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Fabrice Ardhuin
,
T. H. C. Herbers
,
Kristen P. Watts
,
Gerbrant Ph van Vledder
,
R. Jensen
, and
Hans C. Graber

Abstract

Wind-sea generation was observed during two experiments off the coast of North Carolina. One event with offshore winds of 9–11 m s−1 directed 20° from shore normal was observed with eight directional stations recording simultaneously and spanning a fetch from 4 to 83 km. An opposing swell of 1-m height and 10-s period was also present. The wind-sea part of the wave spectrum conforms to established growth curves for significant wave height and peak period, except at inner-shelf stations where a large alongshore wind-sea component was observed. At these short fetches, the mean wave direction θm was observed to change abruptly across the wind-sea spectral peak, from alongshore at lower frequencies to downwind at higher frequencies. Waves from another event with offshore winds of 6–14 m s−1 directed 20°–30° from shore normal were observed with two instrument arrays. A significant amount of low-frequency wave energy was observed to propagate alongshore from the region where the wind was strongest. These measurements are used to assess the performance of some widely used parameterizations in wave models. The modeled transition of θm across the wind-sea spectrum is smoother than that in the observations and is reproduced very differently by different parameterizations, giving insights into the appropriate level of dissipation. Calculations with the full Boltzmann integral of quartet wave–wave interactions reveal that the discrete interaction approximation parameterization for these interactions is reasonably accurate at the peak of the wind sea but overpredicts the directional spread at high frequencies. This error is well compensated by parameterizations of the wind input source term that have a narrow directional distribution. Observations also highlight deficiencies in some parameterizations of wave dissipation processes in mixed swell–wind-sea conditions.

Full access
Björn Lund
,
Brian K. Haus
,
Jochen Horstmann
,
Hans C. Graber
,
Ruben Carrasco
,
Nathan J. M. Laxague
,
Guillaume Novelli
,
Cédric M. Guigand
, and
Tamay M. Özgökmen

Abstract

The Lagrangian Submesoscale Experiment (LASER) involved the deployment of ~1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V F. G. Walton Smith was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at ~500-m resolution up to a maximum range of ~3 km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR–drifter pairs, the root-mean-square error for the current speed is 4 cm and that for the current direction is 12°. The MR samples currents at a greater effective depth than the CARTHE drifters (1–5 m vs ~0.4 m). The mean MR–drifter differences are consistent with a wave- and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 15°). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP.

Open access
Nathan J. M. Laxague
,
Brian K. Haus
,
David G. Ortiz-Suslow
,
Conor J. Smith
,
Guillaume Novelli
,
Hanjing Dai
,
Tamay Özgökmen
, and
Hans C. Graber

Abstract

Estimation of near-surface current is essential to the estimation of upper-ocean material transport. Wind forcing and wave motions are dominant in the near-surface layer [within O(0.01) m of the surface], where the highly sheared flows can differ greatly from those at depth. This study presents a new method for remotely measuring the directional wind and wave drift current profile near to the surface (between 0.01 and 0.001 m for the laboratory and between 0.1 and 0.001 m for the field). This work follows the spectral analysis of high spatial ( 0.002 m) and temporal resolution ( 60 Hz) wave slope images, allowing for the evaluation of near-surface current characteristics without having to rely on instruments that may disturb the flow. Observations gathered in the 15 m × 1 m × 1 m wind-wave flume at the University of Miami’s Surge-Structure-Atmosphere Interaction (SUSTAIN) facility show that currents retrieved via this method agree well with the drift velocity of camera-tracked dye. Application of this method to data collected in the mouth of the Columbia River (MCR) indicates the presence of a near-surface current component that departs considerably from the tidal flow and may be steered by the wind stress. These observations demonstrate that wind speed–based parameterizations alone may not be sufficient to estimate wind drift and to hold implications for the way in which surface material (e.g., debris or spilled oil) transport is estimated when atmospheric stress is of relatively high magnitude or is steered off the mean wind direction.

Full access
Henry Potter
,
Hans C. Graber
,
Neil J. Williams
,
Clarence O. Collins III
,
Rafael J. Ramos
, and
William M. Drennan

Abstract

One of the scientific objectives of the U.S. Office of Naval Research–sponsored Impact of Typhoons on the Ocean in the Pacific (ITOP) campaign was improved understanding of air–sea fluxes at high wind speeds. Here the authors present the first-ever direct measurements of momentum fluxes recorded in typhoons near the surface. Data were collected from a moored buoy over 3 months during the 2010 Pacific typhoon season. During this period, three typhoons and a tropical storm were encountered. Maximum 30-min sustained wind speeds above 26 m s−1 were recorded. Data are presented for 1245 h of direct flux measurements. The drag coefficient shows evidence of a rolloff at wind speeds greater than 22 m s−1, which occurred during the passage of a single typhoon. This result is in agreement with other studies but occurs at a lower wind speed than previously measured. The authors conclude that this rolloff was caused by a reduction in the turbulent momentum flux at the frequency of the peak waves during strongly forced conditions.

Full access
Lynn K. Shay
,
Thomas M. Cook
,
Zachariah R. Hallock
,
Brian K. Haus
,
Hans C. Graber
, and
Jorge Martinez

Abstract

As part of the Naval Research Laboratory and Office of Naval Research sponsored Physics of Coastal Remote Sensing Research Program, an experiment was conducted in September–October 1996 off Virginia Beach. Ocean surface currents were measured using the high-frequency (25.4 MHz) mode of the Ocean Surface Current Radar at 20-min intervals at a horizontal resolution of 1 km over an approximate 30 km × 44 km domain. Comparisons to subsurface current measurements at 1–2 m beneath the surface from two broadband acoustic Doppler current profilers (ADCP) revealed good agreement to the surface currents. Regression analyses indicated biases of 4 and −3 cm s−1 for cross-shelf and along-shelf currents, respectively, where slopes were O(1) with correlation coefficients of 0.8.

Nine months of sea level heights from the NOAA National Ocean Survey Chesapeake Bay Bridge Tunnel tidal station revealed an energetic M 2 tidal component having an amplitude of 37.5 cm and a phase of 357°. The S 2 tidal constituent had an amplitude of 7 cm and a phase of 49°. By contrast, the diurnal band (K 1, O 1) tidal constituents were considerably weaker with amplitudes of 1–5 cm. From 19 days of HF-derived surface currents, the M 2 and S 2 tidal current amplitudes had a maximum of about 50 and 8 cm s−1 at the Chesapeake Bay mouth, respectively. Explained variances associated with these four tidal current constituents were a maximum of 60% at the mouth and decreased southward. Analyses at the ADCP moorings indicated that the semidiurnal tidal currents were predominantly barotropic with cross-shelf and along-shelf currents of 18 and 10 cm s−1. Energetic semidiurnal tidal currents were highly correlated over the HF-radar domain, and the phase angles indicated a consistent anticyclonic veering of the M 2 tidal current with along-shelf distance from the mouth. Normalized tidal current vorticities by the local Coriolis parameter (f), which represent a proxy for the Rossby number, were ±0.25f near the mouth and ±0.05f in the southern part of the domain for the M 2 constituent. Simulations from a linear, barotropic model were highly correlated with observed M 2 tidal currents at 85 points within the HF-radar domain, consistent with the premise of weakly nonlinear flows.

Full access
David E. Weissman
,
Fuk K. Li
,
Shu-hsiang Lou
,
Son V. Nghiem
,
Gregory Neumann
,
Robert E. McIntosh
,
Steven C. Carson
,
James R. Carswell
,
Hans C. Graber
, and
Robert E. Jensen

Abstract

Scatterometer model functions that directly estimate friction velocity have been developed and are being tested with radar and in situ data acquired during the Surface Wave Dynamics Experiment (SWADE) of 1991. Ku-band and C-band scatterometers were operated simultaneously for extensive intervals for each of 10 days during SWADE. The model function developed previously from the FASINEX experiment converts the Ku-band normalized radar cross-section (NRCS) measurements into friction velocity estimates. These are compared to in situ estimates of surface wind stress and direction across a wide area both on and off the Gulf Stream (for hourly intervals), which were determined from buoy and meteorological measurements during February and March 1991. This involved the combination of a local, specially derived wind field, with an ocean wave model coupled through a sea-state-dependent drag coefficient. The Ku-band estimates u∗ magnitude are in excellent agreement with the in situ values. The C-band scatterometer measurements were coincident with the Ku-band NRCSs, whose u∗ estimates are then used to calibrate the C band. The results show the C-band NRCS dependence at 20°, 30°, 40°, and 50° to be less sensitive to friction velocity than the corresponding cases for Ku band. The goal is to develop the capability of making friction velocity estimates (and surface stress) from radar cross-sectional data acquired by satellite scatterometers.

Full access
Björn Lund
,
Ruben Carrasco
,
Hanjing Dai
,
Hans C. Graber
,
Cédric M. Guigand
,
Brian K. Haus
,
Jochen Horstmann
,
John A. Lodise
,
Guillaume Novelli
,
Tamay Özgökmen
,
Michael A. Rebozo
,
Edward H. Ryan
, and
Michael Streßer

Abstract

Our unmanned aerial system (UAS) current mapping is based on optical video data of the sea surface. We use three-dimensional fast Fourier transform and least squares fitting to measure the surface waves’ phase velocities and the currents via the linear dispersion relationship. Our UAS is a low-cost off-the-shelf quadcopter with inaccurate camera position and attitude measurements, which may cause spurious currents as large as the signal. We present a novel wave-based UAS heading and position correction, improving the image rectification accuracy by a factor of ~3.5 and the current measurements’ temporal repeatability by factors of 1.8–4.8. This validation study maps the currents at high spatiotemporal resolution (5 m and 4 s) across the ~700-m-wide tidally dominated Bear Cut channel in Miami, Florida. The UAS currents are compared to flotsam tracks, obtained through automated UAS video object detection and tracking, drifter tracks, and acoustic Doppler current profiler measurements. The root-mean-square errors of the cross- and along-channel currents are better than 0.03 m s−1 for the flotsam comparison and better than 0.06 m s−1 for the drifter comparison; the latter revealed a 0.06 m s−1 along-wind bias due to wind- and wave-driven vertical current shear. UAS current mapping could be used to monitor river discharge, buoyant pollutants, or submesoscale fronts and eddies; the proposed wave-based heading and position correction enables its use in areas without ground control points.

Open access
Brian K. Haus
,
David G. Ortiz-Suslow
,
James D. Doyle
,
David D. Flagg
,
Hans C. Graber
,
Jamie MacMahan
,
Lian Shen
,
Qing Wang
,
Neil J. Willams
, and
Caglar Yardim

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.

Full access
Nirnimesh Kumar
,
James A. Lerczak
,
Tongtong Xu
,
Amy F. Waterhouse
,
Jim Thomson
,
Eric J. Terrill
,
Christy Swann
,
Sutara H. Suanda
,
Matthew S. Spydell
,
Pieter B. Smit
,
Alexandra Simpson
,
Roland Romeiser
,
Stephen D. Pierce
,
Tony de Paolo
,
André Palóczy
,
Annika O’Dea
,
Lisa Nyman
,
James N. Moum
,
Melissa Moulton
,
Andrew M. Moore
,
Arthur J. Miller
,
Ryan S. Mieras
,
Sophia T. Merrifield
,
Kendall Melville
,
Jacqueline M. McSweeney
,
Jamie MacMahan
,
Jennifer A. MacKinnon
,
Björn Lund
,
Emanuele Di Lorenzo
,
Luc Lenain
,
Michael Kovatch
,
Tim T. Janssen
,
Sean R. Haney
,
Merrick C. Haller
,
Kevin Haas
,
Derek J. Grimes
,
Hans C. Graber
,
Matt K. Gough
,
David A. Fertitta
,
Falk Feddersen
,
Christopher A. Edwards
,
William Crawford
,
John Colosi
,
C. Chris Chickadel
,
Sean Celona
,
Joseph Calantoni
,
Edward F. Braithwaite III
,
Johannes Becherer
,
John A. Barth
, and
Seongho Ahn

Abstract

The inner shelf, the transition zone between the surfzone and the midshelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from September–October 2017, conducted from the midshelf, through the inner shelf, and into the surfzone near Point Sal, California. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves, and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the midshelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean.

Full access
Nirnimesh Kumar
,
James A. Lerczak
,
Tongtong Xu
,
Amy F. Waterhouse
,
Jim Thomson
,
Eric J. Terrill
,
Christy Swann
,
Sutara H. Suanda
,
Matthew S. Spydell
,
Pieter B. Smit
,
Alexandra Simpson
,
Roland Romeiser
,
Stephen D. Pierce
,
Tony de Paolo
,
André Palóczy
,
Annika O’Dea
,
Lisa Nyman
,
James N. Moum
,
Melissa Moulton
,
Andrew M. Moore
,
Arthur J. Miller
,
Ryan S. Mieras
,
Sophia T. Merrifield
,
Kendall Melville
,
Jacqueline M. McSweeney
,
Jamie MacMahan
,
Jennifer A. MacKinnon
,
Björn Lund
,
Emanuele Di Lorenzo
,
Luc Lenain
,
Michael Kovatch
,
Tim T. Janssen
,
Sean R. Haney
,
Merrick C. Haller
,
Kevin Haas
,
Derek J. Grimes
,
Hans C. Graber
,
Matt K. Gough
,
David A. Fertitta
,
Falk Feddersen
,
Christopher A. Edwards
,
William Crawford
,
John Colosi
,
C. Chris Chickadel
,
Sean Celona
,
Joseph Calantoni
,
Edward F. Braithwaite III
,
Johannes Becherer
,
John A. Barth
, and
Seongho Ahn
Full access