Spatial and Temporal Variability During Radiatively-Driven Convection in Lake Superior

Abstract

In dimictic freshwater lakes, a period known as radiatively-driven convection can occur when the surface mixed layer or full water column is below the freshwater temperature of maximum density. During this period, incoming shortwave radiation warms either surface water or water beneath a layer of ice, generating density gradients and convective cells that then descend into the water column. Little is understood about the lateral variability during this period, and most previous studies have considered the process in ice-covered lakes. Using a unique two-dimensional mooring with both horizontal and vertical temperature resolution along with a nearby meteorological buoy and ADCP mooring from a 2019 deployment in Lake Superior, this analysis presents an in depth look at convective cell formation, propagation, and lateral scales. The convective cells observed were found to be dependent on shortwave radiation, wind speed, and thermal expansion coefficient, and to propagate along with background currents. The local time rate of change of temperature at fixed points was calculated and compared to the lateral advective term, which was calculated utilizing both the horizontal resolution of the 2D mooring and currents from the local ADCP; it was discovered that these two terms agreed strongly during periods when the calculation was possible, suggesting that both diffusion and heating of convective cells is insignificant on shorter time scales. This allowed for the application of a frozen field approximation, where convective cells were assumed to remain intact as they traveled. Results from this analysis agreed with correlation analysis results, suggesting horizontal scales of convective cells are on the order of 50 m, with varying structure geometry sometimes producing scales over 200 m. Additionally, vertical structure was found to be similar in nature, suggesting convective cells formed during this period remain intact both as they descend and propagate laterally.