A Comparison of Methods for Estimating Reynolds Stress from ADCP Measurements in Wavy Environments

Abstract

Turbulent Reynolds stresses are now routinely estimated from acoustic Doppler current profiler (ADCP) measurements in estuaries and tidal channels using the variance method, yet biases due to surface gravity waves limit its use in the coastal ocean. Recent modifications to this method, including spatially filtering velocities to isolate the turbulence from wave velocities and fitting a cospectral model to the below-wave band cospectra, have been used to remove this bias. Individually, each modification performed well for the published test datasets, but a comparative analysis over the range of conditions in the coastal ocean has not yet been performed. This work uses ADCP velocity measurements from five previously published coastal ocean and estuarine datasets, which span a range of wave and current conditions as well as instrument configurations, to directly compare methods for estimating stresses in the presence of waves. The computed stresses from each were compared to bottom stress estimates from a quadratic drag law and, where available, estimates of wind stress. These comparisons, along with an analysis of the cospectra, indicated that spectral fitting performs well when the wave climate is wide-banded and/or multidirectional as well as when instrument noise is high. In contrast, spatial filtering performs better when waves are narrow-banded, low frequency, and when wave orbital velocities are strong relative to currents. However, as spatial filtering uses vertically separated velocity bins to remove the wave bias, spectral fitting is able to resolve stresses over a larger fraction of the water column.

An Alongshore Momentum Budget Over a Fringing Tropical Fore‐Reef

“…(The confidence intervals were computed by bootstrapping, which sometimes result in asymmetric uncertainties.) As mentioned previously, drag coefficient values from the log fits at 12mS were confirmed with two additional estimates using cospectral fits applied to velocities from both the ADV (Gerbi et al, 2008) and the ADCP (Kirincich & Rosman, 2011;Kirincich et al, 2010).…”

Spatial and temporal variation in the hydrodynamic landscape in intertidal salt marsh systems

“…[A1] It is important to separate wave-induced motion from measurements of velocity when calculating turbulence parameters in coastal and other environments (e.g., Trowbridge 1998, Kirincich andRosman 2011). The objective of the analysis below is to employ a linear filtration technique (Benilov and Filyushkin 1970) to decompose the wave and turbulence components of the covariance and kinetic energy of the fluctuating velocity.…”

Drivers of spring and summer variability in the coastal ocean offshore of Cape Cod, MA

“…The vertical divergences of the horizontal Reynolds stresses are a critical component of the dynamics present in coastal flows as they act to redistribute the input or removal of momentum by boundary forces throughout the water column. Recent efforts have documented how ADCP‐based velocity observations can be used to estimate the horizontal Reynolds stresses [ Stacey et al ., , ; Rosman et al ., ; Kirincich et al ., ; Kirincich and Rosman , ]. Previous studies of coastal dynamics have examined the frictional components of circulation via the depth‐integrated momentum balances [e.g., Allen , ; Lentz et al ., ], where the effects of this momentum transfer is accounted for by the difference between the surface and bottom stresses using drag laws.…”

“…The observational effort, carried out in the spring of 2010 and the spring and summer of 2011 utilized across-shelf hydrographic surveys and the deployment of a series of moorings designed to provide estimates of water column hydrography and velocity as well as enable estimates of turbulent Reynolds stresses. be used to estimate the horizontal Reynolds stresses [Stacey et al, 1999a[Stacey et al, , 1999bRosman et al, 2008;Kirincich and Rosman, 2010]. Previous studies of coastal dynamics have examined the frictional components of circulation via the depthintegrated momentum balances [e.g., Allen, 1980;Lentz et al, 1999], where the effects of this momentum transfer is accounted for by the difference between the surface and bottom stresses using drag laws.…”