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Near-surface turbulence and buoyancy induced by heavy rainfall

Published online by Cambridge University Press:  03 October 2017

E. L. Harrison  and
F. Veron Open the ORCID record for F. Veron [Opens in a new window]
E. L. Harrison
Affiliation:
Naval Surface Warfare Center, Carderock Division, 9500 Macarthur Boulevard, West Bethesda, MD 20817, USA
F. Veron*
Affiliation:
University of Delaware, School of Marine Science and Policy, Newark, DE 19716, USA Université de Bordeaux, I2M, CNRS UMR 5295, 16 avenue Pey-Berland, 33607 Pessac, France
*
†Email address for correspondence: fveron@udel.edu
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Abstract

We present results from experiments designed to measure near-surface turbulence generated by rainfall. Laboratory experiments were performed using artificial rain falling at near-terminal velocity in a wind–wave channel filled with synthetic seawater. In this first series of experiments, no wind was generated and the receiving seawater was initially at rest. Rainfall rates from 40 to $190~\text{mm}~\text{h}^{-1}$ were investigated. Subsurface turbulent velocities of the order of $O(10^{-2})~\text{m}~\text{s}^{-1}$ are generated near the interface below the depth of the cavities generated by the rain drop impacts. The turbulence appears independent of rainfall rates. At depth larger than the size of the cavities, the turbulent velocity fluctuations decay as $z^{-3/2}$. Turbulent length scales also appear to scale with the size of the impact cavities. In these seawater experiments, a freshwater lens is established at the water surface due to the rain. At the highest rain rate studied, the resulting buoyancy flux appears to lead to a shallower subsurface mixed layer and a slight decrease of the turbulent kinetic energy dissipation. Finally, direct measurements and inertial estimates of the turbulent kinetic energy dissipation show that approximately 0.1–0.3 % of the kinetic energy flux from the rain is dissipated in the form of turbulence. This is consistent with existing freshwater measurements and suggests that high levels of dissipation occur at depths and scales smaller than those resolved here and/or that other phenomena dissipate a considerable amount of the total kinetic energy flux provided by rainfall.

JFM classification

Geophysical and Geological Flows: Air/sea interactions Geophysical and Geological Flows: Ocean processes Turbulent Flows: Stratified turbulence
Type
Papers
Information
Journal of Fluid Mechanics , Volume 830 , 10 November 2017 , pp. 602 - 630
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Copyright
© 2017 Cambridge University Press 

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