Direct estimation of buoyancy flux in the lacustrine and marine environment

The vertical transport of properties (e.g. nutrients, pollutants, algae, etc) by turbulence in estuaries and lakes is a key driver of local and larger scale biogeochemical cycles. On the global scale, understanding of vertical mixing is essential for the ability of global climate models to accurately predict future scenarios. Whilst a major goal in oceanography is to be able to accurately represent turbulence and mixing in numerical computer models, there is still a fundamental gap in our knowledge of the relevant processes. Presently, investigators can measure with reasonable certainty the rate at which the mean water flow loses energy to turbulence on a large scale, and the rate at which turbulence loses energy to heat by friction at the millimeter scale. In a stratified environment, turbulent energy is also lost to 'buoyancy flux' - the energy required to mix denser water from lower in the water column against gravity into lighter water above. The rate of energy used by this vertical flux of density is currently assumed to be a certain fraction of the turbulent energy lost to friction. This fraction has been established and refined empirically by numerous experiments, although an exact figure has not yet been agreed upon. My goal of this fellowship is to use 3 collocated instruments to directly measure the buoyancy flux, and compare the results to estimates of the buoyancy flux made using the methods outlined above. The three instruments are; (i) an acoustic Doppler velocimeter (ADV), which makes accurate and rapid velocity measurements at a single point, (ii) a fast response thermistor and (iii) a fast response conductivity meter. The ADV will measure the turbulent vertical velocity fluctuations at a point in space, while the thermistor and conductivity meter will measure the density of the fluid passing through the measured 'cell' of water. The turbulent motions exchange dense water from below and lighter water from above. By correlating the vertical water motions and water density, the vertical buoyancy flux can be determined. These measurements of buoyancy flux will be combined with existing instrumentation used to measure the production and dissipation of turbulence. A complete balance of the energy terms (the energy going into turbulence at the large scale, the energy lost from turbulence as friction at the micro scale and the energy lost to buoyancy flux in the scale between) will be attempted. This will be compared against the state of the art turbulence models used in numerical simulations. The models will be tested and adjusted against the measurements made. Refinements of turbulence models will assist in global, estuarine and lake models.