Abstract
Penetrative convection in the surface layer of a midsize temperate lake (5 km 2 ) was investigated in both summer and winter using a conductivity‐temperature‐depth (CTD) logger mounted on an autonomous underwater vehicle (AUV) flown repeatedly along horizontal transects at selected depths. In summer, the epilimnion cooled differentially during a calm evening (240 and 297 W m −2 on the east and west sides of the lake, respectively). These cooling rates agree well with the average net heat flux of 270 W ths. In summer, the epilimnion cooled differentially during a calm evening (240 and 297 W estimated from meteorological data. Density currents were driven by this differential cooling. In winter, CTD profiles during a sunny day showed four distinct thermal layers beneath the ice (~50 cm thick), consistent with radiative penetrative convection: a stratified diffusive layer just beneath the ice (top 1.6 m); a well‐mixed convective layer (that deepens at 1.14 m d −1 and warms at 0.015°C d −1 during the observation period); an entrainment layer (1.5 m thick); and a weakly stratified quiescent layer (to bottom). AUV transects, flown at constant depths in each layer, revealed a 150‐m wide region displaying evidence of penetrative convection, surrounded by regions with negligible heat changes. These high‐resolution, horizontal CTD measurements provided insight into previously unresolved physical dynamics of the well‐mixed layer of a temperate lake in quasi‐shear‐free conditions that would have been difficult to quantify during summer months and impossible under winter ice cover without the use of an AUV platform.