Note on the bulk estimate of the energy dissipation rate in the bottom boundary layer

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Authors

Xiaozhou Ruan

Abstract

The dissipation of the kinetic energy (KE) associated with oceanic flows is believed to occur primarily in the oceanic bottom boundary layer (BBL) where bottom drag converts the KE from mean flows to heat loss through irreversible mixing at molecular scales. Due to the practical difficulties associated with direct observations on small-scale turbulence close to the seafloor, most up-to-date estimates on bottom drag rely on a simple bulk formula (CdU3) proposed by G.I. Taylor that relates the integrated BBL dissipation rate to a drag coefficient (Cd) as well as a flow magnitude outside of the BBL (U). Using output from several turbulence-resolving Direct Numerical Simulations, it is shown that the true BBL-integrated dissipation rate is about 90% of that estimated using the classic bulk formula, applied here to the simplest scenario where a mean flow is present over a flat and hydrodynamically-smooth bottom. It is further argued that Taylor's formula only provides an upper bound estimate and should be applied with caution in future quantification of BBL dissipation; the performance of the bulk formula depends on the distribution of velocity and shear stress near the bottom, which in the real ocean, could be disrupted by bottom roughness.

DOI

https://doi.org/10.31223/X5090H

Subjects

Oceanography, Physical Sciences and Mathematics

Keywords

Geophysical fluid dynamics, Physical oceanography, Bottom boundary layers

Dates

Published: 2021-04-26 13:53

Last Updated: 2021-04-26 20:53

License

CC BY Attribution 4.0 International

Additional Metadata

Conflict of interest statement:
None

Data Availability (Reason not available):
Data source is provided in the manuscript.

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