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 hp and the corresponding patch Thorpe scales LTp were identified using profiling measurements of temperature microstructure and small-scale shear. The distribution of hp was found to be lognormal with mean and median values of 0.69 m and 0.50 m respectively. The distribution of LTp 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 LTp/hp was approximated by the Weibull probability model with a shape parameter cw ≈ 2, and also by beta probability distribution. For hp > 0.25 m, the ratio LTp/hp 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 m2 s−1 and ∼5 × 10−5 m2 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 hp of several meters