On the Nature of the Frontal Zone of the Choctawhatchee Bay Plume in the Gulf of Mexico

River plumes often feature turbulent processes in the frontal zone and interfacial region at base of the plume, which ultimately impact spreading and mixing rates with the ambient coastal ocean. The degree to which these processes govern overall plume mixing is yet to be quantified with microstructure observations. A field campaign was conducted in a river plume in the northeast Gulf of Mexico in December 2013, in order to assess mixing processes that could potentially impact transport and dispersion of surface material near coastal regions. Current velocity, density, and Turbulent Kinetic Energy Values, ε, were obtained using an Acoustic Doppler Current Profiler (ADCP), a Conductivity Temperature Depth (CTD) profiler, a Vertical Microstructure Profiler (VMP), and two Acoustic Doppler Velocimeters (ADVs). The frontal region contained ε values on the order of 10−5 m2 s−3, which were markedly larger than in the ambient water beneath (O 10−9 m2s−3). An energetic wake of moderate ε values (O 10−6 m2 s−3) was observed trailing the frontal edge. The interfacial region of an interior section of the plume featured opposing horizontal velocities and a ε value on the order of 10−6 m2 s−3. A simplified mixing budget was used under significant assumptions to compare contributions from wind, tides, and frontal regions of the plume. The results from this order of magnitude analysis indicated that frontal processes (59%) dominated in overall mixing. This emphasizes the importance of adequate parameterization of river plume frontal processes in coastal predictive models

Water surface slope spectra in nearshore and river mouth environments

With the ever-growing interest in satellite remote sensing, direct observations of short wave characteristics are needed along coastal margins. These zones are characterized by a diversity of physical processes that can affect sea surface topography. Here we present connections made between ocean wave spectral shape and wind forcing in coastal waters using polarimetric slope sensing and eddy covariance methods; this is based on data collected in the vicinity of the mouth of the Columbia River (MCR) on the Oregon-Washington border. These results provide insights into the behavior of short waves in coastal environments under variable wind forcing; this characterization of wave spectra is an important step towards improving the use of radar remote sensing to sample these dynamic coastal waters. High wavenumber spectral peaks are found to appear for U10 > 6 m s but vanish for τ > 0.1 N m2, indicating a stark difference between how wind speed and wind stress are related to the short-scale structure of the ocean surface. Near-capillary regime spectral shape is found to be less steep than in past observations and to show no discernable sensitivity to wind forcing

A laboratory study of spray generation in high winds

Characterizing the vertical distribution of large spray particles (i.e., spume) in high wind conditions is necessary for better understanding of the development of the atmospheric boundary layer in extreme conditions. To this end a laboratory experiment was designed to observe the droplet concentration in the air above actively breaking waves. The experiments were carried out in hurricane force conditions (U10 equivalent wind speed of 36 to 54 m s) and using both fresh water and salt water. While small differences between fresh and salt water were observed in profiles of radius-integrated spray volume fraction, the profiles tend to converge as the wind forcing increases. This supports the assumption that the physical mechanism for spume production is not sensitive to salinity and its corresponding link to the bubble size distribution

On the nature of the frontal zone of the Choctawhatchee Bay plume in the Gulf of Mexico

River plumes often feature turbulent processes in the frontal zone and interfacial region at base of the plume, which ultimately impact spreading and mixing rates with the ambient coastal ocean. The degree to which these processes govern overall plume mixing is yet to be quantified with microstructure observations. A field campaign was conducted in a river plume in the northeast Gulf of Mexico in December 2013, in order to assess mixing processes that could potentially impact transport and dispersion of surface material near coastal regions. Current velocity, density, and Turbulent Kinetic Energy Values, ε, were obtained using an Acoustic Doppler Current Profiler (ADCP), a Conductivity Temperature Depth (CTD) profiler, a Vertical Microstructure Profiler (VMP), and two Acoustic Doppler Velocimeters (ADVs). The frontal region contained ε values on the order of 10−5 m2 s−3, which were markedly larger than in the ambient water beneath (O 10−9 m2 s−3). An energetic wake of moderate ε values (O 10−6 m2 s−3) was observed trailing the frontal edge. The interfacial region of an interior section of the plume featured opposing horizontal velocities and a ε value on the order of 10−6 m2 s−3. A simplified mixing budget was used under significant assumptions to compare contributions from wind, tides, and frontal regions of the plume. The results from this order of magnitude analysis indicated that frontal processes (59%) dominated in overall mixing. This emphasizes the importance of adequate parameterization of river plume frontal processes in coastal predictive models

Marine X‐Band Radar Currents and Bathymetry: An Argument for a Wave Number‐Dependent Retrieval Method

This study evaluates shipboard marine X‐band radar (MR) near‐surface current and bathymetry measurements under shallow water conditions. The retrieval algorithm is based on the surface wave signal within three‐dimensional wave number frequency MR backscatter intensity variance spectra. The MR data were collected during a research cruise that investigated submesoscale processes and their impact on oil spill transport in the Louisiana Bight. The MR currents and bathymetry are validated using measurements from 500 GPS‐equipped surface drifters, a shipboard acoustic Doppler current profiler, and a shipboard single‐beam echo sounder. Earlier results from the same experiment but using a different set of sensors indicate strong upper ocean vertical current shear over a 3.5 hr period, despite only mild wind forcing. Here, the MR currents are derived as a function of wave number, providing a measure of vertical shear for the full duration of the cruise. Strong vertical shear is frequently observed, with a maximum difference of 0.42 m/s between the MR high (effective depth of ∼0.9 m) and low ( ∼2.4 m) wave number bins. Treating the drifter and echo sounder measurements as truth, the accuracies of the MR near‐surface currents and bathymetry are 0.04–0.07 m/s and 1.2 m (or 7% of the mean water depth). However, it is shown that ∼50% of the MR high wave number and drifter current differences is due to vertical shear. The shallow water MR near‐surface current accuracy thus matches findings from a previous deep water validation where vertical shear was much weaker. Key Points Wave number‐dependent marine X‐band radar current and bathymetry retrieval uncovers strong upper ocean vertical shear in the Louisiana Bight The current and bathymetry retrieval errors are 0.04–0.07 m/s and 1.2 m (or 7% of the mean water depth), respectively Vertical shear reaches 10% of the wind speed over 3.6 m, explaining ∼50% of the current retrieval error

Statistical properties of the surface velocity field in the northern Gulf of Mexico sampled by GLAD drifters

The Grand LAgrangian Deployment (GLAD) used multiscale sampling and GPS technology to observe time series of drifter positions with initial drifter separation of O(100 m) to O(10 km), and nominal 5 min sampling, during the summer and fall of 2012 in the northern Gulf of Mexico. Histograms of the velocity field and its statistical parameters are non-Gaussian; most are multimodal. The dominant periods for the surface velocity field are 1–2 days due to inertial oscillations, tides, and the sea breeze; 5–6 days due to wind forcing and submesoscale eddies; 9–10 days and two weeks or longer periods due to wind forcing and mesoscale variability, including the period of eddy rotation. The temporal e-folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales, 0.25–0.50 days and 0.9–1.4 days, and are the same order as the temporal e-folding scales of observed winds from nearby moored National Data Buoy Center stations. The Lagrangian integral time scales increase from coastal values of 8 h to offshore values of approximately 2 days with peak values of 3–4 days. The velocity variance is large, O(1)m2/s2, the surface velocity statistics are more anisotropic, and increased dispersion is observed at flow bifurcations. Horizontal diffusivity estimates are O(103)m2/s in coastal regions with weaker flow to O(105)m2/s in flow bifurcations, a strong jet, and during the passage of Hurricane Isaac. The Gulf of Mexico surface velocity statistics sampled by the GLAD drifters are a strong function of the feature sampled, topography, and wind forcingEnvironmental Fluid Mechanic

Scaling laws for the upper ocean temperature dissipation rate

Our understanding of temperature dissipation rate ? within the upper ocean boundary layer, which is critical for climate forecasts, is very limited. Near-surface turbulence also affects dispersion of contaminants and biogeochemical tracers. Using high-resolution optical turbulence measurements, scaling laws for ? are investigated under forcing states where either the daytime heat flux or the wind stress forcing is dominant. We find that ? remains constant over 1.5 times the significant wave height, while over a layer below, ? decays based on the local surface forcing. When the heat flux is dominant, traditional scaling based on the Monin-Obukhov similarity theory remains valid; ? ? z?1. When the wind stress dominates, we observe the emergence of a new scaling, ? ? z?1/2, which is explained by invoking the effect of small-scale coherent structures on vertical heat transport. These results have implications for improved modeling of the ocean's heat and CO2 intake.Hydraulic EngineeringCivil Engineering and Geoscience

Ocean current estimation using a Multi-Model Ensemble Kalman Filter during the Grand Lagrangian Deployment experiment (GLAD)

In the summer and fall of 2012, during the GLAD experiment in the Gulf of Mexico, the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) used several ocean models to assist the deployment of more than 300 surface drifters. The Navy Coastal Ocean Model (NCOM) at 1km and 3km resolutions, the US Navy operational NCOM at 3km resolution (AMSEAS), and two versions of the Hybrid Coordinates Ocean Model (HYCOM) set at 4km were running daily and delivering 72-h range forecasts. They all assimilated remote sensing and local profile data but they were not assimilating the drifter’s observations. This work presents a non-intrusive methodology named Multi-Model Ensemble Kalman Filter that allows assimilating the local drifter data into such a set of models, to produce improved ocean currents forecasts. The filter is to be used when several modeling systems or ensembles are available and/or observations are not entirely handled by the operational data assimilation process. It allows using generic in situ measurements over short time windows to improve the predictability of local ocean dynamics and associated high-resolution parameters of interest for which a forward model exists (e.g. oil spill plumes). Results can be used for operational applications or to derive enhanced background fields for other data assimilation systems, thus providing an expedite method to non-intrusively assimilate local observations of variables with complex operators. Results for the GLAD experiment show the method can improve water velocity predictions along the observed drifter trajectories, hence enhancing the skills of the models to predict individual trajectories