Microstructure measurements and heat flux calculations of a triple-diffusive process in a lake within the diffusive layer convection regime

摘要

Microstructure measurements of a triple-diffusive staircase with a stability ratio of 1.1 are presented. Data were recorded at Lake Banyoles, a small lake in Catalonia, Spain, with a warm, salty, and turbid underground inflow. Turbulent scales are well resolved in the two observed convective layers and allow determination of the dissipation rates of the turbulent kinetic energy, ε, and of the turbulent temperature fluctuations, χ, which are found to be 3.3 × 10-7 C2/s and 2.7 × 10-9 W/kg for one of the layers and 5.9 × 10-7 C2/s and 3.8 × 10-9 W/kg for the other. Thermal spectra for the convective layers look universal in viscous-diffusive and viscous-convective subranges. Characteristic vertical displacements within convective layers could also be obtained on the basis of Thorpe scales and were found to be 0.3 times the layer thickness. Vertical convective fluctuations were estimated and found to be of the order of 10-4 m/s. When turbulent fluxes were determined within the convective layers on the basis of microstructure data and compared to those at the diffusive interfaces, they showed a stationary state with a mean thermal flux of 3.5 × 10-6 C m/s. A comparison of experimental heat fluxes to different models favors the scaling model of Grossman and Lohse (2000) for Rayleigh-Bénard convection and the double-diffusive convection model of Fernando (1979a, 1979b) for low stability. If the 4/3 power law is assumed, then the parameterization proposed by Taylor (1988) for diffusive interfaces at low values of the density ratio is also in accordance with our data.