ConvectionâDiffusion Competition Within Mixed Layers of Stratified Natural Waters
In stratified natural waters, convective processes tend to form nearly homogeneous mixed layers. However, shearâdriven turbulence generated by largeâscale background flow often rapidly smooths them through mixing with the stratified surroundings. Here we studied the effect of background turbulence on convectively driven mixed layers for the case of bioconvection in Lake Cadagno, Switzerland. Along with microstructure measurements, a diffusiveâshape model for the mixed layers allowed us to define (i) mixed layer thickness and (ii) diffusive transition length. Further microstructure analysis was performed allowing estimation of convective turbulence in the mixed layer and shearâdriven turbulence quantified by eddy diffusion in their surroundings. Based upon these results, we propose a PĂŠclet number scaling that relates mixed layer shape to the opposing effects of convection and diffusion. We further validate this quantitative approach by applying it to two other distinct convective systems representative of doubleâdiffusive convection and radiatively driven underâice convection.
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