Inclusion of surface gravity wave effects in vertical mixing parameterizations with application to Chesapeake Bay, USA

Alexander William Fisher, UMCES Horn Point Laboratory, Cambridge, MD, United States, Lawrence P Sanford, University of Maryland Center for Environmental Science Appalachian Laboratory, Frostburg, MD, United States, Malcolm E Scully, Woods Hole Oceanographic Institution, Woods Hole, MA, United States and Steven E Suttles, U.S. Geological Survey, Woods Hole, MA, United States
Abstract:
Enhancement of wind-driven mixing by Langmuir turbulence (LT) may have important

implications for exchanges of mass and momentum in estuarine and coastal waters, but

the transient nature of LT and observational constraints make quantifying its impact on

vertical exchange difficult. Recent studies have shown that wind events can be of first

order importance to circulation and mixing in estuaries, prompting this investigation into

the ability of second-moment turbulence closure schemes to model wind-wave enhanced

mixing in an estuarine environment.

An instrumented turbulence tower was deployed in middle reaches of Chesapeake Bay in

2013 and collected observations of coherent structures consistent with LT that occurred

under regions of breaking waves. Wave and turbulence measurements collected from a

vertical array of Acoustic Doppler Velocimeters (ADVs) provided direct estimates of

TKE, dissipation, turbulent length scale, and the surface wave field. Direct measurements

of air-sea momentum and sensible heat fluxes were collected by a co-located ultrasonic

anemometer deployed ~3m above the water surface. Analyses of the data indicate that the

combined presence of breaking waves and LT significantly influences air-sea momentum

transfer, enhancing vertical mixing and acting to align stress in the surface mixed layer in

the direction of Lagrangian shear.

Here these observations are compared to the predictions of commonly used second-moment

turbulence closures schemes, modified to account for the influence of wave

breaking and LT. LT parameterizations are evaluated under neutrally stratified

conditions and buoyancy damping parameterizations are evaluated under stably stratified

conditions. We compare predicted turbulent quantities to observations for a variety of

wind, wave, and stratification conditions. The effects of fetch-limited wave growth,

surface buoyancy flux, and tidal distortion on wave mixing parameterizations will also be

discussed.

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