A climatology of collective lake disturbances

This study examines the frequency and intensity with which collective lake disturbances (COLDs) develop. These disturbances develop when cold air overspreads the Great Lakes region in winter. The heat and moisture that is transferred from the Great Lakes aggregate into the lower atmosphere, and that spreads across a large region, allows eventually for the development of a meso-α-scale pressure perturbation and circulation. Cases from the period 1980-90 were identified based on the existence of a surface trough or closed low over the Great Lakes region in the presence of cold air. Output from the Limited-Area Fine Mesh (LFM) model was used rather than performing numerous with-lake and no-lake numerical simulations to determine whether the feature was indeed the result of aggregate heating by the lakes. The LFM did not include the lakes in its simulations, so the 24-h forecast served as an optimal no-lakes simulation. Subtracting the initialization sea level pressure (SLP) field valid at the same time allowed for an assessment of the COLD events in terms of the SLP perturbation. An average of 33 events per year with an average SLP perturbation of 3-4 hPa was found for the 10-yr period. The synoptic-scale conditions for weak events with SLP perturbations less than 3 hPa differed significantly from those for strong events with SLP perturbations greater than 9 hPa. The weak scenario was characterized by a weak trough over the Great Lakes with high static stability and weak cold advection below 500 hPa and weak vorticity advection at 500 hPa. The strong scenario was characterized by a nearly closed low over the Great Lakes with low static stability and strong cold advection below 500 hPa and strong positive vorticity advection at 500 hPa. The current study is the first attempt to measure the frequency and intensity with which the Great Lakes collectively generate meso-α-scale disturbances in winter. The LFM-based technique provides a result that cannot likely be obtained without a herculean effort from a numerical modeling standpoint. Future numerical studies using the identified scenarios, however, will be extremely useful to better understand the sensitivities of COLD events to the large-scale conditions.