Gregory P. Gerbi

Observations of Turbulence in the Ocean Surface Boundary Layer: Energetics and Transport

Observations of turbulent kinetic energy (TKE) dynamics in the ocean surface boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean surface boundary layer than it would be in a flow past a rigid boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean surface boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-boundary boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.

Measurements of Momentum and Heat Transfer across the Air–Sea Interface

Abstract This study makes direct measurements of turbulent fluxes in the mixed layer in order to close heat and momentum budgets across the air–sea interface and to assess the ability of rigid-boundary turbulence models to predict mean vertical gradients beneath the ocean’s wavy surface. Observations were made at 20 Hz at nominal depths of 2.2 and 1.7 m in ∼16 m of water. A new method is developed to estimate the fluxes and the length scales of dominant flux-carrying eddies from cospectra at frequencies below the wave band. The results are compared to independent estimates of those quantities, with good agreement between the two sets of estimates. The observed temperature gradients were smaller than predicted by standard rigid-boundary closure models, consistent with the suggestion that wave breaking and Langmuir circulation increase turbulent diffusivity in the upper ocean. Similarly, the Monin–Obukhov stability function ϕh was smaller in the authors’ measurements than the stability functions used in rig...

Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA

The mechanism that allowed many tens of km of movement of the enormous block slide floored by the rootless Heart Mountain detachment fault in NW Wyoming has long been a puzzle. Carbonat-rich microbreccia that is widespread along the fault and in dikes in the upper plate contains accreted grains indistinguishable from those observed as fallout from volcanic eruption clouds (accretionary lapilli) and impact ejecta clouds and in intrusive diatremes. In these settings and also in industrial processing, accreted grains form when particles in a turbulent gaseous suspension containing limited water adhere to a nucleating grain or to each other. Elongate grains in thick microbreccia have strong but diverse shap-preferred orientations unlike those reported from other fault rocks but instead suggestive of turbulent flow, and the microbreccia contains layering and other features of sedimentary character that appear to record deposition from suspension rather than frictional processes along a fault. We suggest that frictional heating led to dissociation of carbonate rock along the fault, producing supercritical CO 2 as the suspending medium. High CO 2 pressure drastically reduced friction along the fault and allowed continuation of catastrophic movement, probably initiated by a volcanic or phreatomagmatic explosion, resulting in very large displacement on a low-dipping surface. Earlier slower sliding may have occurred but final emplacement was rapid (minutes) and spectacular.

Calculating Reynolds Stresses from ADCP Measurements in the Presence of Surface Gravity Waves Using the Cospectra-Fit Method

Recently, the velocity observations of acoustic Doppler current profilers (ADCPs) have been successfully used to estimate turbulent Reynoldsstresses in estuaries and tidal channels. However,the presence of surface gravity waves can significantly bias stress estimates, limiting application of the technique in the coastal ocean. This work describes a new approach to estimate Reynolds stresses from ADCP velocities obtained in the presence of waves. The method fits an established semiempirical model of boundary layer turbulence to the measured turbulent cospectra at frequencies below those of surface gravity waves to estimate the stress. Applied to ADCP observations made in weakly stratified waters and variable significant wave heights, estimated near-bottom and near-surface stresses using this method compared well with independent estimates of the boundary stresses in contrast to previous methods. Additionally, the vertical structure of tidal stress estimated using the new approach matched that inferred from a linear momentum balance at stress levels below the estimated stress uncertainties. Because the method makes an estimate of the horizontal turbulent length scales present as part of the modelfit, these results can also enable a direct correction for the mean bias errors resulting from instrument tilt, if these scales are long relative to the beam separation.

Breaking Surface Wave Effects on River Plume Dynamics during Upwelling-Favorable Winds

This study examines the dynamics of a buoyant river plume in upwelling-favorable winds, concentrating on the time after separation from the coast. A set of idealized numerical simulations is used to examine the effects of breaking surface gravity waves on plume structure and cross-shore dynamics. Inclusion of a wavebreaking parameterization in the two-equation turbulence submodel causes the plume to be thicker and narrower, and to propagate offshore more slowly, than a plume in a simulation with no wave breaking. In simulations that include wave breaking, the plume has much smaller vertical gradients of salinity and velocity than in the simulation without breaking. This leads to decreased importance of shear dispersion in the plumes with wave breaking.Much of the wideningrate of the plume is explainedby divergentEkmanvelocities at the off- and onshore edges. Some aspects of plume evolution in all cases are predicted well by a simple theory based on a critical Richardson number and an infinitely deep ocean. However, because the initial plume in thesesimulationsisincontactwiththeseafloorintheinnershelf,somedetailsarepoorlypredicted,especially around the time that the plume separates from the coast.

Hydrodynamic sensing and behavior by oyster larvae in turbulence and waves.

ABSTRACT Hydrodynamic signals from turbulence and waves may provide marine invertebrate larvae with behavioral cues that affect the pathways and energetic costs of larval delivery to adult habitats. Oysters (Crassostrea virginica) live in sheltered estuaries with strong turbulence and small waves, but their larvae can be transported into coastal waters with large waves. These contrasting environments have different ranges of hydrodynamic signals, because turbulence generally produces higher spatial velocity gradients, whereas waves can produce higher temporal velocity gradients. To understand how physical processes affect oyster larval behavior, transport and energetics, we exposed larvae to different combinations of turbulence and waves in flow tanks with (1) wavy turbulence, (2) a seiche and (3) rectilinear accelerations. We quantified behavioral responses of individual larvae to local instantaneous flows using two-phase, infrared particle-image velocimetry. Both high dissipation rates and high wave-generated accelerations induced most larvae to swim faster upward. High dissipation rates also induced some rapid, active dives, whereas high accelerations induced only weak active dives. In both turbulence and waves, faster swimming and active diving were achieved through an increase in propulsive force and power output that would carry a high energetic cost. Swimming costs could be offset if larvae reaching surface waters had a higher probability of being transported shoreward by Stokes drift, whereas diving costs could be offset by enhanced settlement or predator avoidance. These complex behaviors suggest that larvae integrate multiple hydrodynamic signals to manage dispersal tradeoffs, spending more energy to raise the probability of successful transport to suitable locations. Summary: Turbulence and waves induce oyster larvae to swim faster upward or to dive. These behaviors are energetically costly but could reduce predation mortality and enhance larval delivery to adult habitats.

Directional flow sensing by passively stable larvae

ABSTRACT Mollusk larvae have a stable, velum-up orientation that may influence how they sense and react to hydrodynamic signals applied in different directions. Directional sensing abilities and responses could affect how a larva interacts with anisotropic fluid motions, including those in feeding currents and in boundary layers encountered during settlement. Oyster larvae (Crassostrea virginica) were exposed to simple shear in a Couette device and to solid-body rotation in a single rotating cylinder. Both devices were operated in two different orientations, one with the axis of rotation parallel to the gravity vector, and one with the axis perpendicular. Larvae and flow were observed simultaneously with near-infrared particle-image velocimetry, and behavior was quantified as a response to strain rate, vorticity and centripetal acceleration. Only flows rotating about a horizontal axis elicited the diving response observed previously for oyster larvae in turbulence. The results provide strong evidence that the turbulence-sensing mechanism relies on gravity-detecting organs (statocysts) rather than mechanosensors (cilia). Flow sensing with statocysts sets oyster larvae apart from zooplankters such as copepods and protists that use external mechanosensors in sensing spatial velocity gradients generated by prey or predators. Sensing flow-induced changes in orientation, rather than flow deformation, would enable more efficient control of vertical movements. Statocysts provide larvae with a mechanism of maintaining their upward swimming when rotated by vortices and initiating dives toward the seabed in response to the strong turbulence associated with adult habitats. Summary: Oyster larvae exhibit behavioral responses to flow-induced rotation of the body relative to gravity; in turbulence, these responses will enhance control of vertical motion.

The Role of Whitecapping in Thickening the Ocean Surface Boundary Layer

AbstractThe effects of wind-driven whitecapping on the evolution of the ocean surface boundary layer are examined using an idealized one-dimensional Reynolds-averaged Navier–Stokes numerical model. Whitecapping is parameterized as a flux of turbulent kinetic energy through the sea surface and through an adjustment of the turbulent length scale. Simulations begin with a two-layer configuration and use a wind that ramps to a steady stress. This study finds that the boundary layer begins to thicken sooner in simulations with whitecapping than without because whitecapping introduces energy to the base of the boundary layer sooner than shear production does. Even in the presence of whitecapping, shear production becomes important for several hours, but then inertial oscillations cause shear production and whitecapping to alternate as the dominant energy sources for mixing. Details of these results are sensitive to initial and forcing conditions, particularly to the turbulent length scale imposed by breaking wa...

Interpreting Fixed-Location Observations of Turbulence Advected by Waves: Insights from Spectral Models

AbstractAssigning a physical interpretation to turbulent fluctuations beneath waves is complex because eddies are advected by unsteady wave orbital motion. Here, the kinematic effects of wave orbital motion on turbulent fluctuations at a fixed location were investigated using model turbulence spatial spectra (κ spectra) together with a general form of the frozen turbulence approximation. Model autospectra and cospectra included an inertial subrange, a rolloff at energy-containing scales (L = 2π/κ0), and a dissipation range. Turbulence was advected by a background flow composed of waves (rms orbital velocity σw, peak frequency ωw, and spectral width Δωw) propagating parallel to a current uc. Expressions were derived for turbulence frequency spectra (ω spectra), and parameters were varied across ranges typical in the coastal ocean. Except close to the wave band, the ω-spectrum shape collapses with two dimensionless parameters, a velocity ratio σw/uc, and a time-scale ratio ucκ0/ωw, which can be used to diag...

Validation of Ocean Color Remote Sensing Reflectance Using Autonomous Floats

AbstractThe use of autonomous profiling floats for observational estimates of radiometric quantities in the ocean is explored, and the use of this platform for validation of satellite-based estimates of remote sensing reflectance in the ocean is examined. This effort includes comparing quantities estimated from float and satellite data at nominal wavelengths of 412, 443, 488, and 555 nm, and examining sources and magnitudes of uncertainty in the float estimates. This study had 65 occurrences of coincident high-quality observations from floats and MODIS Aqua and 15 occurrences of coincident high-quality observations floats and Visible Infrared Imaging Radiometer Suite (VIIRS). The float estimates of remote sensing reflectance are similar to the satellite estimates, with disagreement of a few percent in most wavelengths. The variability of the float–satellite comparisons is similar to the variability of in situ–satellite comparisons using a validation dataset from the Marine Optical Buoy (MOBY). This, combi...