Elevated Mixed Layers during Great Lake Lake-Effect Events: An Investigation and Case Study from OWLeS

Data from rawinsondes launched during intensive observation periods (IOPs) of the Ontario Winter Lake-Effect Systems (OWLeS) field project reveal that elevated mixed layers (EMLs) in the lower troposphere were relatively common near Lake Ontario during OWLeS lake-effect events. Conservatively, EMLs exist in 193 of the 290 OWLeS IOP soundings. The distribution of EML base pressure derived from the OWLeS IOP soundings reveals two classes of EML, one that has a relatively low-elevation base (900-750 hPa) and one that has a relatively high-elevation base (750-500 hPa). It is hypothesized that the former class of EML, which is the focus of this research, is, at times, the result of mesoscale pro-cesses related to individual Great Lakes. WRF reanalysis fields from a case study during the OWLeS field project provide evidence of two means by which low-elevation base EMLs can originate from the lake-effect boundary layer convection and associated mesoscale circulations. First, such EMLs can form within the upper-level outflow branches of mesoscale so-lenoidal circulations. Evacuated Great Lakes-modified convective boundary layer air aloft then lies above ambient air of a greater static stability, forming EMLs. Second, such EMLs can form in the absence of a mesoscale solenoidal circulation when Great Lake-modified convective boundary layers overrun ambient air of a greater density. The reanalysis fields show that EMLs and layers of reduced static stability tied to Great Lakes-modified convective boundary layers can extend downwind for hundreds of kilometers from their areas of formation. Operational implications and avenues for future re-search are discussed.