An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes |
| |
Authors: | Michel A J de Nijs Julie D Pietrzak |
| |
Institution: | (1) Environmental Fluid Mechanics Section, Delft University of Technology, PO Box 5048, Stevinweg 1, 2600 GA Delft, the Netherlands |
| |
Abstract: | Measurements of turbulent fluctuations of velocity, salinity, and suspended particulate matter (SPM) are presented. The data
show persistent countergradient buoyancy fluxes. These countergradient fluxes are controlled by the ratio of vertical turbulent
kinetic energy (VKE) and available potential energy (APE) terms in the buoyancy flux equation. The onset of countergradient
fluxes is found to approximately coincide with larger APE than VKE. It is shown here that the ratio of VKE to APE can be written
as the square of a vertical Froude number. This number signifies the onset of the dynamical significance of buoyancy in the
transport of mass. That is when motions driven by buoyancy begin to actively determine the vertical turbulent transport of
mass. Spectral and quadrant analyses show that the occurrence of countergradient fluxes coincides with a change in the relative
importance of turbulent energetic structures and buoyancy-driven motions in the transport of mass. Furthermore, these analyses
show that with increasing salinity-induced Richardson number (Ri), countergradient contributions expand to the larger scales of motions and the relative importance of outward and inward
interactions increases. At the smaller scales, at moderate Ri, the countergradient buoyancy fluxes are physically associated with an asymmetry in transport of fluid parcels by energetic
turbulent motions. At the large scales, at large Ri, the countergradient buoyancy fluxes are physically associated with convective motions induced by buoyancy of incompletely
dispersed fluid parcels which have been transported by energetic motions in the past. Moreover, these convective motions induce
restratification and enhanced settling of SPM. The latter is generally the result of salinity-induced convective motions,
but SPM-induced buoyancy is also found to play a role. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|