Characteristics of Atypical Damaging Mesovortices in Quasi-Linear Convective Systems: Science and Warning Challenges

Science has progressed recently on explaining the origins of mesovortices in quasi-linear convective systems (QLCSs). Atkins and St. Laurent (2009) and Trapp and Weisman (2003) offer different explanations for the generation of the leading-edge mesovortices generated in their simulations, but both explanations rely on tilting and stretching of horizontal vorticity parallel to the gust front by updrafts and downdrafts.

Despite the advances in the science, radar evidence suggests the origin and structure of mesovortices may span a broader spectrum than are commonly discussed in the literature. Federal Aviation Administration (FAA) Terminal Doppler Weather Radars (TDWRs), offering a finer spatial and temporal data resolution than is achievable with the National Weather Service Weather Surveillance Radar - 1988 Doppler (NWS WSR-88D), have provided numerous cases of damaging mesovortices that don't fit the behavioral mold of tornadic mesovortices as documented in the literature. In the WFO Baltimore/Washington (LWX) County Warning Area (CWA), damaging mesovortices have been associated with, for example, supercell structure in a linear-structure storm segment, rearward-generating and path-crossing damaging mesovortices and damaging, non-supercellular mesovortices well behind the leading edge of a linear storm.

While these unusual damaging mesovortices also develop upward, unlike their better-documented counterparts they have relatively short longevity (on the order of 2-3 minutes), much smaller horizontal dimension (~1km diameter or smaller) and comparably short developmental lead times. At times, these mesovortices may even be associated with non-surface-based convection, suggesting the possibility of a more dynamically-driven genesis process. The damage paths with these mesovortices almost invariably are scalloped in shape and have a low aspect ratio (25-125 yards wide and a few miles long), a scale much more typical of tornadic paths than straight-line wind damage.

Documented tornadic mesovortices have been shown to have greater longevity, depth and rotational velocity than their non-tornadic counterparts, and often are proximal to swaths of non-tornadic divergent wind damage (which at times can be extreme). The unusual damaging mesovortices described in this study do not seem to fit the typical characteristics of tornadic mesovortices given in the literature, yet are comparably strong in intensity and rotational velocity.

Given the short duration, shallow depth and small dimension of these vortices, it is understandable that such mesovortices may be difficult to warn for. Environmental awareness becomes key to anticipating the potential for their formation, and local studies at WFO Baltimore/Washington have aided in this effort. But several logistical questions remain: Are these damaging mesovortices in fact tornadoes? Is a tornado warning the best service option to warn of the associated threats? How can these vortices be warned for if an office lacks TDWR data? These questions will be discussed in the context of this presentation.