Kinetic energy and anisotropy of  convective turbulence under lake ice

Convective turbulence driven by gravitational instability is a fundamental mixing mechanism in geophysical flows, but in situ estimation of its characteristics is obscured by the background flows and the relatively slow temporal scales. We present characteristics of the full Reynolds tensor from a convective surface boundary layer of an ice-covered lake. The results were obtained by using an original method of measuring the full set of turbulent stresses by a combined use of two ADCPs. The strong horizontal shear stress was revealed as a characteristic feature of free convection differing from the  "conventional"  turbulent boundary flows. The ratio of normal stresses along vertical and horizontal axes remained below 1/4, demonstrating anisotropic character of turbulence asymptotically approaching the axisymmetric two-component “pancake” form. The vertical r.m.s. velocity fluctuations obeyed the buoyancy flux scaling with the coefficient of 1/3, which is at the lower boundary of previously reported values, while horizontal fluctuations followed the same scaling with a unity coefficient