Author(s): Georgiy Kirillin; Ilya Aslamov; Vladimir Kozlov; Nikolay Granin
Linked Author(s): Georgiy Kirillin
Keywords: No keywords
Abstract: Ice cover on polar and temperate lakes creates seasonally developing boundary layer at the ice base with specific features: fixed temperature at the solid boundary and stable density stratification beneath. Turbulent transport in the boundary layer determine the ice growth and melting conditions at the ice-water interface, especially in large lakes and marginal seas, where large-scale water circulation can produce highly variable mixing conditions. Since the boundary mixing under ice is difficult to measure, existing models of ice cover dynamics usually neglect or parameterize it in a very simplistic form. We present first detailed observations on mixing under ice of Lake Baikal, obtained with the help of advanced acoustic methods. The dissipation rate of the turbulent kinetic energy (TKE) was derived from spectra of velocity fluctuations at a single point under ice, and from correlations (structure functions) of current velocities along vertical profiles in the boundary layer. The range of the dissipation rate variability covered 2 orders of magnitude. Intensity of mixing was closely connected to mean speeds of the under-ice currents, the latter being of geostrophic origin and having lake-wide scales. Turbulence directly affected heat flux at the ice-water interface and, by this, the ice growth and melting rate. To quantify these effects, we develop a parameterization of the water-ice heat flux based on the Kolmogorov microscale involving the dissipation rate of TKE. The parameterization describes well the thickness of the viscous sub-layer at the ice base, and the conductive heat flux across it, determined from the fine-scale temperature measurements in the boundary layer. The parameterization can be implemented in coupled lake-ice models, which typically use TKE dissipation rate as a prognostic variable.
Year: 2018