Misovortices and Boundaries within the 7 January 2014 Long Lake-Axis-Parallel Lake-effect Snow Band during the Ontario Winter Lake-effect Systems (OWLeS) Project

Recent field projects that have focused on lake-effect snow in the eastern Great Lakes region [e.g., Long Lake-Axis-Parallel Project (LLAP 2010-2011) and Ontario Winter Lake-effect Systems (OWLeS 2013-2014)] have revealed the presence of vortices with diameters between 40 and 4000 m within LLAP bands in the vicinity of Lake Ontario. These vortices usually develop along horizontal shear zones found within these bands. In most of these cases, the shear zone is co-located with a sharp horizontal gradient in radar reflectivity and is located on the side of the band closer to the long axis band-parallel shore of Lake Ontario.

One particular case from the OWLeS project with an abundance of misovortices occurred on 7 January 2014. Westerly flow was in place across the Eastern Great Lakes behind a departing synoptic-scale low pressure system. Arctic air, characterized by 850-hPa temperatures around -25°C, was in place, leading to a deep, unstable boundary layer over the lake, setting the stage for the formation of a significant LLAP band. A sharp horizontal gradient in reflectivity along with a series of vortices was found on the northern edge of this band. This band and the vortices can be traced back to Georgian Bay. The upstream connection was most prominent between 02 - 07 UTC. After this time, the connection was lost as a 500-hPa shortwave crossed the region and 700-hPa ridging impinged from the west, backing the winds over Georgian Bay from northwesterly to westerly. Once the upstream connection was lost, the string of vortices over Lake Ontario vanished and did not return for the rest of the event; however, rare instances of isolated vortices still were observed. This leads to the hypothesis that the upstream connection was crucial for the development of the misovortices in this case.

While it is likely that the misovortices formed owing to horizontal shear instability (HSI) along the low-level convergence zone in the band, it is uncertain as to why this convergence boundary and associated horizontal shear zone formed in the first place, or why the connection to Georgian Bay was so crucial to the formation of vortices in this case. Upper-air soundings, surface weather observations, mobile and operational Doppler radar data, along with dual-Doppler wind syntheses are analyzed and presented herein to highlight some of the key features involved in the formation and dissipation of these vortices.