Lake-aggregate mesoscale disturbances. Part IV: Development of a mesoscaleaggregate vortex

Many studies have noted that cyclone development in the Great Lakes region during winter is the result of strong diabatic heating and low-level destab ilization from the lakes. The exact mechanisms, however, by which this heat ing and moistening lead to sea level pressure fairs, and to weak cyclones o ver the lakes (e.g., mesoscale aggregate vortices), have not been investiga ted previously. In this study, model output that includes all of the Great Lakes and none o f the Great Lakes is analyzed to understand more completely the importance of synoptic-scare forcing, diabatic heating, and perturbation-synoptic-scal e processes for the development of a mesoscale aggregate vortex over the re gion 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 devel opment were not simply the hydrostatic result of heat from the Great Lakes "spreading" over a large region. Rather, the synoptic-scale flow contribute d to vortex development during the first 24 h by providing strong cold nort hwesterly flow, which generated significant surface heat fluxes; and during the second 24 h by providing low-level warm advection and midlevel positiv e vorticity advection from southwesterly flow, which enhanced large-scale a scent and horizontal perturbation heat flux convergence near the surface. T he eventual collocation of strong cyclonic perturbation southerly winds at 900 hPa, strong anticyclonic perturbation southerly winds at 700 hPa, and e ast-west-oriented isotherms in between greatly enhanced the warm advection and vortex development in the region. Finally, the intensifying cyclonic pe rturbation flow contributed significantly to surface sensible and latent he at fluxes and to further vortex development when it phased with the synopti c-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.