Statistics of microstructure patchiness in a stratified lake

Abstract

[1] Statistics of microstructure patches in a sheared, strongly stratified metalimnion of Lake Banyoles (Catalonia, Spain), which occupied ∼40% of the total lake depth of 12 m, are analyzed. Light winds (<3 m s −1 ) dominated the periods of observation in late June and early July of 2009. The patch sizes h p and the corresponding patch Thorpe scales L Tp were identified using profiling measurements of temperature microstructure and small-scale shear. The distribution of h p was found to be lognormal with mean and median values of 0.69 m and 0.50 m respectively. The distribution of L Tp within the patches was also fitted to a lognormal model and the mean and median values found to be close to 0.1 m. The probability distribution of the ratio L Tp /h p was approximated by the Weibull probability model with a shape parameter c w ≈ 2, and also by beta probability distribution. For h p > 0.25 m, the ratio L Tp /h p depends on the patch Richardson and mixing Reynolds numbers following the parameterization of Lozovatsky and Fernando (2002). Analysis of the dynamics of mixing reveals that averaged vertical diffusivities ranged between ∼1 × 10 −4 m 2 s −1 and ∼5 × 10 −5 m 2 s −1, depending on the phase of the internal waves. Episodic wind gusts (wind speed above 6 m s −1 ) transfer ∼1.6% of the wind energy to the metalimnion and ∼0.7% to the hypolimnion, generating large microstructure patches with h p of several meters.Citation: Planella Morato, J., E. Roget, and I. Lozovatsky (2011), Statistics of microstructure patchiness in a stratified lake,

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