High Winds Generated by Bow Echoes. Part II: The Relationship between the Mesovortices and Damaging Straight-Line Winds

Abstract

Airborne radar analysis of a mesovortex that developed near the apex of a bow echo is presented. The mesovortex was shown to play a critical role in determining the location of intense “straight-line” wind damage at the surface. The perturbation pressure gradient force (in natural coordinates) along the parcel path accelerated the horizontal winds; however, intense mesovortices modified the low-level outflow and largely determined the locations where the strongest winds occurred. Regions of maximum winds are accounted for as a superposition of the vortex and the flow in which it is embedded. The strongest winds occur on the side of the vortex where translation and rotation effects are in the same direction. This model explains the observed tongue of high wind speeds that were confined to the periphery of the mesovortex. The origin of the mesovortex is also examined. Similarities and differences of this bow echo event with recent modeling studies are presented.

High-resolution full-scale measurements of thunderstorm outflow winds

“…For example, mesoscale vertical vorticity maxima (or mesovorticies; Trapp and Weisman, 2003;Wakimoto et al, 2006) associated with bow echoes have recently been implicated in accelerating outflow winds to the level of producing F1 damage (Wakimoto et al, 2006). While such studies have contributed to the broad understanding of thunderstorm outflow dynamics and sources of severe wind, the lack of near surface data and a focus on larger atmospheric scales have limited the information that can be incorporated into design considerations .…”

The Genesis of Mesovortices within a Real-Data Simulation of a Bow Echo System

“…Severe straight-line winds near the surface, sometimes called derechoes (Johns and Hirt 1987), are often observed in association with quasi-linear convective systems (QLCSs), such as squall lines and bow echoes (Atkins et al 2004;Wakimoto et al 2006a). Recent observational and modeling studies show that these damaging near-surface winds are often related to low-level (below 1 km AGL) mesovortices (MVs) within QLCSs Atkins et al 2005;Wheatley et al 2006).…”

“…Here, a mesovortex is defined as the meso-g-scale (Orlanski 1975) circulations forming at low levels on the gust front of QLCSs (Atkins and St. Laurent 2009a). Surface wind damage of Fujita-scale (Fujita 1981) F0 to F1 tornado intensity can be produced through the superposition of the mesovortex vortical flow with strong ambient translational flow (Wakimoto et al 2006b;Atkins and St. Laurent 2009a). MVs are also known to lead to nonsupercell tornadoes (Forbes and Wakimoto 1983;Funk et al 1999;Atkins et al 2005;Schenkman et al 2012).…”

Predicting Small-Scale, Short-Lived Downbursts: Case Study with the NWP Limited-Area ALARO Model for the Pukkelpop Thunderstorm

“…It is not known how broadly applicable these depths are to smaller-scale storms or in other regional environments. The descending cold air causes a pressure increase at the surface, called a surface mesohigh, of which the strength depends on the temperature and depth of the cold pool (Wakimoto 1982). Engerer et al (2008) investigated the surface characteristics of cold pools from MCS using observations from the Oklahoma Mesonet.…”

“…Gusty winds in the cold pool originate from downward momentum transport and surface pressure perturbations (e.g., Vescio and Johnson 1992). Furthermore, mesoscale vortices can play a role in generating low-level wind gusts near squall lines and bow echoes (Atkins et al 2004;Wakimoto et al 2006;Wheatley and Trapp 2008;Atkins and St. Laurent 2009).…”