Observations of a quasi shear‐free lacustrine convective boundary layer: Stratification and its implications on turbulence

Abstract

Convection due to surface cooling has been investigated in the surface boundary layer of a small lake under near‐ideal conditions undisturbed by wind, large‐scale currents, and differential cooling. Successive temperature microstructure profiles revealed a stable stratification in the lower bulk of the convective boundary layer despite strong cooling over the entire layer. Lateral heat fluxes as well as heat exchange with the thermocline could be excluded as driving mechanism of the observed stratification. Acoustic Doppler current profiler (ADCP) measurements indicated an asymmetric pattern of vertical velocities: strong downward plumes were observed upon a background of slow upflow. We suggest that this asymmetry allows cold water from the super‐adiabatic surface layer to intrude at the base of the convective layer, thus causing the observed stratification. This in particular involves downward plumes that feature a relatively low entrainment rate in the center bulk of the convective layer. Corresponding large‐eddy simulation studies, as well as thermistor string and ADCP data, were found consistent with this scenario. The dissipation rate of turbulent kinetic energy (TKE) was estimated on the basis of the temperature microstructure profiles and from the ADCP data. Mean dissipation rates of only about 20% of the surface buoyancy flux were typically identified. This finding is attributed to respectively low buoyant production rates of TKE, attenuated by strong convective plumes penetrating through the stable stratification in the lower bulk of the convective layer into the thermocline.