Observations of radiatively driven convection plumes in a deep, unstratified, ice-free lake

Radiatively-driven convection is a physical process that occurs in freshwater below the temperature of maximum density wherein volumetric heating of surface waters by solar radiation creates a diurnal, spatially distributed, destabilizing buoyancy flux that drives penetrative convection. While this process has typically been studied under ice-covered conditions, it can also occur in open water during springtime warming leading up to overturn, and in such systems, it may serve as the dominant process driving mixing of nutrients and biota. Despite the ecological significance and unique physical dynamics of radiatively-driven convection, little is understood regarding the spatial heterogeneity and three-dimensional structure of the process. The addition of wind shear also modifies radiatively-driven convection dynamics in open water conditions, yet observations have not yet been used to quantify the relative scales and importance of these separate forcings in driving mixing and turbulence. This study examines data collected with a buoyancy-driven autonomous underwater vehicle (aka glider) during a period of active radiatively-driven convection and low surface wind shear in early springtime in Lake Superior. Conductivity, temperature and depth (CTD) measurements reveal distinct convective plumes of anomalously warm downwelling water with width scales on the order of 100 m and temperature anomalies of ~0.1 °C. Shear and temperature microstructure measurements indicate turbulence kinetic energy (TKE) dissipation rates exceeding 10-8 W/kg, orders of magnitude greater than laterally adjacent waters. This is the first known observation of lateral variability in TKE dissipation rates during radiatively-driven convection. Spatially and temporally averaged TKE budgets illustrate buildup, vertical transport, and dissipation of TKE, while the ~3 hr lag between buoyancy forcing and dissipation is consistent with the Deardorff convective timescale. These observations demonstrate that radiatively-driven convection can dominate vertical mixing dynamics even in deep, open water systems.