Publications

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2017
Pawlak, G, Moline MA, Terrill EJ, Colin PL.  2017.  Hydrodynamic influences on acoustical and optical backscatter in a fringing reef environment. Journal of Geophysical Research-Oceans. 122:322-335.   10.1002/2016jc012497   AbstractWebsite

Observations of hydrodynamics along with optical and acoustical water characteristics in a tropical fringing reef environment reveal a distinct signature associated with flow characteristics and tidal conditions. Flow conditions are dominated by tidal forcing with an offshore component from the reef flat during ebb. Measurements span variable wave conditions enabling identification of wave effects on optical and acoustical water properties. High-frequency acoustic backscatter (6 MHz) is strongly correlated with tidal forcing increasing with offshore directed flow and modulated by wave height, indicating dominant hydrodynamic influence. Backscatter at 300 and 1200 kHz is predominantly diurnal suggesting a biological component. Optical backscatter is closely correlated with high-frequency acoustic backscatter across the range of study conditions. Acoustic backscatter frequency dependence is used along with changes in optical properties to interpret particle-size variations. Changes across wave heights suggest shifts in particle-size distributions with increases in relative concentrations of smaller particles for larger wave conditions. Establishing a connection between the physical processes of a fringing tropical reef and the resulting acoustical and optical signals allows for interpretation and forecasting of the remote sensing response of these phenomena over larger scales.

2016
Campana, J, Terrill EJ, de Paolo T.  2016.  The development of an inversion technique to extract vertical current profiles from X-band radar observations. Journal of Atmospheric and Oceanic Technology. 33:2015-2028.   10.1175/jtech-d-15-0145.1   AbstractWebsite

The influence of wave-current interactions on time series of marine X-band radar backscatter maps at the mouth of the Columbia River (MCR) near Astoria, Oregon, is examined. The energetic wave environment at the MCR, coupled with the strong tidally forced currents, provides a unique test environment to explore the limitations in accurately determining the magnitude and vertical structure of upper-ocean currents from wavefield measurements. Direct observation in time and space of the wave-induced radar backscatter and supporting acoustic Doppler current profiler (ADCP) current measurements provide a rich dataset for investigating how currents shift the observed wave dispersion relationship. First, current extraction techniques that assume a specific current-depth profile are tested against ADCP measurements. These constrained solutions prove to have inaccuracies because the models do not properly account for vertical shear. A forward solution using measured current profiles to predict the wavenumber-Doppler shift relationship for the range of ocean waves sensed by the radar is introduced. This approach confirms the ocean wavefield is affected by underlying vertical current shear. Finally, a new inversion method is developed to extract current profiles from the wavenumber-dependent Doppler shift observations. The success of the inversion model is shown to be sensitive to the range of wavenumbers spanned by observed Doppler shifts, with skill exceeding 0.8 when wavenumbers span more than 0.1 radm 21. This agreement when observations successfully capture the broadband wavefield suggests the X-band backscatter is a viable means of remotely estimating current shear.

2015
Harada, AE, Lindgren EA, Hermsmeier MC, Rogowski PA, Terrill E, Burton RS.  2015.  Monitoring spawning activity in a Southern California marine protected area using molecular identification of fish eggs. Plos One. 10   10.1371/journal.pone.0134647   AbstractWebsite

In order to protect the diverse ecosystems of coastal California, a series of marine protected areas (MPAs) have been established. The ability of these MPAs to preserve and potentially enhance marine resources can only be assessed if these habitats are monitored through time. This study establishes a baseline for monitoring the spawning activity of fish in the MPAs adjacent to Scripps Institution of Oceanography (La Jolla, CA, USA) by sampling fish eggs from the plankton. Using vertical plankton net tows, 266 collections were made from the Scripps Pier between 23 August 2012 and 28 August 2014; a total of 21,269 eggs were obtained. Eggs were identified using DNA barcoding: the COI or 16S rRNA gene was amplified from individual eggs and sequenced. All eggs that were successfully sequenced could be identified from a database of molecular barcodes of California fish species, resulting in species-level identification of 13,249 eggs. Additionally, a surface transport model of coastal circulation driven by current maps from high frequency radar was used to construct probability maps that estimate spawning locations that gave rise to the collected eggs. These maps indicated that currents usually come from the north but water parcels tend to be retained within the MPA; eggs sampled at the Scripps Pier have a high probability of having been spawned within the MPA. The surface transport model also suggests that although larvae have a high probability of being retained within the MPA, there is also significant spill-over into nearby areas outside the MPA. This study provides an important baseline for addressing the extent to which spawning patterns of coastal California species may be affected by future changes in the ocean environment.

Terrill, EJ, Taylor GRL.  2015.  Entrainment of air at the transoms of full-scale surface ships. Journal of Ship Research. 59:49-65.   10.5957/josr.59.1.140014   AbstractWebsite

We report on the results from a series of full-scale trials designed to quantify the air entrainment at the stern of an underway vessel. While an extremely complex region to model air entrainment due to the confluence of the breaking transom wave, bubbles from the bow, turbulence from the hull boundary layer, and bubbles and turbulence from propellers, the region is a desirable area to characterize and understand because it serves as the initial conditions of a ship's far-field bubbly wake. Experiments were conducted in 2003 from RN Revel le and 2004 from RN Athena II using a custom-built conductivity probe vertical array that could be deployed at the blunt transom of a full-scale surface ship to measure the void fraction field. The system was designed to be rugged enough to withstand the full speed range of the vessels. From the raw time-series data, the entrainment of air at speeds ranging from 2.1 to 7.2 m/s is computed at various depths and beam locations. The data represent the first such in-situ measurements from a full-scale vessel and can be used to validate two-phase ship hydrodynamic CFD codes and initialize far-field, bubbly wake CFD models.

Rogowski, PA, Terrill E, Schiff K, Kim SY.  2015.  An assessment of the transport of southern California stormwater ocean discharges. Marine Pollution Bulletin. 90:135-142.   10.1016/j.marpolbul.2014.11.004   AbstractWebsite

The dominant source of coastal pollution adversely affecting the regional coastal water quality is the seasonally variable urban runoff discharged via southern California's rivers. Here, we use a surface transport model of coastal circulation driven by current maps from high frequency radar to compute two-year hindcasts to assess the temporal and spatial statistics of 20 southern California stormwater discharges. These models provide a quantitative, statistical measure of the spatial extent of the discharge plumes in the coastal receiving waters, defined here as a discharge's "exposure". We use these exposure maps from this synthesis effort to (1) assess the probability of stormwater connectivity to nearby Marine Protected Areas, and (2) develop a methodology to estimate the mass transport of stormwater discharges. The results of the spatial and temporal analysis are found to be relevant to the hindcast assessment of coastal discharges and for use in forecasting transport of southern California discharges. (C) 2014 Elsevier Ltd. All rights reserved.

2014
Rogowski, P, Terrill E, Chen JL.  2014.  Observations of the frontal region of a buoyant river plume using an autonomous underwater vehicle. Journal of Geophysical Research-Oceans. 119:7549-7567.   10.1002/2014jc010392   AbstractWebsite

To characterize the transitional region from the near-field to far-field of a river plume entering coastal waters, we conducted four surveys using an autonomous underwater vehicle (AUV) to target the outflow of the New River Inlet, North Carolina, during maximum ebb tide. The utilization of a mobile sensor to synoptically observe current velocity data in tandem with natural river plume tracers (e.g., colored dissolved organic matter, salinity) was essential in understanding the mechanisms driving the observed circulation and mixing patterns within these waters. We find that this region is regularly impacted by two primary processes: (1) the interaction of an old dredged channel plume with the main discharge and (2) the recirculation of the discharge plume by an eddy that persistently forms between the old channel and main discharge location. Wind-driven processes in the nearshore can enhance the interaction of these two plumes resulting in unstable regions where mixing of the merged plume with the receiving waters is accelerated. We also conduct comparisons between AUV velocity observations from two surveys and their corresponding velocity outputs from a parallelized quasi-3-D model. We conclude that the ability to observe the estuarine outflow transitional region at near-synoptic temporal scales and resolutions discussed in this paper is key in providing the mechanisms driving local circulation which is essential for proper parameterization of high-resolution numerical coastal models. Key Points Wind-driven processes in the nearshore can enhance mixing of discharge plumes Synoptic observations are key to understanding the structure of dynamic plume discharges Choosing appropriate hydrodynamic model parameters can be accomplished using AUV data

Lund, B, Collins CO, Graber HC, Terrill E, Herbers THC.  2014.  Marine radar ocean wave retrieval's dependency on range and azimuth. Ocean Dynamics. 64:999-1018.   10.1007/s10236-014-0725-6   AbstractWebsite

The strength of the surface wave signal in marine X-band radar (MR) images strongly depends on range and azimuth (i.e., the angle between antenna look and peak wave direction). Traditionally, MR wave analysis is carried out in a set of rectangular windows covering the radar field of view (FOV). The FOV is typically partially obstructed, e.g., due to the coastline or ship superstructures. Especially for ships that are subject to regular course changes, this results in an increased variability or error associated with wave parameters. Using MR measurements from R/P FLIP, acquired off California during the 2010 US Office of Naval Research (ONR) high resolution air-sea interaction (Hi-Res) experiment, this study quantifies the dependency of the radar-based 2D wave spectrum and parameters on range and azimuth. With the help of reference data from a nearby Datawell Waverider buoy, we propose empirical methods to remove the dependency and we illustrate their efficacy.

2013
Rogowski, P, Terrill E, Otero M, Hazard L, Middleton W.  2013.  Ocean outfall plume characterization using an autonomous underwater vehicle. Water Science and Technology. 67:925-933.   10.2166/wst.2012.635   AbstractWebsite

A monitoring mission to map and characterize the Point Loma Ocean Outfall (PLOO) wastewater plume using an Autonomous Underwater Vehicle (AUV) was performed on 3 March 2011. The mobility of an AUV provides a significant advantage in surveying discharge plumes over traditional cast-based methods, and when combined with optical and oceanographic sensors, provides a capability for both detecting plumes and assessing their mixing in the near and far-fields. Unique to this study is the measurement of Colored Dissolved Organic Matter (CDOM) in the discharge plume and its application for quantitative estimates of the plume's dilution. AUV mission planning methodologies for discharge plume sampling, plume characterization using onboard optical sensors, and comparison of observational data to model results are presented. The results suggest that even under variable oceanic conditions, properly planned missions for AUVs equipped with an optical CDOM sensor in addition to traditional oceanographic sensors, can accurately characterize and track ocean outfall plumes at higher resolutions than cast-based techniques.

2012
Rogowski, P, Terrill E, Otero M, Hazard L, Middleton W.  2012.  Mapping ocean outfall plumes and their mixing using autonomous underwater vehicles. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007804   AbstractWebsite

This paper reports on developing autonomous underwater vehicle (AUV) survey methods for ocean outfall discharge plumes and new insights gained on plume mixing. Unique to the study is mapping the discharge mixing using colored dissolved organic matter (CDOM) calibrated for effluent dilution. AUV mission planning methodologies for discharge plume sampling, plume characterization using onboard temperature, salinity and optical sensors, and comparison of observational data to model results are presented for the Point Loma Ocean Outfall offshore of San Diego, CA. The results are expected to be applicable to the general theme of mixing of submerged buoyant discharges. In the near-field, the plume is found to mix to a height consistent with the predictions of buoyant jet engineering models. At the far-field, the fine spatial scales of the plume resolved by the vehicle suggests that shear instabilities caused by internal waves can enhance plume mixing and elevate the discharge plume above the predicted equilibrium rise height. These results suggest that even under variable oceanic conditions, properly planned missions for AUVs equipped with an optical CDOM sensor in addition to traditional physical oceanographic sensors, can accurately map the mixing of ocean outfall plumes at resolutions not possible with traditional boat-based techniques. Variations of oceanic conditions are found to influence the mixing and fate of the plume at time scales generally not considered in the design of these discharges.

2011
Kim, SY, Terrill EJ, Cornuelle BD, Jones B, Washburn L, Moline MA, Paduan JD, Garfield N, Largier JL, Crawford G, Kosro PM.  2011.  Mapping the U.S. West Coast surface circulation: A multiyear analysis of high-frequency radar observations. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006669   AbstractWebsite

The nearly completed U. S. West Coast (USWC) high-frequency radar (HFR) network provides an unprecedented capability to monitor and understand coastal ocean dynamics and phenomenology through hourly surface current measurements at up to 1 km resolution. The dynamics of the surface currents off the USWC are governed by tides, winds, Coriolis force, low-frequency pressure gradients (less than 0.4 cycles per day (cpd)), and nonlinear interactions of those forces. Alongshore surface currents show poleward propagating signals with phase speeds of O(10) and O(100 to 300) km day(-1) and time scales of 2 to 3 weeks. The signals with slow phase speed are only observed in southern California. It is hypothesized that they are scattered and reflected by shoreline curvature and bathymetry change and do not penetrate north of Point Conception. The seasonal transition of alongshore surface circulation forced by upwelling-favorable winds and their relaxation is captured in fine detail. Submesoscale eddies, identified using flow geometry, have Rossby numbers of 0.1 to 3, diameters in the range of 10 to 60 km, and persistence for 2 to 12 days. The HFR surface currents resolve coastal surface ocean variability continuously across scales from submesoscale to mesoscale (O(1) km to O(1000) km). Their spectra decay with k(-2) at high wave number (less than 100 km) in agreement with theoretical submesoscale spectra below the observational limits of present-day satellite altimeters.

2010
Harlan, J, Terrill E, Hazard L, Keen C, Barrick D, Whelan C, Howden S, Kohut J.  2010.  The Integrated Ocean Observing System High-Frequency Radar Network: Status and Local, Regional, and National Applications. Marine Technology Society Journal. 44:122-132. AbstractWebsite

A national high-frequency radar network has been created over the past 20 years or so that provides hourly 2-D ocean surface current velocity fields in near real time from a few kilometers offshore out to approximately 200 km. This preoperational network is made up of more than 100 radars from 30 different institutions. The Integrated Ocean Observing System efforts have supported the standards-based ingest and delivery of these velocity fields to a number of applications such as coastal search and rescue, oil spill response, water quality monitoring, and safe and efficient 1 marine navigation. Thus, regardless of the operating institution or location of the radar systems, emergency response managers, and other users, can rely on a common source and means of obtaining and using the data. Details of the history, the physics, and the application of high-frequency radar are discussed with successes of the integrated network highlighted.

Kim, SY, Cornuelle BD, Terrill EJ.  2010.  Decomposing observations of high-frequency radar-derived surface currents by their forcing mechanisms: Decomposition techniques and spatial structures of decomposed surface currents. Journal of Geophysical Research-Oceans. 115   10.1029/2010jc006222   AbstractWebsite

Surface current observations from a high-frequency radar network deployed in southern San Diego are decomposed according to their driving forces: pure tides and their neighboring off-band energy, local winds, and low frequency. Several superposed ocean responses are present as a result of the complicated bottom topography and relatively weak winds off southern San Diego, as opposed to coastal regions where circulation can be explained by a dominant forcing mechanism. This necessitates an application of a statistical decomposition approach. Surface currents coherent with pure tides are calculated using harmonic analysis. Locally wind-driven surface currents are estimated by regression of observed winds on observed surface currents. The dewinded and detided surface currents are filtered by weighted least-squares fitting assuming white noise and three colored signal bands: low-frequency band (less than 0.4 cycles per day) and near-tidal peaks at the diurnal (K-1) and semidiurnal (M-2) frequencies. The spatial and temporal variability of each part of the decomposed surface currents is investigated in terms of ocean response to the driving forces. In addition, the spatial correlations of individual components exhibit Gaussian and exponential shapes with varying decorrelation length scales.

Kim, SY, Cornuelle BD, Terrill EJ.  2010.  Decomposing observations of high-frequency radar-derived surface currents by their forcing mechanisms: Locally wind-driven surface currents. Journal of Geophysical Research-Oceans. 115   10.1029/2010jc006223   AbstractWebsite

The wind impulse response function and transfer function for high-frequency radar-derived surface currents off southern San Diego are calculated using several local wind observations. The spatial map of the transfer function reflects the influence of the coast on wind-current dynamics. Near the coast (within 20 km from the shoreline), the amplitudes of the transfer function at inertial and diurnal frequencies are reduced due to effects of coastline and bottom bathymetry. Meanwhile, the amplitude of low-frequency currents increases near the coast, which is attributed to the local geostrophic balance between cross-shore pressure gradients against the coast and currents. Locally wind-driven surface currents are estimated from the data-derived response function, and their power spectrum shows a strong diurnal peak superposed on a red spectrum, similar to the spectra of observed winds. Current magnitudes and veering angles to a quasi-steady wind are typically 2-5% of the wind speed and vary 50 degrees-90 degrees to the right of the wind, respectively. A wind skill map is introduced to present the fractional variance of surface currents explained by local winds as a verification tool for wind data quality and relevance. Moreover, the transfer functions in summer and winter are presented to examine the seasonal variation in ocean surface current response to the wind associated with stratification change.

2009
Hoteit, I, Cornuelle B, Kim SY, Forget G, Kohl A, Terrill E.  2009.  Assessing 4D-VAR for dynamical mapping of coastal high-frequency radar in San Diego. Dynamics of Atmospheres and Oceans. 48:175-197.   10.1016/j.dynatmoce.2008.11.005   AbstractWebsite

The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is investigated using a three-dimensional hydrodynamic model of the San Diego coastal region and an adjoint-based assimilation method. The HF radar provides near-real-time radial velocities from three sites covering the region offshore of San Diego Bay. The hydrodynamical model is the Massachusetts Institute of Technology general circulation model (MITgcm) with 1 km horizontal resolution and 40 vertical layers. The domain is centered on Point Loma, extending 117 km offshore and 120 km alongshore. The reference run (before adjustment) is initialized from a single profile of T and S and is forced with wind data from a single shore station and with zero heat and fresh water fluxes. The adjoint of the model is used to adjust initial temperature, salinity, and velocity, hourly temperature, salinity and horizontal velocities at the open boundaries, and hourly surface fluxes of momentum, heat and freshwater so that the model reproduces hourly HF radar radial velocity observations. Results from a small number of experiments suggest that the adjoint method can be successfully used over 10-day windows at coastal model resolution. It produces a dynamically consistent model run that fits HF radar data with errors near the specified uncertainties. In a test of the forecasting capability of the San Diego model after adjustment, the forecast skill was shown to exceed persistence for up to 20 h. (C) 2008 Elsevier B.V. All rights reserved.

Kim, SY, Terrill EJ, Cornuelle BD.  2009.  Assessing Coastal Plumes in a Region of Multiple Discharges: The US-Mexico Border. Environmental Science & Technology. 43:7450-7457.   10.1021/es900775p   AbstractWebsite

The San Diego/Tijuana border region has several environmental challenges with regard to assessing water quality impacts resulting from local coastal ocean discharges for which transport is not hindered by political boundaries. While an understanding of the fate and transport of these discharged plumes has a broad audience, the spatial and temporal scales of the physical processes present numerous challenges in conducting assessment with any fidelity. To address these needs, a data-driven model of the transport of both shoreline and offshore discharges is developed and operated in a hindcast mode for a four-year period to analyze regional connectivity between the discharges and the receiving of waters and the coastline. The plume exposure hindcast model is driven by surface current data generated by a network of high-frequency radars. Observations provided by both boat-based CTD measurements and fixed oceanographic moorings are used with the Roberts-Snyder-Baumgartner model to predict the plume rise height. The surface transport model outputs are compared with shoreline samples of fecal indicator bacteria (FIB), and the skill of the model to assess low water quality is evaluated using receiver operating characteristic (ROC) analysis.

Kim, SY, Cornuelle BD, Terrill EJ.  2009.  Anisotropic Response of Surface Currents to the Wind in a Coastal Region. Journal of Physical Oceanography. 39:1512-1533.   10.1175/2009JPO4013.1   Abstract

Analysis of coastal surface currents measured off the coast of San Diego for two years suggests an anisotropic and asymmetric response to the wind, probably as a result of bottom/coastline boundary effects, including pressure gradients. In a linear regression, the statistically estimated anisotropic response explains approximately 20% more surface current variance than an isotropic wind-ocean response model. After steady wind forcing for three days, the isotropic surface current response veers 42 degrees +/- 2 degrees to the right of the wind regardless of wind direction, whereas the anisotropic analysis suggests that the upcoast (onshore) wind stress generates surface currents with 10 degrees +/- 4 degrees (71 degrees +/- 3 degrees) to the right of the wind direction. The anisotropic response thus reflects the dominance of alongshore currents in this coastal region. Both analyses yield wind-driven currents with 3%-5% of the wind speed, as expected. In addition, nonlinear isotropic and anisotropic response functions are considered, and the asymmetric current responses to the wind are examined. These results provide a comprehensive statistical model of the wind-driven currents in the coastal region, which has not been well identified in previous field studies, but is qualitatively consistent with descriptions of the current response in coastal ocean models.

Piskozub, J, Stramski D, Terrill E, Melville WK.  2009.  Small-scale effects of underwater bubble clouds on ocean reflectance: 3-D modeling results. Optics Express. 17:11747-11752.   10.1364/OE.17.011747   AbstractWebsite

We examined the effect of individual bubble clouds on remote-sensing reflectance of the ocean with a 3-D Monte Carlo model of radiative transfer. The concentrations and size distribution of bubbles were defined based on acoustical measurements of bubbles in the surface ocean. The light scattering properties of bubbles for various void fractions were calculated using Mie scattering theory. We show how the spatial pattern, magnitude, and spectral behavior of remote-sensing reflectance produced by modeled bubble clouds change due to variations in their geometric and optical properties as well as the background optical properties of the ambient water. We also determined that for realistic sizes of bubble clouds, a plane-parallel horizontally homogeneous geometry (1-D radiative transfer model) is inadequate for modeling water-leaving radiance above the cloud. (C) 2009 Optical Society of America

Moline, MA, Blackwell SM, Case JF, Haddock SHD, Herren CM, Orrico CM, Terrill E.  2009.  Bioluminescence to reveal structure and interaction of coastal planktonic communities. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:232-245.   10.1016/j.dsr2.2008.08.002   AbstractWebsite

Ecosystem function will in large part be determined by functional groups present in biological communities. The simplest distinction with respect to functional groups of an ecosystem is the differentiation between primary and secondary producers. A challenge thus far has been to examine these groups simultaneously with sufficient temporal and spatial resolution for observations to be relevant to the scales of change in coastal oceans. This study takes advantage of general differences in the bioluminescence flash kinetics between planktonic dinoflagellates and zooplankton to measure relative abundances of the two groups within the same-time space volume. This novel approach for distinguishing these general classifications using a single sensor is validated using fluorescence data and exclusion experiments. The approach is then applied to data collected from an autonomous underwater vehicle surveying > 500 km in Monterey Bay and San Luis Obispo Bay, CA during the summers of 2002-2004. The approach also reveals that identifying trophic interaction between the two planktonic communities may also be possible. (C) 2008 Elsevier Ltd. All rights reserved.

Zedler, SE, Niiler PP, Stammer D, Terrill E, Morzel J.  2009.  Ocean's response to Hurricane Frances and its implications for drag coefficient parameterization at high wind speeds. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc005205   AbstractWebsite

The drag coefficient parameterization of wind stress is investigated for tropical storm conditions using model sensitivity studies. The Massachusetts Institute of Technology (MIT) Ocean General Circulation Model was run in a regional setting with realistic stratification and forcing fields representing Hurricane Frances, which in early September 2004 passed east of the Caribbean Leeward Island chain. The model was forced with a NOAA-HWIND wind speed product after converting it to wind stress using four different drag coefficient parameterizations. Respective model results were tested against in situ measurements of temperature profiles and velocity, available from an array of 22 surface drifters and 12 subsurface floats. Changing the drag coefficient parameterization from one that saturated at a value of 2.3 x 10(-3) to a constant drag coefficient of 1.2 x 10(-3) reduced the standard deviation difference between the simulated minus the measured sea surface temperature change from 0.8 degrees C to 0.3 degrees C. Additionally, the standard deviation in the difference between simulated minus measured high pass filtered 15-m current speed reduced from 15 cm/s to 5 cm/s. The maximum difference in sea surface temperature response when two different turbulent mixing parameterizations were implemented was 0.3 degrees C, i.e., only 11% of the maximum change of sea surface temperature caused by the storm.

2008
Kim, SY, Terrill EJ, Cornuelle BD.  2008.  Mapping surface currents from HF radar radial velocity measurements using optimal interpolation. Journal of Geophysical Research-Oceans. 113   10.1029/2007jc004244   AbstractWebsite

An optimal interpolation (OI) method to compute surface vector current fields from radial velocity measurements derived from high-frequency (HF) radars is presented. The method assumes a smooth spatial covariance relationship between neighboring vector currents, in contrast to the more commonly used un-weighted least-squares fitting (UWLS) method, which assumes a constant vector velocity within a defined search radius. This OI method can directly compute any quantities linearly related to the radial velocities, such as vector currents and dynamic quantities (divergence and vorticity) as well as the uncertainties of those respective fields. The OI method is found to be more stable than the UWLS method and reduces spurious vector solutions near the baselines between HF radar installations. The OI method produces a covariance of the uncertainty of the estimated vector current fields. Three nondimensional uncertainty indices are introduced to characterize the uncertainty of the vector current at a point, representing an ellipse with directional characteristics. The vector current estimation using the OI method eliminates the need for multiple mapping steps and optimally fills intermittent coverage gaps. The effects of angular interpolation of radial velocities, a commonly used step in the preprocessing of radial velocity data prior to vector current computation in the UWLS method, are presented.

Beckenbach, E, Terrill E.  2008.  Internal tides over abrupt topography in the Southern California Bight: Observations of diurnal waves poleward of the critical latitude. Journal of Geophysical Research-Oceans. 113   10.1029/2006jc003905   AbstractWebsite

[1] In the ocean, a mechanism for baroclinic internal tide generation is through the interaction of barotropic tidal currents with topography. Velocity profiles from a bottom-mounted acoustic Doppler current profiler are used to characterize the tidal generation of diurnal and semidiurnal internal modes over a ridge atop an abrupt 30° bathymetric slope in the Southern California Bight. This location is poleward of the critical latitude for diurnal frequency internal waves, yet the circulation is dominated by the diurnal internal tide. Overall, the structure of the diurnal internal tide resembles the first baroclinic mode. However, the mode is topographically distorted in the lower water column where it is amplified and polarized in the cross-ridge direction. The behavior of the diurnal response is contrasted with the nearly ideal appearance of the semidiurnal response. The diurnal baroclinic mode occurs as a persistent response to tidal forcing rather than because of the diurnal sea breeze as has also been suggested as an explanation for internal diurnal tides in other areas of the Southern California Bight.

2007
Terrill, EJ, Otero M, Hazard L.  2007.  Mapping surface currents around US coasts - A network of high-frequency radar for the Integrated Ocean Observing System. Sea Technology. 48:29-+.   10.1029/2007JC004244   AbstractWebsite

[1] An optimal interpolation (OI) method to compute surface vector current fields from radial velocity measurements derived from high-frequency (HF) radars is presented. The method assumes a smooth spatial covariance relationship between neighboring vector currents, in contrast to the more commonly used un-weighted least-squares fitting (UWLS) method, which assumes a constant vector velocity within a defined search radius. This OI method can directly compute any quantities linearly related to the radial velocities, such as vector currents and dynamic quantities (divergence and vorticity) as well as the uncertainties of those respective fields. The OI method is found to be more stable than the UWLS method and reduces spurious vector solutions near the baselines between HF radar installations. The OI method produces a covariance of the uncertainty of the estimated vector current fields. Three nondimensional uncertainty indices are introduced to characterize the uncertainty of the vector current at a point, representing an ellipse with directional characteristics. The vector current estimation using the OI method eliminates the need for multiple mapping steps and optimally fills intermittent coverage gaps. The effects of angular interpolation of radial velocities, a commonly used step in the preprocessing of radial velocity data prior to vector current computation in the UWLS method, are presented.

Warrick, JA, DiGiacomo PM, Welsberg SB, Nezlin NP, Mengel M, Jones BH, Ohlmann JC, Washburn L, Terrill EJ, Farnsworth KL.  2007.  River plume patterns and dynamics within the Southern California Bight. Continental Shelf Research. 27:2427-2448.   10.1016/j.csr.2007.06.015   AbstractWebsite

Stormwater river plumes are important vectors of marine contaminants and pathogens in the Southern California Bight. Here we report the results of a multi-institution investigation of the river plumes across eight major river systems of southern California. We use in situ water samples from multi-day cruises in combination with MODIS satellite remote sensing, buoy meteorological observations, drifters, and HF radar current measurements to evaluate the dispersal patterns and dynamics of the freshwater plumes. River discharge was exceptionally episodic, and the majority of storm discharge occurred in a few hours. The combined plume observing techniques revealed that plumes commonly detach from the coast and turn to the left, which is the opposite direction of Coriolis influence. Although initial offshore velocity of the buoyant plumes was similar to 50 cm/s and was influenced by river discharge inertia (i.e., the direct momentum of the river flux) and buoyancy, subsequent advection of the plumes was largely observed in an alongshore direction and dominated by local winds. Due to the multiple day upwelling wind conditions that commonly follow discharge events, plumes were observed to flow from their respective river mouths to down-coast waters at rates of 20-40 km/d. Lastly, we note that suspended-sediment concentration and beam-attenuation were poorly correlated with plume salinity across and within the sampled plumes (mean r(2) = 0.12 and 0.25, respectively), while colored dissolved organic matter (CDOM) fluorescence was well correlated (mean r(2) = 0.56), suggesting that CDOM may serve as a good tracer of the discharged freshwater in subsequent remote sensing and monitoring efforts of plumes. Published by Elsevier Ltd.

Black, PG, D'Asaro EA, Drennan WM, French JR, Niiler PP, Sanford TB, Terrill EJ, Walsh EJ, Zhang JA.  2007.  Air-sea exchange in hurricanes - Synthesis of observations from the coupled boundary layer air-sea transfer experiment. Bulletin of the American Meteorological Society. 88:357-+.   10.1175/bams-88-3-357   AbstractWebsite

The Coupled Boundary Layer Air-Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air-sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (C-D) values derived from CBLAST momentum flux measurements show C-D becoming invariant with wind speed near a 23 m s(-1) threshold rather than a hurricane-force threshold near 33 m s-(1.) Values above 23 m s(-1) are lower than previous open-ocean measurements. The Dalton number estimates (C-E) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s(-1), which is in approximate agreement with previous measurements at lower winds. These observations imply a C-E/C-D ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be "roll vortex" or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9-1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery. As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and sub-surface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2 degrees C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s(-1) were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

Kim, SY, Terrill E, Cornuelle B.  2007.  Objectively mapping HF radar-derived surface current data using measured and idealized data covariance matrices. Journal of Geophysical Research-Oceans. 112   10.1029/2006jc003756   AbstractWebsite

Surface currents measured by high-frequency radars are objectively mapped using covariance matrices computed from hourly surface current vectors spanning two years. Since retrievals of surface radial velocities are inherently gappy in space and time, the irregular density of surface current data leads to negative eigenvalues in the sample covariance matrix. The number and the magnitude of the negative eigenvalues depend on the degree of data continuity used in the matrix computation. In a region of 90% data coverage, the negative eigenvalues of the sample covariance matrix are small enough to be removed by adding a noise term to the diagonal of the matrix. The mapping is extended to regions of poorer data coverage by applying a smoothed covariance matrix obtained by spatially averaging the sample covariance matrix. This approach estimates a stable covariance matrix of surface currents for regions with the intermittent radar coverage. An additional benefit is the removal of baseline errors that often exist between two radar sites. The covariance matrices and the correlation functions of the surface currents are exponential in space rather than Gaussian, as is often assumed in the objective mapping of oceanographic data sets. Patterns in the decorrelation length scale provide the variabilities of surface currents and the insights on the influence of topographic features (bathymetry and headlands). The objective mapping approach presented herein lends itself to various applications, including the Lagrangian transport estimates, dynamic analysis through divergence and vorticity of current vectors, and statistical models of surface currents.