Near-surface turbulence and buoyancy induced by heavy rainfall

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...

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Bibliographic Details
Published in:Journal of fluid mechanics 2017-11, Vol.830, p.602-630
Main Authors: Harrison, E. L., Veron, F.
Format: Article
Language:English
Subjects:
Atmospheric precipitations
Buoyancy
Buoyancy flux
Cavities
Chemical analysis
Energy dissipation
Energy exchange
Energy transfer
Experiments
Fluctuations
Fluids
Flux
Freshwater
Holes
Inland water environment
Kinetic energy
Laboratories
Mixed layer
Rain
Rainfall
Seawater
Studies
Terminal velocity
Turbulence
Velocity
Vortices
Water analysis
Wind
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Summary: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.
ISSN:0022-1120
1469-7645
DOI:10.1017/jfm.2017.602