Turbulence Characteristics of Ice-Free Radiatively Driven Convection in a Deep, Unstratified Lake

Abstract

This study examines data collected with an autonomous underwater glider during a period of vigorous radiatively driven convection (RDC) and low winds in deep, unstratified Lake Superior. Conductivity, temperature and depth (CTD) measurements reveal distinct convective plumes of warm downwelling water with temperature anomalies of $\sim 0.1°$C and width scales on the order of $10-100$ m, consistent with theoretical scalings for the unstratified convective regime. Shear and temperature microstructure measurements indicate turbulent kinetic energy (TKE) dissipation $\left(\epsilon \right)$ and temperature variance dissipation rates $\left({\chi }_T\right)$ orders of magnitude greater in thermal plumes than laterally adjacent waters. Decay timescales of $\epsilon$ indicate highly efficient mixing is sustained throughout the night. Energetics, mixing efficiency, and constraints on convective plume scales are also discussed. These observations demonstrate that RDC can dominate vertical mixing dynamics even in deep ice-free systems, and these systems can serve as a real-scale laboratory for investigation of convective dynamics.