Radar observations of mesoscale circulations induced by a small lake under varying synoptic‐scale flows

Abstract

This paper examines Doppler radar observations of springtime lake breeze (LB) circulations produced by a small reservoir (mean width of ∼2 km), Wheeler Lake, on the Tennessee River in northern Alabama. The analysis allows for the formation of a general understanding of the flow structures as a composite of many LB circulation events. It is found that LB circulation, once formed, is robust and persistent over a range of background or synoptic‐scale wind flows despite the small size of Wheeler Lake and the relatively small differences between land and lake temperature (often a few degrees Celsius) that drive the LB circulation. The formation, location, strength, and inland penetration of the Wheeler Lake breeze are highly sensitive to the background wind speed and direction with respect to the lakeshore, resulting in complex spatial and temporal variations in the LB characteristics. The LB front is most prominent as a radar fine line when the ambient flow is offshore (opposing the LB background wind) and perpendicular to the orientation (120°–300°) of the lake. For wind speeds <4–5 m s−1, a radar‐detectable thin line delineating the LB front appears primarily on the upwind side of the lake, either to the north or south, while synoptic‐scale winds stronger than ∼5 m s−1 tend to destroy the breeze front entirely. If the background wind is parallel to lake orientation and the wind is ≤4 m s−1, the LB circulations often develop on both sides of the lake. The radar fine line often appears patchy and broad for low‐wind conditions near 2 m s−1. Overall, the findings of this study show that small lakes can generate persistent local circulations that follow similar patterns of behavior and respond to the synoptic flow as large lakes or sea breezes.