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

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

Challenges for Mesoscale Numerical Models in the Littoral Environment

100th American Meteorological Society Annual MeetingHigh-resolution numerical weather prediction (NWP) in the littoral zone remains an outstanding challenge due to the complexity of the surface physical and thermodynamic properties, coastline representation and turbulence quantification. Prevailing approaches to surface flux parameterization in mesoscale NWP are susceptible to error in the littoral zone in part due to the surface heterogeneity and the impact of wind direction on the surface fluxes. The Coastal Land-Air-Sea Interaction (CLASI) project conducted a pilot field campaign during two weeks in summer 2016 around Monterey Bay, California, USA, to collect in-situ and remote measurements of meteorological and oceanographic quantities on-shore, off-shore and slightly inland to capture the cross-shore and along-shore gradient of key atmospheric fields and turbulent fluxes in a variety of conditions. The region was modeled using mesoscale NWP with the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) at horizontal resolutions as fine as 333 m. Whereas COAMPS performed well with predictions of near surface scalar and momentum fields verified by surface mesonet stations 4 to 5 km in-land, the pilot project revealed several sources of error in mesoscale NWP along the coastline. Such errors include a strong positive model bias of surface momentum flux and a lack of sensitivity of surface momentum flux to wind direction, a result expected since surface flux parameterizations are designed for homogeneous conditions. This presentation will show results of new high-resolution NWP model studies performed in preparation for an impending more extensive CLASI field campaign planned for summer 2020 in Monterey Bay. The goal of this latest modeling work is to identify the optimal locations and other constraints for observations needed as part of the larger field campaign objective to develop and validate new NWP model surface flux parameterizations. The COAMPS runs will be compared against large-eddy simulation of Monterey Bay littoral environments to validate NWP surface flux parameterizations in the littoral zone and better understand NWP model sensitivity. The presentation will also outline planned CLASI field campaign efforts for 2020

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

An Evaluation of Kolmogorov's −5/3 Power Law Observed Within the Turbulent Airflow Above the Ocean

The data used in this study are publicly available through an open access repository: https://nps.box.com/ shared/static/di5887nl4g3thgz 6bz67z3l44tj77qur.zip.The article of record as published may be found at https://doi.org/10.1029/ 2019GL085083In 1941, Kolmogorov postulated that the energy distribution of turbulence, across a particular range of eddy sizes cascading to dissipation, could be uniquely described as a universal −5/3 power law. This theory was readily accepted as the basis for conceptualizing the phenomenological characteristics of turbulence and remains central to continued experimental and theoretical developments in turbulence study. However, the theory's own validity lacks final certainty. Here we present the first observation‐based evaluation of Kolmogorov's power law within the atmospheric flow above the ocean. Using a unique platform and a novel analytical approach, we found that the expected power law varies systematically with height above the surface and the local environmental state. Our findings suggest that Kolmogorov's idealized value (−5/3) is approximately valid but, under certain conditions, may depend strongly on the unique processes and dynamics near the ocean surface. This discovery should motivate a reevaluation of how Kolmogorov'sU.S. Office of Naval ResearchDirected Energy Joint Technology Office (DEJTO)This work was supported through the Coupled Air‐Sea Processes and Electromagnetic ducting Research (CASPER) project funded by the U.S. Office of Naval Research grant N0001419WX01369 under its Multidisciplinary University Research Initiative (MURI). Q. W. is also supported by the Quantifying and Understanding Environmental Turbulence Affecting Lasers (QueTal) project funded by the Directed Energy Joint Technology Office (DEJTO) grant (F2KBAB8159G002)

An Evaluation of the Constant Flux Layer in the Atmospheric Flow Above the Wavy Air‐Sea Interface

The article of record as published may be found at https://doi.org/10.1029/2020JD032834The constant flux layer assumption simplifies the problem of atmospheric surface layer (ASL) dynamics and is an underlying assumption of Monin‐Obukhov Similarity Theory, which is ubiquitously applied to model interfacial exchange and atmospheric turbulence. Within the marine environment, the measurements necessary to confirm the local ASL as a constant flux layer are rarely available, namely: direct observations of the near‐surface flux gradients. Recently, the Research Platform FLIP was deployed with a meteorological mast that resolved the momentum and heat flux gradients from 3 to 16 m above the ocean surface. Here, we present findings of a study assessing the prevalence of the constant flux layer within the ASL, using an approach that accounts for wave‐coherent turbulence, defines the wave boundary layer height, and empirically quantifies the observed flux divergence. Our analysis revealed that only 30‐40% of momentum flux gradients were approximately constant; for the heat fluxes, this increased to 50‐60%. The stationarity of local turbulence was critical to the constant flux layer's validity, but resulted in excising a large proportion of the observed profiles. Swell‐wind alignment was associated with momentum flux profile divergence under moderate wind speeds. In conjunction, our findings suggest that the constant flux layer, as it is conventionally defined, is not generally valid within the marine ASL. This holds significant implications for measuring air‐sea fluxes from single point sources and the application of Monin‐Obukhov similarity theory over the ocean.Office of Naval Research (ONR)This research was funded by the Office of Naval Research (ONR) Grant N0001418WX01087 under its Multidisciplinary University Research Initiative (MURI)

CLASI: Coordinating Innovative Observations and Modeling to Improve Coastal Environmental Prediction Systems

Abstract The Coastal Land–Air–Sea Interaction (CLASI) project aims to develop new “coast-aware” atmospheric boundary and surface layer parameterizations that represent the complex land–sea transition region through innovative observational and numerical modeling studies. The CLASI field effort involves an extensive array of more than 40 land- and ocean-based moorings and towers deployed within varying coastal domains, including sandy, rocky, urban, and mountainous shorelines. Eight Air–Sea Interaction Spar (ASIS) buoys are positioned within the coastal and nearshore zone, the largest and most concentrated deployment of this unique, established measurement platform. Additionally, an array of novel nearshore buoys and a network of land-based surface flux towers are complemented by spatial sampling from aircraft, shore-based radars, drones, and satellites. CLASI also incorporates unique electromagnetic wave (EM) propagation measurements using a coherent array, drone receiver, and a marine radar to understand evaporation duct variability in the coastal zone. The goal of CLASI is to provide a rich dataset for validation of coupled, data assimilating large-eddy simulations (LES) and the Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). CLASI observes four distinct coastal regimes within Monterey Bay, California (MB). By coordinating observations with COAMPS and LES simulations, the CLASI efforts will result in enhanced understanding of coastal physical processes and their representation in numerical weather prediction (NWP) models tailored to the coastal transition region. CLASI will also render a rich dataset for model evaluation and testing in support of future improvements to operational forecast models

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