Laboratory observations of double-diffusive convection using high-frequency broadband acoustics

Abstract

High-frequency broadband (200-300 kHz) acoustic scattering techniques have been used to observe the diffusive regime of double-diffusive convection in the laboratory. Pulse compression signal processing techniques allow 1) centimetre-scale interface thickness to be rapidly, remotely, and continuously measured, 2) the evolution, and ultimate merging, of multiple interfaces to be observed at high-resolution, and 3) convection cells within the surrounding mixed layers to be observed. The acoustically measured interface thickness, combined with knowledge of the slowly-varying temperatures within the surrounding layers, in turn allows the direct estimation of double-diffusive heat and buoyancy fluxes. The acoustically derived interface thickness, interfacial fluxes and migration rates are shown to support established theory. Acoustic techniques complement traditional laboratory sampling methods and provide enhanced capabilities for observing the diffusive regime of double-diffusion in the ocean.

Semiconvection: theory

“…Layer thickness is therefore a quantity that cannot be discussed independently of the history of the system. The process has not been studied very extensively (but see Wirtz & Reddy 1979, McDougall 1981, Young & Rosner 2000, Ross & Lavery 2009, and the coffee table experiment in ZS13). The layer thickness increases by a process of merging of neighboring layers.…”

Semiconvection: numerical simulations

“…This requires a model for the layer thickness, for which no good theory is available. Observations in laboratory experiments and in geophysical cases show that layer thickness is not constant, but grows in time by processes in the merging of neighbouring layers (McDougall 1981;Ross & Lavery 2009, see also the numerical experiment in Young & Rosner 2000). Layer thickness can therefore not be treated independently of the history of the system.…”

“…The layers slowly merge, either by fading of contrast between neighbouring layers, or the vertical migration of interfaces towards adjacent layers. The details of this process have not been studied much (but see McDougall 1981;Young & Rosner 2000;Ross & Lavery 2009). A plausible estimate of the rate of growth of the layer thickness can be given in terms of the effective solute diffusivity of the system, however (S13, and Sect.…”

Semiconvection: numerical simulations

“…The layers slowly merge, either by fading of contrast between neighboring layers, or the vertical migration of interfaces towards adjacent layers. The details of this process have not been studied much (but see McDougall 1981, Young & Rosner 2000, Ross & Lavery 2009. A plausible estimate of the rate of growth of the layer thickness can be given in terms of the effective solute diffusivity of the system, however (S13, and Sect.…”

“…This requires a model for the layer thickness, for which no good theory is available. Observations in laboratory experiments and in geophysical cases show that layer thickness is not constant, but grows in time by processes of merging of neighboring layers (McDougall 1981, Ross & Lavery 2009, see also the numerical experiment in Young & Rosner 2000). Layer thickness can therefore not be treated independently of the history of the system.…”