Low-level wind and thermodynamic observations of a mesovortex in the 15 May 2009 VORTEX2 squall line

In the last decade, increasing attention has been paid to the development of embedded mesovortices within the leading edges of squall lines. Such mesovortices have been implicated in the production of squall line tornadoes and have also been observed to be locations of maximized non-tornadic surface wind damage. There are a number of plausible hypotheses for the formation of squall line mesovortices, including: downward tilting of system-generated crosswise horizontal vorticity; upward tilting of environmental streamwise horizontal vorticity; and, barotropic shearing instability along a squall line's outflow boundary. Most of these mechanisms have been proposed based upon results from numerical simulations. Although a few single-Doppler radar case studies of mesovortices exist in the literature, high-resolution measurements have heretofore been uncommon.

On 15 May 2009, a squall line mission was undertaken during the Verification of the Origins of Tornadoes Experiment 2009-2010 (VORTEX2). The target squall line was sampled by an array of Doppler radars and a fleet of vehicle-borne surface observing systems (“mobile mesonets”). Around the time that the squall line entered the dual-Doppler lobes of the VORTEX2 armada, a pronounced low-level mesovortex was noted along its leading edge, with a distinctive cusp in the system's outflow boundary/gust front. In this study we exploit the high-resolution VORTEX2 dual-Doppler wind retrievals and mobile mesonet observations in order to characterize the low-level structure of the outflow and the embedded mesovortex. An assessment of the current squall line mesovortex paradigms will be presented through the lens of the radar-synthesized 3D wind and vorticity fields.