Ice-Free Radiative Convection Drives Spring Mixing in a Large Lake

In this work we highlight the importance of radiative convection as a mixing mechanism in a large, ice-free lake (Lake Michigan, USA), where solar heating of waters below the temperature of maximum density drives vertical convection during the vernal turnover. Measurements taken over a 2-week period at a 55-m deep site demonstrate the ability of radiative convection to mix the entire water column. Observations show a diurnal cycle in which solar heating drives a steady deepening of the convective mixed layer throughout the day (dH(CML)/dt = 12.8 m/hr), followed by surface-cooling-induced restratification during the night. Radiative convection is linked to a dramatic enhancement in turbulence characteristics, including both turbulent kinetic energy dissipation (epsilon: 10(-9)-10(-7) W/kg) and turbulent scalar diffusivity (K-z: 10(-3)-10(-1) m(2)/s), suggesting that radiative convection plays a major role in driving vertical mixing throughout the water column during the isothermal spring.