Institut für Ostseeforschung Warnemünde
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Dynamics of rotating gravity currents

Density and turbulence structure in a channelized gravity current
Density and turbulence structure in a gravity current

Gravity currents in rotating reference frames are a facinating and, due to their importance for the ocean general circulation,intensively studied topic. Gravity currents in the virtually tideless Baltic Sea occur at much shallower depths and exhibit smaller spatial scales than their large-scale counterparts in the deep ocean. Nevertheless, they share many features with these large-scale overflows, e.g. the high Reynolds numbers, strong rotational effects, and subcritical Froude numbers. The smaller spatial scales make the gravity currents in the Baltic more easily accessible for high-resolution measurements of hydrographic and turbulence parameters.We exploit this advantage, and use the Baltic Sea as a natural laboratory for deriving generally applicable models of rotating bottom gravity currents (Umlauf and Arneborg 2009a,b, Arneborg et al., 2007).

As an example for the available data sets, the figure to the right shows a complete microstructure and CTD transect across a gravity current passing through an approximately 10 km wide channel in the Western Baltic. The high sampling density (more than 70 profiles for this figure) allows us to compute detailed transects of turbulence parameters across the whole width of the gravity current, from which the essential non-dimensional numbers describing rotating gravity currents can be derived: the Froude number, the Ekman number, the entraiment rate, the drag coefficient, etc). Clearly visible in the density structure shown in the figure (black contour lines) is also the characteristic pinching and spreading of the pycnocline, an effect resulting from the transverse transport inside the interface as analyzed by Umlauf and Arneborg (2009a,b) and Umlauf et al., (2007).

 

Tracer Release Experiment in the Baltic (BaTRE)

Image of Gotland Basin and tracer distribution
Eastern Gotland Basin (left) with tracer and CTD profiles

One of the outstanding questions for the Baltic Sea ecosystem is how physical and bio-geochemical properties of the deep layers communicate with the surface mixed layer where production takes place. In a joint project of IOW and IFM-GEOMAR (Kiel) these mixing process are currently investigated in the framework of the Baltic Sea Tracer Release Experiment (BaTRE), conducted in 2007-2010.

In collaboration with the tracer research group of Jim Ledwell (WHOI),  a new type of tracer, SF5CF3, was injected in September 2007 into the deep waters of the Eastern Gotland Basin with the help of the new Oceanic Tracer Injection System (OTIS) built for this project  (see Umlauf et al. 2008). The figure to the right shows the study area, the vertical distribution of the tracer (markers in highlighted area), and some hydrographic parameters as measured approximately two weeks after the injection.

The experiment is accompagnied by extensive turbulence measurments in order to obtain direct mixing estimates for comparision with the basin-scale mixing inferred from the vertical spreading of the tracer cloud. Combined with additional moored instrumentation (ADCPs, current meters, CTD loggers), a coherent data set will be available for the identification of processes responsible for the overall mixing. 

 

Internal wave mixing

Internal waves in the Bornholm Basin
Internal waves in the Bornholm Basin

The "International Graduate School for Internal Waves in the Atmosphere and Ocean" (ILWAO)  is a joint research initiative of several German research instiutes. The goal is to understand the role of internal waves for mixing and transport in an interdisciplinary approach, including internal waves in the atmosphere, in the ocean, in the laboratory, and in plasmas.

The oceanic part carried out by IOW includes two PhD projects working on the field and numerical research program, respectively. The field work supervised by me focuses on the contribution  of near-inertial waves (see figures) for boundary mixing, and on the behavior of high-frequency internal waves near the slopes. Our study areas are the Bornholm Basin and the Stolpe Channel in the southern part of the Baltic Sea. The field program combines high-resolution moored instrumentation with densely-spaced microstructure and velocity transects across the sloping topography of the basin.

 

Convection in Bottom Boundary Layers

Schematic view of shear-induced BBL convection
Schematic view of shear-induced BBL convection

In a joint project with the University of Konstanz (Germany) and the EAWAG (Switzerland), we investigate the dynamics of gravitationally unstable bottom boundary layers in lakes of different sizes, and in the Baltic Sea. Such unstable bottom boundary layers occur if, as illustrated in the figure, internal wave motions (here due to internal seiching) move stratified fluid up the slope. The near-bottom shear created by these motions favours the advection of dense fluid above less dense fluid. Under these circumstances, convection forms an additional energy source for turbulence that may substantially modify turbulence and mixing in the bottom boundary layer (Lorke et al. 2008).Conversely, if water is moved down the slope, the near-bottom shear has a tendency for the creation of stable stratification. This process is similar to the periodic creation of unstable and stable stratification due to 'tidal straining' in regions of fresh-water influence (ROFIs) on the continental shelf.

If the scales are large enough, the boundary layer dynamics is affected by rotation. The physics of this process will be studied with data from several cruises conducted in 2008-2010 in the Baltic Sea, and with the help of an extensive modeling program.

 

Turbulence Modeling

My focus in this area of research is the development and testing of turbulence models for stratified turbulent flows. The class of models I am mainly interested in are so-called one-point turbulence closure models, in particular second-moment closures.

Among the models recently developed by our group is a version of the k-ω model for stratified flows (Umlauf et al., 2003), and a generic length scale model (GLM) from which almost all traditional models (e.g. the k-ε model and the Mellor-Yamada model) can be recovered as special cases (Umlauf and Burchard, 2003). More information about these and related models can be found in the review article by Umlauf and Burchard (2005).

The turbulence models developed by us and other groups are implemented in our public domain turbulence library GOTM (Umlauf et al, 2005). Either via an interface to GOTM or as stand-alone versions, these turbulence models have been implemented in a number of three-dimensional ocean models (ROMS, OPA, MOM4, POLCOMS, GETM, etc.). 

For more information about GOTM and our three-dimensional circulation model GETM, check out our web sites at www.gotm.net and www.getm.eu