Lake-Aggregate Mesoscale Disturbances. Part IV: Development of a Mesoscale Aggregate Vortex

Abstract

Many studies have noted that cyclone development in the Great Lakes region during winter is the result of strong diabatic heating and low-level destabilization from the lakes. The exact mechanisms, however, by which this heating and moistening lead to sea level pressure falls, and to weak cyclones over the lakes (e.g., mesoscale aggregate vortices), have not been investigated previously. In this study, model output that includes all of the Great Lakes and none of the Great Lakes is analyzed to understand more completely the importance of synoptic-scale forcing, diabatic heating, and perturbation?synoptic-scale processes for the development of a mesoscale aggregate vortex over the region during a 48-h period between 0000 UTC 13 and 0000 UTC 15 November 1982. The analysis indicates that the sea level pressure falls and vortex development were not simply the hydrostatic result of heat from the Great Lakes ?spreading? over a large region. Rather, the synoptic-scale flow contributed to vortex development during the first 24 h by providing strong cold northwesterly flow, which generated significant surface heat fluxes; and during the second 24 h by providing low-level warm advection and midlevel positive vorticity advection from southwesterly flow, which enhanced large-scale ascent and horizontal perturbation heat flux convergence near the surface. The eventual collocation of strong cyclonic perturbation southerly winds at 900 hPa, strong anticyclonic perturbation southerly winds at 700 hPa, and east?west-oriented isotherms in between greatly enhanced the warm advection and vortex development in the region. Finally, the intensifying cyclonic perturbation flow contributed significantly to surface sensible and latent heat fluxes and to further vortex development when it phased with the synoptic-scale flow at the surface. The one case that has been examined does not likely serve as an explanation for all mesoscale aggregate vortices. More studies are needed to determine the climatology of these vortices that develop over the Great Lakes region in winter.