A Lake Breeze on the Eastern Shore of Lake Michigan: Observations and Model

Abstract

This study was undertaken to determine the physical and dynamical characteristics of the lake breeze wind system at a site on the eastern shore of Lake Michigan. Lake breeze development and progress inland were observed by hourly pilot balloon ascents in a vertical plane perpendicular to the lake shore. The fields of moisture and temperature in this vertical plane on a day when a clearly defined lake breeze occurred were obtained using an instrumented aircraft. Observations were made during periods when prevailing external meteorological conditions would exert minimum influence on the local thermal circulation. Over the land the depth of the layer of onshore flow is approximately 750 m and a maximum velocity of 5–7 m sec⁻¹ is observed within 250 m of the surface directly over the lake shore. Above the lake breeze current a well defined return flow is apparent by midafternoon. The layer of return flow is about twice as deep as the lake breeze and velocities in the return flow axe proportionately lower. The local wind system extends through a depth exceeding 2500 m. Using climatological records it is demonstrated that flow can be in opposite directions on opposite sides of the lake, at least near the surface. Homogeneity of the lake breeze along the shoreline and the orderly progress of the lake breeze front to a distance exceeding 16 km inland are clearly apparent in the data. Evidence suggests that the structure and dimensions of the lake breeze wind system over the water are similar to those over the land. Turning of the local wind in response to the Coriolis force is also demonstrated by the observations. One cell of a symmetric lake breeze has been modelled numerically using the techniques developed by Estoque for the sea breeze with appropriate boundary condition modifications to account for the finite extent of the water surface. The temperature wave over the land was specified in accordance with observed temperatures on days when a lake breeze did occur. Results of the numerical forecast after 6 hr and after 9 hr of meteorological time are compared with the observed lake breeze at corresponding times.