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

Recent lake-effect snow field projects 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 misovortices with diameters between 40 and 4000 m within LLAP bands in the vicinity of Lake Ontario. These misovortices usually develop along horizontal shear zones that form within these bands. In most of these cases, the shear zone is co-located with a sharp horizontal gradient in radar reflectivity and is found 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. A westerly flow regime was in place across the region behind a departing synoptic-scale low pressure system. Arctic air, characterized by 850-hPa temperatures around -25 ˚C, traversed the long-axis of Lake Ontario, setting the stage for the formation of a significant LLAP-band. Additionally, a vigorous 500-hPa shortwave trough traversed Lake Ontario between 06-08 UTC, which assisted in the development of a deep boundary layer, sampled by areal rawinsondes. A sharp horizontal gradient in reflectivity was found on the northern edge of the LLAP band along with an upstream connection with Georgian Bay in Canada. This “string” of vortices, which were all also along the northern edge of the LLAP-band, originated over Georgian Bay and propagated downstream over Lake Ontario. The upstream connection was evident from around 02 UTC to 07 UTC before diminishing as 700-hPa ridging impinged on the region from the west, backing winds 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 form owing to horizontal shear instability (HSI) along the low-level convergence zones in the bands, it is uncertain as to why the convergence boundaries and shear zones form 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 stationary 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 of these convergence boundaries and misovortices.