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Title: How we compute N matters to estimates of mixing in stratified flows

We know that most commonly used models for turbulent mixing in the ocean rely on a background stratification against which turbulence must work to stir the fluid. While this background stratification is typically well defined in idealized numerical models, it is more difficult to capture in observations. Here, a potential discrepancy in ocean mixing estimates due to the chosen calculation of the background stratification is explored using direct numerical simulation data of breaking internal waves on slopes. There are two different methods for computing the buoyancy frequency$N$$, one based on a three-dimensionally sorted density field (often used in numerical models) and the other based on locally sorted vertical density profiles (often used in the field), are used to quantify the effect of$$N$$on turbulence quantities. It is shown that how$$N$$is calculated changes not only the flux Richardson number$$R_{f}$$, which is often used to parameterize turbulent mixing, but also the turbulence activity number or the Gibson number$$Gi$$, leading to potential errors in estimates of the mixing efficiency using$$Gi$-based parameterizations.
Authors:
ORCiD logo [1] ; ORCiD logo [2] ;  [3] ;  [3]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  2. Colorado State Univ., Fort Collins, CO (United States). Dept. of Civil and Environmental Engineering; Stanford Univ., CA (United States). Bob and Normal Street Environmental Fluid Mechanics Lab., Dept. of Civil and Environmental Engineering
  3. Stanford Univ., CA (United States). Bob and Normal Street Environmental Fluid Mechanics Lab., Dept. of Civil and Environmental Engineering
Publication Date:
Report Number(s):
LLNL-JRNL-733364
Journal ID: ISSN 0022-1120; applab; TRN: US1800685
Grant/Contract Number:
AC52-07NA27344
Type:
Accepted Manuscript
Journal Name:
Journal of Fluid Mechanics
Additional Journal Information:
Journal Volume: 831; Journal ID: ISSN 0022-1120
Publisher:
Cambridge University Press
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
42 ENGINEERING; 58 GEOSCIENCES; 97 MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; internal waves; stratified turbulence; turbulent mixing
OSTI Identifier:
1414351

Arthur, Robert S., Venayagamoorthy, Subhas K., Koseff, Jeffrey R., and Fringer, Oliver B.. How we compute N matters to estimates of mixing in stratified flows. United States: N. p., Web. doi:10.1017/jfm.2017.679.
Arthur, Robert S., Venayagamoorthy, Subhas K., Koseff, Jeffrey R., & Fringer, Oliver B.. How we compute N matters to estimates of mixing in stratified flows. United States. doi:10.1017/jfm.2017.679.
Arthur, Robert S., Venayagamoorthy, Subhas K., Koseff, Jeffrey R., and Fringer, Oliver B.. 2017. "How we compute N matters to estimates of mixing in stratified flows". United States. doi:10.1017/jfm.2017.679. https://www.osti.gov/servlets/purl/1414351.
@article{osti_1414351,
title = {How we compute N matters to estimates of mixing in stratified flows},
author = {Arthur, Robert S. and Venayagamoorthy, Subhas K. and Koseff, Jeffrey R. and Fringer, Oliver B.},
abstractNote = {We know that most commonly used models for turbulent mixing in the ocean rely on a background stratification against which turbulence must work to stir the fluid. While this background stratification is typically well defined in idealized numerical models, it is more difficult to capture in observations. Here, a potential discrepancy in ocean mixing estimates due to the chosen calculation of the background stratification is explored using direct numerical simulation data of breaking internal waves on slopes. There are two different methods for computing the buoyancy frequency$N$, one based on a three-dimensionally sorted density field (often used in numerical models) and the other based on locally sorted vertical density profiles (often used in the field), are used to quantify the effect of$N$on turbulence quantities. It is shown that how$N$is calculated changes not only the flux Richardson number$R_{f}$, which is often used to parameterize turbulent mixing, but also the turbulence activity number or the Gibson number$Gi$, leading to potential errors in estimates of the mixing efficiency using$Gi$-based parameterizations.},
doi = {10.1017/jfm.2017.679},
journal = {Journal of Fluid Mechanics},
number = ,
volume = 831,
place = {United States},
year = {2017},
month = {10}
}