonic mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. As the convective system evolved into a bow echo, cyclonicanticyclonic mesovortex pairs were also observed. The vortex couplet was produced by a local updraft maximum that tilted baroclinically generated vortex lines upward into arches. The local updraft maximum was created by a convective-scale downdraft that produced an outward bulge in the gust front position. Cyclonic-only mesovortices were predominantly observed as the convective system evolved into the mature bow echo stage. Similar to the early cellular stage, these mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. The downdraft parcels descended within the rear-inflow jet. The generality of the mesovortex genesis mechanisms was assessed by examining the structure of observed mesovortices in Doppler radar data. The mesovortex genesis mechanisms were also compared to others reported in the literature and the genesis of low-level mesocyclones in supercell thunderstorms.

Processes Controlling the Structure and Longevity of Two Quasi-Linear Convective Systems Crossing the Southern New England Coast

“…17h). The development of only a cyclonically rotating vortex resembles those documented by Atkins and St. Laurent (2009) as observed during the BAMEX experiment.…”

Evolution of a Quasi-Linear Convective System Sampled by Phased Array Radar

“…Atkins and St. Laurent (2009) suggest slightly different mesovortex genesis mechanisms. In quasi-idealized simulations of a bow echo on 10 June 2003 (see Atkins et al 2005), both cyclonic vortices and cyclonic-anticyclonic vortex couplets were observed. The cyclonic-only vortices were observed to form at all stages of bow echo evolution.…”

“…Based on numerical simulations, Weisman and Trapp (2003) and Trapp and Weisman (2003) propose that mesoy-scale vortices, or mesovortices within the bow echo are responsible for these narrow areas of damage. Several radar-based studies have confirmed the existence of damage-producing mesovortices in QLCSs (e.g., Atkins et al 2005;Wakimoto et al 2006a;Wheatley et al 2006). Trapp and Weisman (2003) suggest that vorticity couplets form within bow echoes (preferentially north of the bow apex) when baroclinically generated vorticity at the leading edge of the cold pool is tilted downward by subsystem-scale downdrafts and subsequently stretched, increasing the resulting vertical vorticity magnitude.…”

“…The cyclonic-only vortices were observed to form at all stages of bow echo evolution. Atkins and St. Laurent (2009) propose that these cyclonic vortices form when air descending roughly parallel to the gust front acquires the horizontal vorticity induced by the leading edge of the cold pool. This horizontal vorticity is subsequently tilted and stretched by an updraft along the gust front.…”

“…The temporal resolution of WSR-88Ds is also a significant limitation and could preclude adequate sampling of OLCS circulations. Atkins et al (2005) noted that several tornadoes produced in association with the 10 June 2003 St. Louis bow echo had lifetimes shorter than the time required for a typical WSR-88D volume scan. The PAR sampled the 2 April 2010 Rush Springs circulation with much higher temporal resolution, completing volume scans approximately every 2 min, compared to every 4.5 min for a WSR-88D operating in precipitation mode.…”