Naval Oceanographic Office Research Articles

On the limits of distinguishing seabed types via ambient acoustic sound

This article presents a theoretical analysis of optimally distinguishing among environmental parameters from ocean ambient sound. Recent approaches to this problem either focus on parameter estimation or attempt to classify the environment into one of many known types through machine learning. This classification problem is framed as one of hypothesis testing on the received ambient sound snapshots. The resulting test depends on the Kullback-Leibler divergence (KLD) between the distributions corresponding to different environments or sediment types. Analysis of the KLD shows the dependence on the signal-to-noise ratio, the underlying signal subspace, and the distribution of eigenvalues of the respective covariance matrices. This analysis provides insights into both when and why successful hypothesis testing is possible. Experiments demonstrate that our analysis provides insight as to why certain environmental parameters are more difficult to distinguish than others. Experiments on sediment types from the Naval Oceanographic Office Bottom Sediment type database show that certain types are indistinguishable for a given array configuration. Further, the KLD can be used to provide a quantitative alternative to examining bottom loss curves to predict array processing performance.

Basin-scale propagation modeling of MLS signals between Kauai and Monterey

Long distance underwater acoustic propagation is of interest for a variety of applications including underwater navigation, yet modeling such propagation is challenging due to the large degree of environmental uncertainty. In this presentation, the propagation of 75 Hz center frequency maximum length sequence (MLS) signals emitted from a submerged source near Kauai and received at the Monterey Accelerated Research System (MARS) observatory are modeled with the Bellhop ray tracing and the Monterey-Newport Parabolic Equation (MNPE) models. The range-dependent sound speed environment is based on historical profiles from the World Ocean Atlas database and reanalysis data from the hybrid coordinate ocean model (HYCOM). Bathymetry profiles along the axis of propagation are interpolated from the General Bathymetric Chart of the Oceans (GEBCO) 2021 database and the Naval Oceanographic Office Digital Bathymetric Data Base Variable Resolution (DBDB-V) data product. Simulated channel impulse responses are compared to observations for transmission loss and signal coherence.

Underwater Optical Path Loss after Passage of a Tropical Storm

Underwater wireless optical communications (UWOCs) have attracted considerable attention in recent years as an alternative means for acoustic communication. However, optical path loss of light propagation from attenuation is in large part due to absorption and scattering in various water conditions. Identification of environmental effects, especially tropical storms on underwater optical path loss, is key to the success of using optics for UWOCs. Underwater inherent optical properties (IOPs), such as the beam attenuation coefficient for 470 nm light in the western North Pacific Ocean, were measured from U.S. Naval Oceanographic Office Seagliders deployed after Super Typhoon Guchol’s (June 7–20, 2012) passage from June 25 to June 30, 2012 and without any typhoon passage from January 9 to February 28, 2014. The two observed sets (with and without the super typhoon) of IOPs are taken as input for a recently developed radiative transfer equation solver. The simulated normalized received powers for the two durations show a large impact of typhoon passage on UWOCs.

Barotropic boundary conditions and tide forcing in split-explicit high resolution coastal ocean models

A persisting problem in ocean modeling around coastal areas has been the adequate imposition of open boundary conditions, especially in the presence of complex bathymetric features commonly found in shelf seas. Ocean models with horizontal resolutions of O(1km) can generate submesoscale current variability not captured by coarser regional models which resolve mesoscale flows approximately in geostrophic and thermal-wind balance. In practice, higher-resolution systems will provide analyses with enhanced spatial detail but may be less skillful at predicting the evolution of mesoscale eddies. This study aims at assessing the role of open boundary conditions, tidal forcing and tide filtering techniques in a coastal ocean model using a split-explicit time marching scheme with horizontal resolutions of O(1km) and its effect in coastal sea level dynamics and ocean currents. Numerical experiments have been performed using the Regional Ocean Modeling System (ROMS) with different uniform horizontal resolution: 3.6km, 1.2km and 740m. Initial and lateral boundary conditions are derived from the U.S. Naval Oceanographic Office (NAVOCEANO) operational AmSeas model forecast, a 3.6-km resolution application of the regional Navy Coastal Ocean Model (NCOM) that encompasses the Gulf of Mexico and Caribbean Sea. Meteorological conditions are interpolated from the Navy’s Coastal Ocean-Atmosphere Mesoscale Prediction System (COAMPS) model with the exception of surface stresses, which are computed from a 2-km application of the Weather Research and Forecasting (WRF) model used by National Center for Environmental Protection’s (NCEP) National Digital Forecast Database (NDFD). Tidal variability is imposed in two ways: 1) by specifying the time evolution of sea level at the open boundaries and 2) spectrally by specifying the harmonic phases and amplitudes from the TPXO global inverse tide solutions at the boundaries. Sub-tidal low frequency conditions are imposed by filtering high frequencies out of the regional NCOM solution. A spectral analysis of the sea surface height and barotropic velocity is performed via Fourier’s transform, showing significant differences at higher frequencies. Tide signals are then reconstructed and removed from the open boundary condition’s (OBC’s) in 2 ways: 1) using Rich Pawlowicz’s t_tide package (i.e., classic harmonic analysis) (Pawlowicz, 2002 [1]) and 2) with traditional low-pass filters. The spectral analysis provides a comprehensive characterization of the response in coastal sea level dynamics, which is difficult to elucidate in the time domain. Modeled results of sea surface height and ocean currents are validated with NOAA tide gauges and Acoustic Doppler Current Profilers. The tide filtering approach with the low-pass filter yields significant improvement of water levels in coastal areas, and similar performance in the modeled currents. The grid sensitivity analysis shows significant reduction of error when increasing the resolution from 3.6km to 1.2km, but no further improvement is found at higher resolution. Power spectra shows that further increasing the resolution yields better agreement for higher frequencies but no significant reduction in error.

Underwater optical detection after passage of tropical storm

Optical detection systems have the potential to get around some limitations of acoustic detection systems, especially with increased fleet and port security in noisy littoral waters. Identification of environmental effects especially tropical storms on underwater optical detection is a key to the success. A typhoon-influenced area is chosen in the western North Pacific Ocean with high ocean transparency and low seasonal optical variability. Underwater inherent optical properties (IOPs) such as the beam attenuation coefficient for 470 nm light are measured in the selected region from the U.S. Naval Oceanographic Office sea gliders deployed after super typhoon Guchol’s (June 7 to 20, 2012) passage from June 25 to 30, 2012, and with no typhoon activity from January 9 to February 28, 2014. The observed two sets (with and without typhoon) of IOPs are taken as the input into the Navy’s electro-optical detection simulator. The simulation shows low detection after the super typhoon Guchol-2012’s passage and high detection without typhoon passage.

Development and validation of a coastal ocean forecasting system for Puerto Rico and the U.S. virgin islands

An ocean forecasting system has been developed for the coastal area of Puerto Rico and the U.S. Virgin Islands. This paper presents the development and validation of the Puerto Rico Operational Regional Ocean Modeling System (PRO-ROMS) developed by the Caribbean Coastal Ocean Observing System (CARICOOS), which aims at improving currently available short-term forecasts of ocean currents for this region, and to resolve sub-mesoscale variability absent from the currently operational regional systems. The coastal ocean forecasting system is based on the Regional Ocean Modeling System (ROMS), and provides a 3-day forecast of ocean currents, water levels, temperature and salinity. Initial and lateral boundary conditions are derived from the U.S. Naval Oceanographic Office (NAVOCEANO) operational AmSeas forecasting system, a 3.6-km resolution application of the regional Navy Coastal Ocean Model (NCOM) that encompasses the Gulf of Mexico and Caribbean Sea. Meteorological conditions are interpolated from the Navy’s Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) with the exception of surface stresses, which are computed from a 2-km application of the Weather Research and Forecasting (WRF) model used by National Centers for Environmental Protection’s (NCEP) National Digital Forecast Database (NDFD). Tidal variability is imposed using ROMS spectral forcing by specifying the harmonic phases and amplitudes from the TPXO global inverse tide solutions at the boundaries and sub-tidal conditions are imposed by filtering out high frequencies of barotropic variables from the lateral boundary conditions interpolated from AmSeas. Modeled results of sea surface height are validated with NOAA tide gauges, ocean currents are validated with Acoustic Doppler Current Profilers (ADCP) and High Frequency Radars (HFR). Computed statistics show improvement of the ocean current forecast in PRO-ROMS compared to AmSeas, especially at higher frequencies dominated by the tides and in small channels where the bathymetry is better represented by the higher resolution coastal ocean forecasting system.

Gulf Stream Detection from SAR Doppler Anomaly

This work presents a Gulf Stream (GS) North Wall (GSNW) detection algorithm applicable to Sentinel-1 Radial surface Velocity (RVL) products derived from Doppler centroid analysis of synthetic aperture radar (SAR) data collected off the east coast of Canada from February 2017 to August 2017. Visual comparison of the extracted location of the GSNW (obtained by evaluating the peak RVL gradient in the azimuth direction), and the estimated location of the GSNW as determined by the U.S. Naval Oceanographic Office and a GSNW search region (GSNWSR), indicates that the algorithm is capable of detecting the GSNW in ∼80% of the cases evaluated. Results are dependent on the geophysical orientation of the GS across the SAR swath, such that the GSNW detector performed most reliably when the GS was oriented across the swath resulting in maximized surface velocities in the range direction. When the GS meandered, or when GS eddies appeared in the RVL data, the GSNW was often partially detected or detected multiple times, reducing confidence in the extracted location. It is anticipated that the results obtained using the Sentinel-1 RVL products will be applicable to SAR data from other platforms.

Forecasting Storm Surge and Inundation: Model Validation

Abstract Coastal regions are increasingly vulnerable to damage from storm surge and inundation. Delft3D is used by the Naval Oceanographic Office to model the ocean dynamics in the near shore. In this study, the performance of Delft3D in predicting the surge and inundation during Hurricane Ike, which impacted the northern Gulf of Mexico in September 2008, is examined. Wave height, water level, and high-water mark comparisons with a number of observations confirm that the model does well in predicting the surge and inundation during extreme events. The impact of using forecast winds based on the best-track data as opposed to hindcast winds is also investigated, and it is found that the extent of inundation is represented reasonably well with the forecast winds. In Delft3D, waves can be coupled to the hydrodynamic component using the radiation stress gradient method or the dissipation method. Comparing the results of using the two shows that for low-resolution grids such as that needed for a forecast model the dissipation method works better at reproducing the water levels and inundation.

Monitoring and Tracking the Green Tide in the Yellow Sea With Satellite Imagery and Trajectory Model

A massive green tide (i.e., Ulva prolifera bloom) event in summer 2015 in the Yellow Sea was investigated using satellite imagery and a trajectory model. First, the occurrence and evolution of the macroalgal bloom were studied based on time series of cloud-free Moderate Resolution Imaging Spectroradiometer imagery during May to August using the Floating Algae Index detection method. A Lagrangian spill trajectory model, i.e., the General NOAA (National Oceanic and Atmospheric Administration) Operational Modeling Environment (GNOME) model, which was originally designed to simulate the transport of oil spills, was then implemented to produce trajectories of the macroalgae. The model was forced by the 3-hourly ocean surface current data from the Naval Oceanographic Office global-scale operational ocean prediction system, and 6-hourly blended surface wind products from NOAA/National Climatic Data Center. The simulated transport of the green tide agrees fairly well with satellite observations over the time span of about 10 days, indicating that the combination of GNOME model and satellite data can be employed for rapid algal bloom response and environmental impact assessment. The results of numerical experiments also show that the surface winds play a significant role in the movement of the macroalgae in the Yellow Sea where the surface currents in summer are primarily driven by sea winds.

Validation of S-NPP VIIRS Sea Surface Temperature Retrieved from NAVO

The validation of sea surface temperature (SST) retrieved from the new sensor Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-Orbiting Partnership (S-NPP) Satellite is essential for the interpretation, use, and improvement of the new generation SST product. In this study, the magnitude and characteristics of uncertainties in S-NPP VIIRS SST produced by the Naval Oceanographic Office (NAVO) are investigated. The NAVO S-NPP VIIRS SST and eight types of quality-controlled in situ SST from the National Oceanic and Atmospheric Administration in situ Quality Monitor (iQuam) are condensed into a Taylor diagram. Considering these comparisons and their spatial coverage, the NAVO S-NPP VIIRS SST is then validated using collocated drifters measured SST via a three-way error analysis which also includes SST derived from Moderate Resolution Imaging Spectro-radiometer (MODIS) onboard AQUA. The analysis shows that the NAVO S-NPP VIIRS SST is of high accuracy, which lies between the drifters measured SST and AQUA MODIS SST. The histogram of NAVO S-NPP VIIRS SST root-mean-square error (RMSE) shows normality in the range of 0–0.6 °C with a median of ~0.31 °C. Global distribution of NAVO VIIRS SST shows pronounced warm biases up to 0.5 °C in the Southern Hemisphere at high latitudes with respect to the drifters measured SST, while near-zero biases are observed in AQUA MODIS. It means that these biases may be caused by the NAVO S-NPP VIIRS SST retrieval algorithm rather than the nature of the SST. The reasons and correction for this bias need to be further studied.

Initial considerations on effects of braided river beds on long-range acoustic propagation in shallow water

A braided river consists of multiple small channels that divide and recombine numerous times. They form when the sediment load and transporting energies are such that coarser sediment can be deposited as shifting islands or bars between the channels. Braided rivers formed on continental shelves during sea level lowstands during the last 700,000 years when the shelf was sub-aerially exposed to greater than 125 m below present day sea level. A salient feature is that the stream bed (which can be many kilometers wide) is expected to exhibit extremely high geoacoustic variability, given that the islands and bars exhibit much coarser material than that in the channels. The variability is expected both laterally (highest variability or smallest scales perpendicular to the stream flow) and also vertically inasmuch as the river channels experience different avulsion patterns over time. Chirp sonar and core data provide some insight into the underlying geologic processes and the associated scales of variability. Modeling gives some initial clues about the effects of braided river beds on acoustic propagation. [Work supported by the Naval Oceanographic Office and the Office of Naval Research, Ocean Acoustics program.]

Initial evaluations of a Gulf of Mexico/Caribbean ocean forecast system in the context of the Deepwater Horizon disaster

In response to the Deepwater Horizon (DwH) oil spill event in 2010, the Naval Oceanographic Office deployed a nowcast-forecast system covering the Gulf of Mexico and adjacent Caribbean Sea that was designated Americas Seas, or AMSEAS, which is documented in this manuscript. The DwH disaster provided a challenge to the application of available ocean-forecast capabilities, and also generated a historically large observational dataset. AMSEAS was evaluated by four complementary efforts, each with somewhat different aims and approaches: a university research consortium within an Integrated Ocean Observing System (IOOS) testbed; a petroleum industry consortium, the Gulf of Mexico 3-D Operational Ocean Forecast System Pilot Prediction Project (GOMEX-PPP); a British Petroleum (BP) funded project at the Northern Gulf Institute in response to the oil spill; and the Navy itself. Validation metrics are presented in these different projects for water temperature and salinity profiles, sea surface wind, sea surface temperature, sea surface height, and volume transport, for different forecast time scales. The validation found certain geographic and time biases/errors, and small but systematic improvements relative to earlier regional and global modeling efforts. On the basis of these positive AMSEAS validation studies, an oil spill transport simulation was conducted using archived AMSEAS nowcasts to examine transport into the estuaries east of the Mississippi River. This effort captured the influences of Hurricane Alex and a non-tropical cyclone off the Louisiana coast, both of which pushed oil into the western Mississippi Sound, illustrating the importance of the atmospheric influence on oil spills such as DwH.

The Navy's Application of Ocean Forecasting to Decision Support

Abstract : The Naval Oceanographic Office (NAVOCEANO) provides daily operational global, regional, and coastal ocean model forecasts and their associated prediction products. The models utilized include three-dimensional circulation, wave and ice forecasting systems that have been developed to meet Navy requirements; the models are forced by Navy atmospheric models and constrained by Navy-developed bottom topography. In addition, NAVOCEANO acquires, quality controls, and delivers real-time ocean observations from both in situ and remote-sensing resources for assimilation by the ocean models. These observations are also used to assess model skill and develop ocean climatologies. A major supercomputing capacity is needed to run this ocean modeling suite, as well as a uniquely skilled model operations team that keeps the systems running. Dedicated ocean forecasters interpret the predictions and apply the information to Navy operations. The path from requirements to development to operations demonstrates the close links between research and development production, and operational Navy applications. This path also provides innovative future ocean modeling plans directed to improve oceanographic support to the Navy.

The US Navy Coupled Ocean-Wave Prediction System

A new coupled ocean-wave model has been developed and tested as a new component of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS(circle)). The modeling system is comprised of the Simulating WAves Nearshore (SWAN) wave model and the Navy Coastal Ocean Model (NCOM). The models are two-way coupled using the Earth System Modeling Framework (ESMF). The ocean model has been modified to incorporate the effect of the Stokes drift current, wave radiation stresses due to horizontal gradients of the momentum flux of surface waves, enhancement of bottom drag in shallow water, and enhanced vertical mixing due to Langmuir turbulence. The wave model ingests surface currents (wave-current interaction) and water levels. The system is designed to support the Navy's ocean forecast requirements for regional and coastal domains. Validation studies for the Florida Straits and Virginia coastal area are presented. The system will run at the Naval Oceanographic Office and at the Fleet Numerical Meteorology and Oceanography Center.

US Navy Global and Regional Wave Modeling

Abstract : This article reviews the prediction of wind-generated surface gravity waves over the world ocean by the US Navy. The numerical wave model WAVEWATCH III? is used operationally for this purpose at the two primary Navy operational centers, the Fleet Numerical Meteorology and Oceanography Center and the Naval Oceanographic Office. This model is briefly described, and an overview is given of the current operational and near-operational features of global- and regional scale wave models at the two centers. Planned features are summarized.

US Navy Operational Global Ocean and Arctic Ice Prediction Systems

Abstract : The US Navy's operational global ocean nowcast/forecast system is presently comprised of the 0.08 HYbrid Coordinate Ocean Model (HYCOM) and the Navy Coupled Ocean Data Assimilation (NCODA). Its high horizontal resolution and adaptive vertical coordinate system make it capable of producing nowcasts (current state) and forecasts of oceanic weather, which includes three-dimensional ocean temperature, salinity, and current structure; surface mixed layer depth; and the location of mesoscale features such as eddies, meandering currents, and fronts. It runs daily at the Naval Oceanographic Office and provides seven-day forecasts that support fleet operations, provide boundary conditions to higher resolution regional models, and are available to the community. Using a data-assimilative hindcast and series of 14-day forecasts for 2012, the system is shown to have forecast skill of the oceanic mesoscale out to about 10 days for the Gulf Stream region and to 14+ days for the global ocean and other selected subregions. Forecast skill is sensitive to the type of atmospheric forcing (i.e., operational vs. analysis quality). Subsurface temperature bias is small ( 0.25 C) and root mean square error peaks at the depth range of the mixed layer and thermocline. Coupled to the Community Ice CodE (CICE) on the same grid, the HYCOM/CICE/NCODA system (initially restricted to the Arctic) provides sea ice nowcasts and forecasts. Ice edge location errors are improved from the previous sea ice prediction system but are limited in part by the accuracy of the satellite observations it assimilates.

Navy Nearshore Ocean Prediction Systems

Abstract : Knowledge of the nearshore ocean environment is important for naval operations, including military and humanitarian applications. The models used by the US Navy for predicting waves and circulation in the coastal regions are presented here. The wave model of choice for coastal regions is the Simulating WAves Nearshore (SWAN) model, which predicts wave energy as a function of frequency and direction. SWAN is forced by winds as well as waves at the offshore boundaries. For coastal circulation, Delft3D, composed of a number of different modules that can be coupled with each other, is presently used. Most applications for daily operational predictions use only the Delft3D-FLOW module, which predicts currents, mean water levels, temperature, and salinity. Inputs to the model include winds, tides, general ocean circulation, waves, daily river discharges, temperature, and salinity. Delft3D-FLOW is coupled with the Delft3D-WAVE module for areas where wave effects are of importance. A four-dimensional variational assimilation (4DVar) system based on SWAN, the SWANFAR system, is under development for nearshore wave predictions. It will improve wave predictions by using regional wave observations. We present several case studies that illustrate the validation and diverse applications of these models. All operational systems are run at the Naval Oceanographic Office.

User Focus and Simulation Improve Predictions of Piracy Risk

Piracy is an increasingly costly and violent threat to commercial shipping and other vessels off the Horn of Africa. However, because pirates operate in small vessels that cannot navigate or attack in high seas or winds, pirate activity is highly sensitive to environmental conditions. The US Naval Oceanographic Office provides an operational forecast of the pirate threat; counterpiracy forces use this forecast to allocate their efforts over several million square miles. The most recent version uses simulation to model the effects of pirate behavior in interaction with winds, waves, and currents over time to forecast the geographic distribution of the pirate threat. As part of the development of the pirate behavior model, one author traveled to Bahrain to interview counter-piracy forces. We then used carefully designed simulation experiments to identify the variables that are most influential in determining the distribution of predicted pirate activity. The results confirmed the importance of elements of the pirate behavior model that were derived from our operator interviews, informed decisions regarding operational settings for key parameters, and generated insights to guide future updates to the model and intelligence-gathering efforts. The resulting model uses our recommendations, including alternate pirate search patterns. It has been operational since March 2011 and is briefed daily to the senior leadership of US Naval Forces Central Command.

A COMPARISON OF GREAT CIRCLE, GREAT ELLIPSE, AND GEODESIC SAILING

An analytical and numerical comparison of great circle (GC) sailing, great elliptic (GE) sailing, and geodesic (Geod) sailing is presented. The comparison between GC and GE sailing addresses some problems whether the navigator and navigational software developers promptly have to use GE sailing or use hybrid sailing mixed with features of the GC sailing and GE sailing. This fact found here presents that the formulae tackling relationship of latitude and longitude of GC sailing also can be suited to the GE sailing except some calculation of GE sailing involving distance and course. The validity of effectiveness of proposed GE sailing has been verified with numerical tests and compared with extremely accurate geodetic methods (Vincenty's method). The numerical tests calculate the standard deviation of large sample of distance differences comparing GE sailing and Andoyer-Lambert method to Geod sailing. The result reveals that the mean and the standard deviation of distance differences of GE is one half and one sixth of Andoyer-Lambert method. The significance gives the assertion that the accuracy of GE sailing is better than Andoyer-Lambert method which (UK) Royal Navy and (US) Naval Oceanographic Office preferred spheroidal mathematical solution. We also give a dynamic programming recursive algorithm attaining any requirement of accuracy for distance calculation of GE sailing and more compact computational procedure of intermediate points along the GE. The course of GE sailing can be obtained from the proposed course reduction of GC sailing.

Development of a Global Oil Spill Modeling System

This paper describes the development of an oil spill modeling system that is operational on a global scale and can be used for both real-time response, forecast simulations and probabilistic risk analysis based on climatological wind and ocean current data. For ocean and estuarine spills, the system makes use of the General NOAA Operational Modeling Environment (GNOME) oil spill model, Trajectory Analysis Planner and the Automated Data Inquiry for Oil Spills weathering model. Hydrodynamic and meteorological data is obtained from the US Navy and National Oceanic and Atmospheric Administration. Data access is provided through the Naval Oceanographic Office, the Fleet Numerical Meteorological and Oceanographic Center and the GNOME Online Oceanographic Data Server. For riverine spills, the GeoSpatial Stream Flow Model and the Incident Command Tool for Drinking Water Protection are used to respectively, build river networks with associated flows and velocities and, transport and disperse oil spill contamination downstream. Case study examples are presented for both forecast simulations and probabilistic risk analysis.