Characterizing the spatial and temporal scales of convective cells during radiatively-driven convection in a deep lake

Due to the existence of the temperature of maximum density for freshwater (3.98C), springtime shortwave heating of a freshwater lake can result in radiatively-driven convection, where heating at the surface of the lake makes surface waters denser, resulting in full water-column convection. In a deep lake like Lake Superior, this process can dominate the circulation of the lake for two to three months every year, and controls the vertical redistribution of physical, biological, and geochemical properties in the lake during this period. Characteristic features of the convection cells produced are discerned from a set of observations made in Lake Superior over the last several years, from autonomous underwater gliders, turbulence profilers, and moorings, including a large, two-point mooring that supported a two-dimensional array of 48 thermistors 150m on a side. Statically unstable temperature anomalies on the order of 0.1K build up during the day, and the water column fully homogenizes each night. Glider data suggests that warm water produced near the surface convect into the lake in chimney-like structures with lateral scales on the order of 10m, and the cells themselves have lateral scales on the order of 100m. Directly observed vertical velocities are on the order of 1cm/s. Positive temperature anomalies are observed to transit the length of the horizontal mooring frequently during daylight hours. Temperatures at the bottom of the lake (180m) tend to start to increase roughly 6 hours after sunup, also suggesting vertical velocities on the order of 1cm/s. Developing a better understanding of the dynamics of this convective process is essential to our understanding of the annual development of deep, dimictic lakes.