In the atmosphere serve air pollution problems may result from the presence of thermal inversions that inhibit the vertical transport of pollutants. This has important consequences in the design of wastewater and thermal reservoirs. The presence of such an interface, by suppressing fluid turbulence, limits the downward transfer of pollutants and gases such as CO2 and O2 from the upper mixed layer. In environmental fluid mechanics, an understanding of these mixing processes is essential in determining water quality in water bodies containing a sharp density interface. Of particular interest is the mixing in the neighbourhood of relatively sharp density interfaces in the oceans, lakes, reservoirs, and the atmosphere. Turbulent mixing, and especially turbulent mixing in a density stratified fluid, is a difficult problem in geophysical fluid mechanics, as well as in environmental and industrial studies. Turbulence does not invariably occur, however the stratification may be sufficiently great that the Richardson number is large enough everywhere to prevent the onset of dynamical instability. ![]() Turbulence in the oceans can also be generated in more familiar ways, such as in shear zones produced by the confluence of different water messes, particularly in estuaries. Convective motions can develop and, particularly in polar waters, may extend to considerable depths. If, on the other hand, there is substantial surface cooling, there may be a region below the mixed layer in which the density decreases with depth and which is statically unstable. The temperature and salinity in the mixed layer are both virtually uniform as a result of turbulent diffusion, and the continued erosion results in an increasing contrast between the properties of the water in the mixed layer and that immediately below. If the underlying region is statically stable or neutral, the interface between turbulent and non-turbulent fluid is very sharp, and remains so as the turbulence erodes the lower fluid by entrainment. Below the surface, a turbulent mixed layer develops. When the wind blows across the surface of the water, a tangential surface stress is developed both directly from the interfacial stress, and indirectly by the rate of momentum loss from the surface waves by such processes as wave breaking. ![]() ![]() This differentiation is useful, not only conceptually but also observationally since the mechanisms of energy transfer (in both physical and Fourier space) are essentially different. A distinction is drawn between turbulence in a stably stratified fluid on the one hand and random field of internal gravity waves on the other. The characteristic properties of turbulent motions are that they possess a random distribution of vorticity in which there is no unique relation between the frequency and wave number of the Fourier modes that they are diffusive and dissipative. ![]() Not all of the random motions found in the ocean, however, can be described properly as turbulence. Again, the response of the ocean to large-scale wind and thermal disturbances and the development of ocean currents is dependent on the transfer of matter, momentum and energy by irregular smaller scale motions of one kind or another. For example, the interplay between the ocean stratification and the diffusive turbulent motions is often crucial in determining the structure of each. In the ocean, no less than in the atmosphere, is its influence widespread. Turbulence is one of the most ubiquitous phenomena in all of fluid mechanics.
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