Observed Mesoscale Variations in Thermodynamic Properties and Surface Fluxes over Ice-Covered Lake Erie

Contrary to the common thought that extensive ice cover inhibits the development of lake-effect snowstorms over the North American Great Lakes, lake-effect clouds and precipitation have been observed in several cases with nearly ice-covered lakes. These situations can be difficult for regional weather offices to forecast successfully because the knowledge of mesoscale environments over partially ice-covered lakes is limited and current operational numerical models greatly simplify the treatment of ice cover.

The primary goal of the Great Lakes Ice Cover – Atmospheric Flux (GLICAF) project, conducted during February 2004 over Lake Erie, was to collect unprecedented observations to investigate the relationships between surface heat exchanges and variations in pack ice fields on the Great Lakes.

Observations taken by the University of Wyoming King Air on 26 February 2004 show weak positive sensible and latent turbulent heat fluxes, despite significant ice coverage and lake-air temperature differences of less than 2 °C. Boundary layer thermodynamic properties and surface heat fluxes were found to be highly sensitive to variations in upwind atmospheric conditions and over-lake pack ice concentrations.

Correspondence between flight-level (45 m altitude) and lake-surface temperatures (the latter measured by a Heimann KT-19.85 downward-looking radiometer) demonstrate a strong thermal link between the lake and atmospheric boundary layer. Turbulent sensible heat fluxes decreased nonlinearly with increases in lake-surface ice concentration such that the largest decreases occurred when ice cover concentration was over 80%. Latent heat fluxes, although not as strongly correlated with ice concentration, tended to decrease linearly with increasing ice cover concentration. These findings suggest that current representations of ice cover in numerical weather prediction models may significantly underestimate surface sensible heat fluxes in regions of high-concentration pack ice cover.