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Coherent motions and time scales that control heat and mass transfer at wind‐swept water surfaces
Author(s) -
Turney D. E.
Publication year - 2016
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2016jc012139
Subject(s) - mechanics , capillary wave , breaking wave , turbulence , population , wind speed , surface wave , meteorology , physics , atmospheric sciences , wave propagation , optics , demography , sociology
Abstract Forecast of the heat and chemical budgets of lakes, rivers, and oceans requires improved predictive understanding of air‐water interfacial transfer coefficients. Here we present laboratory observations of the coherent motions that occupy the air‐water interface at wind speeds ( U 10 ) 1.1–8.9 m/s. Spatiotemporal near‐surface velocity data and interfacial renewal data are made available by a novel flow tracer method. The relative activity, velocity scales, and time scales of the various coherent interfacial motions are measured, namely for Langmuir circulations, streamwise streaks, nonbreaking wind waves, parasitic capillary waves, nonturbulent breaking wind waves, and turbulence‐generating breaking wind waves. Breaking waves exhibit a sudden jump in streamwise interfacial velocity wherein the velocity jumps up to exceed the wave celerity and destroys nearby parasitic capillary waves. Four distinct hydrodynamic regimes are found to exist between U 10 = 0 and 8.9 m/s, each with a unique population balance of the various coherent motions. The velocity scales, time scales, and population balance of the different coherent motions are input to a first‐principles gas transfer model to explain the waterside transfer coefficient ( k w ) as well as experimental patterns of temperature and gas concentration. The model mixes concepts from surface renewal and divergence theories and requires surface divergence strength ( β ), the Lagrangian residence time inside the upwelling zone (t L u), and the total lifetime of new interface before it is downwelled (t L T). The model's output agrees with time‐averaged measurements k w , patterns of temperature in infrared photographs, and spatial patterns of gas concentration and k w from direct numerical simulations. Several nondimensional parameters, e.g. βt L uandτ st L Twhereτ sis the interfacial shear rate, determine the effectiveness of a particular type of coherent motion for affecting k w .