The diffusive regime of double-diffusive convection is reviewed, with a particular focus on issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not easily represented in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment.

Turbulence generated by vertical oscillations of a horizontal monoplanar grid in homogeneous water contained in a tank was used to study certain properties of nearly isotropic turbulence. The cases of sustained oscillations and the removal of forcing after a period of oscillations were considered. The former was used to evaluate the Eulerian-frequency spectra of nearly isotropic turbulence, and the velocity-time power law and spectral behavior of decaying turbulence were studied using the latter. The experimental observations are compared with available theoretical formulations as well as previous experimental observations.

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A simple model to study the decay of turbulent kinetic energy (TKE) in the convective surface layer is presented. In this model, the TKE is dependent upon two terms, the turbulent dissipation rate and the surface buoyancy fluctuations. The time evolution of the surface sensible heat flux is modelled based on fitting functions of actual measurements from the LITFASS-2003 field campaign. These fitting functions carry an amplitude and a time scale. With this approach, the sensible heat flux can be estimated without having to solve the entire surface energy balance. The period of interest covers two characteristic transition subperiods involved in the decay of convective boundary-layer turbulence. The first sub-period is the afternoon transition, when the sensible heat flux starts to decrease in response to the reduction in solar radiation. It is typically associated with a decay rate of TKE of approximately t −2 (t is time following the start of the decay) after several convective eddy turnover times. The early evening transition is the second sub-period, typically just before sunset when the surface sensible heat flux becomes negative. This sub-period is characterized by an abrupt decay in TKE associated with the rapid collapse of turbulence. Overall, the results presented show a significant improvement of the modelled TKE decay when compared to the often applied assumption of a sensible heat flux decreasing instantaneously or with a very short forcing time scale. In addition, for atmospheric modelling studies, it is suggested that the afternoon and early evening decay of sensible heat flux be modelled as a complementary error function.

This paper describes a laboratory study on the evolution of a point turbulent plume placed at the free surface of a homogeneous fluid layer in the presence of background rotation. It is shown that the plume initially evolves as if there is no rotation. However, the rotational effects become important after the plume descends a vertical distance hc1≈3.3(B/Ω3)1/4 for a normalized time Ωtc1≈2.4, whence the vertical descent rate of the plume is reduced while maintaining approximately the same lateral growth rate. Here Ω is the rate of background rotation and B is the specific buoyancy flux of the plume. The rotational effects inhibit the lateral growth of the plume at a time Ωtc2≈5.5, when the maximum plume width is bc≈1.4(B/Ω3)1/4. Thereafter, the vertical descent continues and the plume evolves into a cylindrical shape while developing a cyclonic circulation in and around it, except near the plume front. Upon reaching the bottom surface after traveling a fluid depth of H, the plume deflects, propagates horizontally, and becomes unstable breaking up into anticyclonic eddies. Studies carried out for the case of H<hc1 show that this instability is initiated at a horizontal length scale proportional to the Rossby deformation radius of the deflected flow, and hence it is of baroclinic type. These eddies appear to align vertically with the cyclonic eddies formed by the barotropic instability of the surface rim current, thus producing heton-like structures. The influence of the diameter d0 of the plume on the flow evolution is also studied, and it is shown that plumes with aspect ratio h/d0<12 (where h is the vertical extent) can be approximated as point plumes. Scaling arguments are advanced to explain the results. Some geophysical applications of the study are also discussed.