Statistics of Overturns, APEF, and TKE Dissipation in Stratified Flows

Relations between Thorpe overturn scales, Ozmidov scales, available potential energy fluctuations (APEF), and turbulence kinetic energy (TKE) dissipation rates were examined. For this purpose we have analyzed a large number of vertical profiles collected in the coastal ocean and in a lake using temperature and velocity shear microstructure profilers. For the analysis, data were grouped into three water-column regimes, representing different physical forcing: the surface mixed-layer (SML), the thermocline region, and the bottom boundary layer (BBL). Density overturns were identified using a scheme proposed by Galbraith and Kelly (1996) and initial results indicate a reasonable correspondence between the TKE dissipation rate and the Thorpe displacement throughout the three water-column regimes. Histograms of the APEF (a surrogate for dissipation rate) and the TKE dissipation rate followed the expected lognormal distributions. The Ozmidov scale was found to be consistently larger than the Thorpe scale in the three regimes for both the oceanic and the lake data. However, a tighter relationship was observed between the Thorpe scale and a buoyancy flux related length scale, similar to what has been observed in an earlier study by Dillon (1982). Average decay times for turbulence in the coastal ocean were found to vary roughly by an order of magnitude from regime to regime in the water-column: SML decay times where on the order of 1 min, those in the thermocline region on the order of 10 min, and in the BBL on the order of 2-3 hrs. Estimates of the turbulence decay times in the lake were rather similar throughout the water column with values on the order of a few minutes. Although this result is under further investigation, examination of APEF and TKE dissipation rates for the oceanic case show a decrease by roughly an order of magnitude in TKE dissipation rates in each regime while the APEF varies on average by a factor smaller than 3. However, for the lake a similar decrease in both the APEF and TKE dissipation rate in the three regimes seems to be consistent with the relatively small variation in the estimated decay times.