Eric Terrill

Air–Sea Exchange in Hurricanes: Synthesis of Observations from the Coupled Boundary Layer Air–Sea Transfer Experiment

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 (CD) values derived from CBLAST momentum flux measurements show CD 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 (CE) 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 CE/CD ratio of approximately 0.7, suggesting that additional energy sour...

Interpretation of Coastal HF Radar–Derived Surface Currents with High-Resolution Drifter Data

Abstract Dense arrays of surface drifters are used to quantify the flow field on time and space scales over which high-frequency (HF) radar observations are measured. Up to 13 drifters were repetitively deployed off the Santa Barbara and San Diego coasts on 7 days during 18 months. Each day a regularly spaced grid overlaid on a 1-km2 (San Diego) or 4-km2 (Santa Barbara) square, located where HF radar radial data are nearly orthogonal, was seeded with drifters. As drifters moved from the square, they were retrieved and replaced to maintain a spatially uniform distribution of observations within the sampling area during the day. This sampling scheme resulted in up to 56 velocity observations distributed over the time (1 h) and space (1 and 4 km2) scales implicit in typical surface current maps from HF radar. Root-mean-square (RMS) differences between HF radar radial velocities obtained using measured antenna patterns, and average drifter velocities, are mostly 3–5 cm s−1. Smaller RMS differences compared wi...

Bubble entrainment by breaking waves and their influence on optical scattering in the upper ocean

Breaking waves at the oceans surface inject bubbles and turbulence into the water column. During periods of rough weather the scales of wave breaking will increase with increasing sea states and result in mixing of the surface waters and the turbulent transport of bubbles to depth. Depending on their concentrations and size distribution, the entrained bubbles can significantly change the optical properties of water, introducing potentially significant errors in retrieval of remotely sensed hyperspectral data products. In this paper, the effects of bubbles on optical scattering in the upper ocean are investigated through optical scattering calculations based on field measurements of bubble populations. The field measurements were obtained offshore Point Conception, California, in June 1997, using an acoustical technique which measured the bubble size distribution at 2 Hz from a surface buoy designed to follow the longer waves. The effects of the bubbles on the bulk optical scattering and backscattering coefficients, b and bb, respectively, are determined by using the acoustically measured size distributions, and size-dependent scattering efficiencies based on Mie scattering calculations. Time series of the bubble distributions measured in rough conditions (wind speed, U10 = 15 m/s, significant wave height, H1/3 = 3.2 m) suggest that the bubble contribution to light scattering is highly variable near the ocean surface, with values spanning roughly 5 decades over time periods of O(10) minutes. Bubble size distributions measured at a 0.7-m depth indicate that the optical effects of the bubbles on bb, and hence the remote sensing reflectance, will be significant at bubble void fractions above 10−6 and that the bubble contribution to total bb will exceed values of 10−2 m−1 inside bubble clouds.

SeaSonde Radial Velocities: Derivation and Internal Consistency

This paper describes the methods presently used to produce unaveraged radial velocity maps from radar voltage cross spectra measured by a SeaSonde, including a discussion of the multiple signal classification (MUSIC) algorithm as it is applied to SeaSonde data and methods employed to alleviate difficulties associated with the use of measured antenna patterns. We also describe internal consistency checks including visual observation of the radial velocity map, consideration of the computed uncertainties and quantitative tests of radial velocities measured by two radars along the baseline between the systems. Baseline analysis is illustrated by application to two SeaSonde networks, with contrasting results that lead to a better understanding of SeaSonde output

Skill Assessment of Resolving Ocean Surface Current Structure Using Compact-Antenna-Style HF Radar and the MUSIC Direction-Finding Algorithm

A skill analysis of the Multiple Signal Characterization (MUSIC) algorithm used in compact-antennastyle HF radar ocean current radial velocity/bearing determination is performed using simulation. The simulation is based upon three collocated antennas (two cross loops and a monopole with ideal gain patterns) in a geometry similar to the 25-MHz SeaSonde HF radar commercially available from Coastal Ocean Dynamics Applications Radar (CODAR) Ocean Sensors, Palo Alto, California. The simulations consider wind wave/current scenarios of varying complexity to provide insight to the accuracy of surface current retrievals and the inherent limitations of the technique, with a focus on the capabilities of the MUSIC algorithm itself. The influence of second-order scatter, interference, and stationary target scatter are not considered. Simple error reduction techniques are explored and their impacts quantified to aid in operational system configuration and encourage areas of further research. Increases in skill between 55% and 100% using spatial averaging, and between 14% and 33% using temporal averaging, are realized, highlighting the utility of these techniques. When these error-flagging and averaging techniques are employed, individual range cell skill metrics are found to be as high as 0.94 for simple currents at a high signal-to-noise ratio (SNR), while more complex currents achieve a maximum skill metric of 0.72 for the same SNR. These simulations, conducted under ideal conditions, provide insight to understanding the variables, which influence the accuracy of surface currents retrieved using MUSIC.

The Integrated Ocean Observing System High-Frequency Radar Network: Status and Local, Regional, and National Applications

A national high-frequency radar network has been created over the past 20 years that provides hourly 2-D ocean surface current velocity fields in near real time from a few km 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 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.

A micro-hole potentiostatic oxygen sensor for oceanic CTDs

A non-membrane, micro-hole, potentiostatic oxygen sensor was designed and tested for use on an oceanic CTD. The sensor consists of three electrodes: a carbon-fiber cathode, a Ag/ AgC1 reference and a platinum anode; all are mounted in a plexiglass flow duct. The carbon-fiber cathode is a bundle of 1000 carbon fibers recessed in epoxy and electroplated with platinum. The sensor calibrates to oxygen concentration (02), not partial pressure. The effect of temperature on sensor output can be modelled with an activation energy term. Pressure decreases current output of these sensors 5% over 5000 dbar. Ten calibration casts were performed as part of the Hawaii Ocean Time Series (HOTS) sampling program in the Pacific Ocean north of Oahu, Hawaii. Vertical profiles of O2 were calibrated using 18–23 Winkler determinations from each cast. The coefficient of determination (r2) for the calibrations ranged from 0.9979 to 1.0000. The standard deviation of the unexplained error between predicted O2 and Winkler OZ ranged from 0.49 to 3.6 μM, with a mean value of 1.3 μM.

Anisotropic Response of Surface Currents to the Wind in a Coastal Region

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 86 28 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 86 48 (71 86 38) to the right of the wind direction. The anisotropic response thus reflects the dominance of alongshore currents in this coastal region. Both analyses yield winddriven 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.

The RED Experiment: An Assessment of Boundary Layer Effects in a Trade Winds Regime on Microwave and Infrared Propagation over the Sea

The Rough Evaporation Duct (RED) experiment was performed off of the Hawaiian Island of Oahu from late August to mid-September 2001 to test the hypothesis that a rough sea surface modifies the evaporation duct. Two land sites were instrumented, one with microwave receivers and the other with an infrared receiver. Two bouys were deployed, a small boat was instrumented and two aircrafts flew various tracks to sense both sea and atmospheric conditions. It was observed that waves do modify the scalars within the air-sea surface layer. There was a lack of agreement of the scalar profile constants and those typically observed over land. Furthermore, evidence was obtained indicating that the Monin-Obukhov similarity theory, combined with high-quality meteorological measurements, can be used by propagation models to accurately predict microwave signal levels.

A Broadband Acoustic Technique for Measuring Bubble Size Distributions: Laboratory and Shallow Water Measurements

The development of a broadband sound velocimeter that allows the simultaneous measurement of sound speed and attenuation over a wide range of frequencies is described. The velocimeter measures the attenuation and dispersion of a broadband acoustic pulse over frequencies ranging from 4 to 100 kHz across a fixed pathlength using a two-transducer system. The resulting data are inverted to arrive at bubble size distributions over bubble radii in the range 30‐800 mm. The instrument was tested in the large wave channel at the Hydraulics Laboratory of Scripps Institution of Oceanography. The channel can generate breaking waves of O(1 m) height using a hydraulically driven wave generator, giving bubble size distributions similar to those found in the field. The presence of the bubbles significantly changes the acoustical properties of the water. Internal consistency checks of the acoustic data and measurements of bubbles using an independent optical sizing technique support the accuracy of the acoustic system in measuring bubble size distributions. A field test of the system was performed off Scripps Pier in water of approximately 6-m depth. Observations demonstrate that bubble transport events with significant temporal and spatial variability are associated with rip currents and introduce significant vertical gradients in the acoustical properties of the water. The performance of the system in the field was found to be comparable to that found in the laboratory.