Open AccessMixing, Dissipation Rate, and Their Overturn-Based Estimates in a Near-Bottom Turbulent Flow Driven by Internal Tides
Author(s) -
Vamsi K. Chalamalla,
Sutanu Sarkar
Publication year - 2015
Publication title -
journal of physical oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.706
H-Index - 143
eISSN - 1520-0485
pISSN - 0022-3670
DOI - 10.1175/jpo-d-14-0057.1
Subject(s) - dissipation , convection , thermal diffusivity , turbulence , eddy diffusion , mechanics , stratification (seeds) , mixing (physics) , convective mixing , maxima , scalar (mathematics) , diffusion , physics , thermodynamics , atmospheric sciences , meteorology , geometry , mathematics , seed dormancy , art , germination , botany , quantum mechanics , dormancy , biology , performance art , art history
Direct numerical simulation (DNS) and large-eddy simulation (LES) are employed to study the mixing brought about by convective overturns in a stratified, oscillatory bottom layer underneath internal tides. The phasing of turbulence, the onset and breakdown of convective overturns, and the pathway to irreversible mixing are quantified. Mixing efficiency shows a systematic dependence on tidal phase, and during the breakdown of large convective overturns it is approximately 0.6, a value that is substantially larger than the commonly assumed value of 0.2 used for calculating scalar mixing from the turbulent dissipation rate. Diapycnal diffusivity is calculated using the irreversible diapycnal flux and, for tall overturns of O (50) m, the diffusivity is found to be almost 1000 times higher than the molecular diffusivity. The Thorpe (overturn) length scale is often used as a proxy for the Ozmidov length scale and thus infers the turbulent dissipation rate from overturns. The accuracy of overturn-based estimates of the dissipation rate is assessed for this flow. The Ozmidov length scale L O and Thorpe length scale L T are found to behave differently during a tidal cycle: L T decreases during the convective instability, while L O increases; there is a significant phase lag between the maxima of L T and L O ; and finally L T is not linearly related to L O . Thus, the Thorpe-inferred dissipation rates are quite different from the actual values. Interestingly, the ratio of their cycle-averaged values is found to be O (1), a result explained on the basis of available potential energy.