An Observational Analysis of Mesovortex Genesis and Maintenance during the PECAN Field Campaign

An Observational Analysis of Mesovortex Genesis and Maintenance during the PECAN Field Campaign

Barrett T. Goudeau and Kevin. R. Knupp, The University of Alabama – Huntsville Dept. of Atmospheric Sciences

  During the overnight hours of 5 June 2015, during IOP-20 of the Plains Elevated Convection at Night (PECAN) field campaign, an elevated nocturnal mesoscale convective system (MCS) produced a series of mesovortices associated with two tornado reports in southeastern South Dakota. Although significant advances have been made in the past decade regarding the overall knowledge and detection of mesovortices embedded in quasi-linear convective system (QLCS) structures, a large portion of the current understanding of these events are confined to “idealized” cases occurring on or just behind the leading edge of a gust front in close proximity to an updraft/downdraft interface, as well as the apparent lack of a horizontal shearing instability . This event featured several distinct differences from what could be considered traditional mesovortex formation, including:

1. Development and maintenance of the mesovortices well behind the gust front within the cold pool;
2. Occurrence within an elevated nocturnal MCS characterized by a largely stable boundary layer; and
3. A below average (~23 kts) 0-3 km line-normal bulk shear in the environment immediately preceding the initial round of convection.

      While this event was characterized by a “non-traditional” environment, it also features several hallmarks of traditioanal mesovortex genesis, including the location of formation just north of a rear inflow jet (RIJ) induced surge.  Given the unique nature of the environment in which these mesovortices occurred, several questions are raised in regards to the efficacy of commonly used techniques such as the “Three Ingredients Method” (Schaumann and Przybylinski, 2012) in identifying non-traditional mesovortices that occur within the cold pool. Additionally, the lack of an upward deep convergence zone (UDCZ) in the region of the mesovortex formation places additional interest in the mechanism of formation for this event given the inhibited vertical stretching of vorticity.

      This presentation presents observations obtained during the PECAN field campaign, in both high spatial and temporal resolution, in an effort to properly quantify the mechanisms responsible for mesovortex and tornado formation in areas well behind the gust front. In addition, the environment both ahead of and behind the cold pool will be analyzed to determine how the mesovortex was able to persist in what would otherwise be considered a non-conducive environment.