Lake-Aggregate Mesoscale Disturbances. Part II: A Case Study of the Effects on Regional and Synoptic-Scale Weather Systems

Abstract

A high-resolution numerical model is employed to examine effects of the Great Lakes aggregate, defined to be the five major Great Lakes, on regional and synoptic-scale weather. Simulations wherein the effects of the lakes are included and then excluded are performed on a selected cold air outbreak episode during late autumn when the lakes are still ice-free. Examination of the differences between the model simulations reveals that several dynamical effects result from heating and moist-ening by the lake aggregate. These effects are manifested primarily in the form of a 4-km-deep, 2000-km-wide, lake-aggregate mesoscale disturbance (circulation) that develops slowly over the region. The simulated lake-aggregate circulation splits a synoptic-scale high into two distinct centers and redirects and intensifies aweak synoptic-scale low, as verified by existing observations. These modifications of the synoptic-scale environment result in additional precipitation over, downstream, and upwind from the lakes. The model simulations also reveal that the developing lake-aggregate circulation influences significantly the lake shore surface winds. In some locations, the surface winds switch from onshore to offshore or vice versa. Because it is well known from observations that the location and orientation of lake-induced snow bands are very sensitive to the low-level wind direction over the lakes, it is concluded that the exact locations of heavy snowfall are the result of a complex multiscale interaction among circulations on three different scales: synoptic, individual lake, and lake aggregate. In addition to the developing primary lake-aggregate circulation, a secondary dynamic response appears at a distant location, adjacent to the eastern seaboard. The organization of this secondary circulation suggests that the lakes may play a direct role in some cases of East Coast cyclogenesis.