Radar Observations of Misovortices within Multiple Long-lake-axis-parallel Lake-effect Bands
The Ontario Winter Lake-effect Systems (OWLeS) project collected a wealth of data on several lake-effect storms in the vicinity of Lake Ontario during the winter of 2013 -14. Of all lake-effect band archetypes, long-lake-axis-parallel (LLAP) bands are the most intense, bringing the highest snowfall rates and greatest snow totals. Numerous previous studies have revealed strong convergence zones owing to secondary circulations within these bands. Horizontal shear along these convergence zones can yield vortex sheets that can break into discrete misovortices through horizontal shearing instability (HSI). These misovortices then can strengthen via stretching by the band updrafts. Recent analysis of a single case from OWLeS documented a string of misovortices within a LLAP band and determined that all of the misovortices and wind shifts were cyclonic and that HSI was likely the primary method of vortex formation in that particular band. Herein, we perform similar analyses on several additional LLAP bands during OWLeS to investigate the robustness of these conclusions.
Our analysis, utilizing single-Doppler WSR-88D data and dual-Doppler wind syntheses of mobile Doppler radar observations reveals that 80-90% of the wind shifts within the LLAP bands were cyclonic, but that anticyclonic shear zones and misovortices also exist. It was found that these structures only form owing to the ingestion of land breeze circulations from the southern shore of Lake Ontario, however. To evaluate the presence of HSI, distributions of Rayleigh’s and Fjørtoft’s instability criterions were analyzed, and it was found that both criteria for HSI were satisfied along the wind shifts within the bands while vortices were present, meaning that HSI is likely the dominant mechanism of vortex formation in these bands as well. Instances of cyclonic-anticyclonic vortex couplets are also observed in some of the bands, but do not form as a result of tilting.