Numerical Study of the Effects of the Great Lakes on a Winter Cyclone

Abstract

A vigorous winter storm over North America is studied by means of an eight-level primitive-equation model. Included are orography, surface and internal friction, long-wave radiation from clouds and water vapor, large-scale release of latent heat, and fluxes of sensible heat and water vapor from water surfaces. Sigma coordinates are employed. The grid size is 190 km at 60°N. Two 36-hr numerical integrations are performed, one with and one without the effects of the Great Lakes and other water surfaces. When these influences are included, lower tropospheric temperatures are raised by more than 7°C and 1000-mb heights are reduced as much as 70 m. Ekman layer wind speeds are modified by up to 6 m/s. The maximum increase in large-scale precipitation over the Great Lakes is 0.5 cm with decreases to the southeast. Below 800 mb, isobaric surfaces are lowered; they are raised at higher levels. Effects on vorticity and divergence also change sign between lower and upper troposphere. Near the earth's surface, the average contribution of the Great Lakes is 1.9?10?5 s?1 to the vorticity and ?1.5?10?5 s?1 to the divergence. The associated effects on both the rotational and divergent wind fields amount to about 3 m/s.