The Influence of a Great Lake Anticyclone on the Atmospheric Circulation

Abstract

An air-water interaction study was performed near Waukegan, Ill., to determine the effect of the cold waters of Lake Michigan on warm air flowing over them. Four days in late May and early June were chosen to analyze the physical nature of the interaction. Many times during spring and early summer, a cold temperate-latitude lake produces intense inversions that limit vertical turbulent energy transfers. Typically these inversions average 1 km in thickness. Coupled with such an inversion there often develops a shallow overwater mesoscale high pressure system, the lake anticyclone. The observations of the cold lake's effect on the atmosphere were taken on an east-west line through Waukegan for a distance 5 km inland (west) and ranging as far as 40 km over the lake (east). Hourly wind soundings to a height of approximately 1500 m were made and air and dew point temperature profiles were measured by wiresonde from the R/V (research vessel) Mysis. Pilot balloon observations at the shoreline and 5 km inland were analyzed together with balloon soundings over the water to depict the diurnal development of the lake's effect on the gradient wind near the shoreline. The low-level wind near the lake was always altered by the prevailing regional wind above the inversion. This lake influence was observed on every sunny day. The major orthogonal gradient winds and their interaction with the lake anticyclones were compared with results from Estoque's numerical model of “sea effect”. The most dramatic effect occurred when a warm offshore wind moved out over a developing inversion above the lake. As the air approached the shoreline it experienced a pronounced upward component of motion, followed by strong subsidence as it moved out over the lake. Parallel-to-shore wind data from Waukegan demonstrated increased vertical motions from the balancing forces of the northerly gradient wind and the lake high over the reverse case of the southerly gradient wind and the lake high. Results under onshore gradient winds, while least dramatic in vertical motion induced by this lake, demonstrated low-level cloud suppression for several tens of kilometers over land downwind.