Wednesday, 10 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Rawinsondes launched during the intensive operating periods (IOPs) of the Ontario Winter Lake-effect Systems (OWLeS) field project unexpectedly revealed that lower tropospheric elevated mixed layers were relatively common near Lake Ontario during lake-effect events. They were detected in at least 60% of the nearly 300 OWLeS IOP soundings. Back trajectories indicate that the vast majority of these lake-effect elevated mixed layers (LEEMLs) occurred in airstreams that had crossed one or more of the upwind Great Lakes. Reanalysis fields resulting from a nested-grid WRF ensemble assimilation run reveal two mechanisms by which LEEMLs form. First, LEEMLs can form within the upper-level outflow branches of lake-modified convective boundary layer solenoidal circulations. The evacuated lake-modified convective boundary layer air aloft is undercut by statically stable air from overland, thus forming LEEMLs. Second, LEEMLs can form when lake-modified convective boundary layers are undercut by statically stable flows associated with background meteorological features. Reanalysis fields also show that LEEMLs from upwind lakes can have one of two effects on downwind lake-modified convective boundary layers, depending on the LEEML’s potential temperature relative to that of the underlying boundary layer. If the potential temperature is warmer, the base of the LEEML acts as a lid, limiting the depth of the lake-effect convection. In contrast, if the potential temperature is cooler, penetration of the LEEML by the underlying convection can result in deepening of that convection. Applications to forecasting and opportunities for future research are discussed.
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