Large-scale structure of convectively-mixed layer in a shallow ice-covered lake

The paper represents findings based on observational data obtained in shallow Lake Vendyurskoe during early-spring under-ice convection development. The use of highly-precise temperature sensors allowed to quantitatively describe dynamics of the convectively-mixed layer and to estimate its integral parameters. Currents within convectively-mixed layer were registered with Acoustic Doppler Current Profilers. Despite relatively low (mm/s) current velocities, convectivelymixed layer dynamics is presented by the wide spectrum of pulsations typical for the developed turbulent regime. Main attention is paid to the study of low-frequency fluctuations, i. e. large-scale convectively-mixed layer structure. The use of hodographs and progressive vector diagrams allowed revealing a system of convective cells functioning as coherent structures. Observed irregularity of pulsations in the low-frequency part of spectrum does not contradict to the existence of quasi-deterministic cells. Dynamics of such pulsations can be considered as an example of the chaos onset in lowdimensional systems. The cell recognition procedure was based on fitting the experimental progressive vector diagrams to the set of diagrams calculated for the case of idealized cell models. Estimation of the cell parameters is performed in frames of the developed procedure of their identification.

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