Operational uses of Spectrum Width to Improve Warnings for Quasi-Linear Convection in the Western Great Lakes

A quasi-linear convective system (QLCS) such as a derecho, bow echo, or squall line can produce significant wind damage over wide areas. It has long been hypothesized that swaths of prolific damaging winds and embedded tornadoes within QLCSs are related to so-called mesovortices that form rapidly along the leading edge of convective systems. The environmental parameters and microphysical evolution of mesovortices are becoming increasingly well understood from a theoretical perspective. In operational settings, however, challenges remain on how to differentiate between damaging and non-damaging mesovortices in time to issue appropriate warnings.

Within a western Great Lakes domain, 78 unique mesovortices were analyzed from eleven recent damaging QLCS events. It was found that damaging circulations tend to have stronger rotational velocities, greater depths, and lifespans longer than non-damaging mesovortices. Furthermore, 82% of the cataloged damaging circulations were co-located with a compact maximum in spectrum width (SW), which the author calls the “SW Signature.” Circulations that possessed the signature themselves tended to have stronger rotational velocities and greater depths than those that did not. Additionally, a separate SW signature in the shape of a “C” or “ring,” deemed the “SW Ring Signature,” was observed with particularly damaging circulations. The rotational velocity and circulation depth for mesovortices with the SW Ring Signature tended to be much greater than circulations with the basic SW Signature or no signature altogether. Furthermore, the structure of the SW Ring Signature may indicate the presence of microvortices within the parent rotation, potentially allowing for forecasters to identify multiple hazards such as simultaneous tornadoes. While SW has been studied and used in operational settings including for the detection of mesovortices, the latter signature, to the author's knowledge, has not been extensively studied or documented in relevant literature. When co-located with strong rotational velocities, both SW signatures appear to be promising ways to identify and possibly increase warning lead times for circulations that have a greater potential to produce damage.