Whereas large, advective motions tend to dominate horizontal mixing, entrainment by turbulence is a major contributor to the vertical exchange between layers in the water column. Mixing, in turn, has implications for water quality. For example, vertical mixing is important to the biological balance of the water because the turbulent entrainment that characterizes it determines the distribution of oxygen, nutrients, and micro-organisms. Density stratification in estuaries and the ocean, however, inhibits vertical mixing. Estuaries become stratified by land-derived fresh water interacting with the ocean's salt water.
Stratification can also develop from incident solar radiation heating the upper levels of the water.
The physics of the mixing process in such a stratified medium are not understood completely, and their parameterizations are inadequate. Computer simulation of such mixing processes is one tool that can help provide insight because it can resolve details of fluid motion that cannot be measured in the laboratory or the field. Since the range of scales in a turbulent flow can vary over several orders of magnitude, resolving both the smallest and largest eddies requires a large number of grid points. Calculating the evolution of the turbulent velocity field and scalar concentration at each grid point in a reasonable amount of time requires a supercomputer with large amounts of memory and storage. But, because a DNS is computationally expensive even on a supercomputer, it is limited to simple topography and boundary conditions.