Quantification of shear-induced convection and bottom-boundary mixing in natural waters
Final Report Abstract
This project was concerned with the investigation of a newly discovered mixing process (shear-induced convective mixing) in the bottom boundary layer of lakes, and potentially also in other stratified basins. More specifically, the goals were: 1. To estimate the magnitude and the importance of convective mixing in the boundary layer on a slope (experimental approach in Lake Constance). 2. To describe this process in a numerical (turbulence) model and to use this model to generalize the experimental results. What is the basin-scale importance of shear-induced BBL mixing in Lake Constance? 3. To establish a general characterization of shear-induced convection in terms of key parameters like vertical stratification, rotation, cross-slope current velocity, and bottom slope. 4. To verify this general characterization in a case study in the Baltic Sea, emphasizing the role of system rotation. For Goal 1, we have conducted an extensive field study in Lake Constance, including high-resolution turbulent measurements in the BBL. This study has indicated that the process of shear-induced convection, so far identified only once in a small Swiss lake (Lake Alpnach), also occurs in other systems. Using the eddy-correlation technique, we were able to explicitly demonstrate the reversal of the buoyancy flux in the BBL during phases of unstable stratification, consistent with the conceptual picture of the process. The results are described in two publications in peer-reviewed journals. In Goal 2, we have used a three-dimensional numerical lake model combined with a second-moment turbulence closure model for two lakes (Lake Alpnach and Lake Constance) to reproduce available measurements, and to study the basin-scale implications of boundary mixing. Using a special numerical technique, we were able, for the first time, to reproduce the generation of shear-induced convection during upslope flow, in good agreement with the measurements. Basin-scale mixing was found to be dominated by boundary mixing, but the contribution of convective regions was found to be small due to a very low mixing efficiency. Results are described in two papers, recently submitted to a high-rank journal. In Goal 3, we have theoretically investigated the basic mechanisms of boundary mixing. One of the key results was a definition of “mixing” in terms of the smoothing rate of small-scale buoyancy variance by molecular effects. In spite of its relative simplicity, the model was able to reproduce the salient features of boundary-layer mixing, including the generation of unstable, convective regions. Strongest mixing occurred during periods of down-slope flow, when turbulence was weaker compared to upslope flow (convection) but much more efficient. Non-dimensional analysis revealed the existence of only 3 non-dimensional parameters that govern the problem, with the strongest effect attributed to the topographic slope. Boundary mixing was found to be most efficient for relatively steep slopes. Results are discussed in one peer-reviewed journal article. In Goal 4, we have compared interior and boundary-layer mixing processes using data from two cruises conducted in September 2008 and March 2010 in the Bornholm Basin of the Baltic Sea. In both cases, mixing was shown to be tightly connected to near-inertial wave shear. Interior mixing was found to follow a MacKinnon-Gregg scaling originally developed for shelf seas.
Publications
- Physical processes at the sediment-water interface. 11th International Symposium on the Interactions between sediment and water, Esperance, Australia, 2008
Lorke, A.
- Stratification and mixing on sloping boundaries, Geophy. Res. Lett., 35, L14610, 2008
Lorke, A., L. Umlauf, and V. Mohrholz
(See online at https://doi.org/10.1029/2008GL034607) - Shear-induced bottom boundary layer convection in stratified basins: A modeling study. 13th International Workshop on Physical Processes in Natural Waters, Palermo, Italy, 2009
Becherer, J., and L. Umlauf
- Diapycnal transport and mixing efficiency in stratified oscillating boundary layers near sloping topography. EGU General Assembly 2010, Vienna, Austria
Umlauf, L., and H. Burchard
- Observation of interior and boundary-layer mixing processes due to near-inertial waves in a stratified basin without tides, EGU General Assembly 2010, Vienna, Austria
van der Lee, E. M., and L. Umlauf
- Estimating turbulent diffusion in a benthic boundary layer. Limnol. Oceanogr.: Methods, 9, 29-41, 2011
Holtappels, M. and A. Lorke
