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

Abstract

Abstract 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 /d t = 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 ( ϵ : 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.