Hans C. Graber

Correlation of oceanographic signatures appearing in synthetic aperture radar and interferometric synthetic aperture radar imagery with in situ measurements

Synthetic aperture radar (SAR) imagery collected over the continental shelf near Cape Hatteras, N. C., is analyzed in conjunction with shipboard hydrographic and current measurements. The SAR measurements were made over a 2-hour period on June 20, 1993, in both standard mapping mode and interferometric synthetic aperture radar (INSAR) mode from a NASA DC-8 aircraft as part of the High-Resolution Remote Sensing Experiment. In situ currents were measured using a surface-towed acoustic Doppler current profiler (ADCP). The measurements were made near the end of a period of Gulf Stream incursion onto the shelf as detected using a shore-based HF radar. Winds were southwesterly at 4-6 m s -1 . Long, curvilinear SAR signatures, resembling earlier SEASAT observations made in the same area, are shown to correspond to narrow, shallow fronts separating water masses that increase in surface density with distance offshore. Across-front changes in surface current inferred from the INSAR data are consistent with 2-m-depth currents measured by the ADCP over scales of tens of meters. Thus frontal current gradients measured by INSAR reflect real changes in surface current and are not due to biases induced by changes in the surface-wave spectrum. This lends support to the detailed INSAR surface maps derived by Graber et al. [1996]. An east-west salinity front having the largest observed surface density and current gradient is corrugated on length scales up to the local Rossby radius of deformation and translates southward between successive images. In data from the longer radar wavelengths the salinity front appears as a dark band downwind of a bright signature, and this is interpreted as a region where Bragg-scale waves regenerate following their dissipation in the frontal region. In addition to the fronts the imagery shows closely spaced packets of southward propagating ocean internal waves occurring in the strongly stratified inshore water mass. This case study further serves to emphasize the potential of SAR imagery for study of a wide range of shelf processes.

EASI: An Air–Sea Interaction Buoy for High Winds

AbstractThis paper describes the new Extreme Air–Sea Interaction (EASI) buoy designed to measure direct air–sea fluxes, as well as mean properties of the lower atmosphere, upper ocean, and surface waves in high wind and wave conditions. The design of the buoy and its associated deep-water mooring are discussed. The performance of EASI during its 2010 deployment off Taiwan, where three typhoons were encountered, is summarized.

Evaluation of ERS-1 scatterometer winds with ocean buoy observations

Wind vectors observed by the ERS-1 AMI scatterometer are compared with wind and wave ocean buoy observations. Effects of oceanographic and atmospheric parameters on the scatterometry are also assessed by using the buoy data. Three ERS-1 AMI wind data products such as the JPL Value-Added Products, the ESA Fast Delivery Products and the IFREMER Off-Line Scatterometer Wind Products, were collocated with buoys located offshore and in deep water from the NDBC network, the TOGA-TAO arrays and the JMA buoys. Time difference and spatial separation between the ERS-1 and buoy observations were limited to less than 60 min and 25 km, respectively. Wind speed, air temperature and humidity measured by the buoys were adjusted to values corresponding to a height of 10 m. Wind speeds comparisons displayed rms differences 2 m s/sup -1/ or less for each data product. The IFREMER winds gave the best agreement among the three data products. Systematic biases depending on the wind speeds are found for the JPL and ESA winds. The JPL winds underestimate at low wind ranges and overestimate at high winds. For the wind directions, large deviations are primarily found at low wind speeds which may be due to less accurate performance of the ambiguity removal algorithms. The comparison of the wind direction for the JPL and IFREMER improves significantly when wind speeds less than 5 m s/sup -1/ are excluded. Dependencies of the wind speed residuals on oceanic and atmospheric parameters observed by the buoys exhibited negative correlations with the sea surface temperature and positive correlations with the significant wave height.

Remote Observation of the Spatial Variability of Surface Waves Interacting With an Estuarine Outflow

This paper explores the application of phased-array high-frequency (HF) radars to identify locations of enhanced local waveheights. Measurements of the near-surface current velocities and waveheights were obtained from HF radars deployed near the mouth of the Chesapeake Bay in the fall of 1997. The radar-derived near-surface velocities were compared with the upper bin (2-m depth) of four upward-looking acoustic Doppler current profilers (ADCPs). The slopes of the linear correlations were close to one and the root-mean-square (rms) differences were similar to previous studies. Significant waveheight (Hs) estimates from both radars were compared with a laser height gauge. The largest differences were observed during low winds due to overestimates at one of the radar stations and during storms when the laser measurement failed. Further analysis focused on the HF radar results from the more reliable of the two sites. The rms difference between this radar and the in situ sensor was 0.29 m. Synoptic observations of Hs over the Chesapeake Bay revealed regions of current-induced wave shoaling and refraction. Hs over the estuarine outflow increased between 19-50% relative to the incident Hs in light onshore winds (~5 m/s). In stronger winds (>10 m/s), Hs also increased by up to 25% when there was a tidal outflow in the surface layer, although the near-surface currents were responding to both the wind and the ebbing tide. Hs was not enhanced when the outflow was below a thicker layer (>5 m) of wind-forced onshore flow

Doppler Correction of Wave Frequency Spectra Measured by Underway Vessels

Abstract“Sea State and Boundary Layer Physics of the Emerging Arctic Ocean” is an ongoing Departmental Research Initiative sponsored by the Office of Naval Research (http://www.apl.washington.edu/project/project.php?id=arctic_sea_state). The field component took place in the fall of 2015 within the Beaufort and Chukchi Seas and involved the deployment of a number of wave instruments, including a downward-looking Riegl laser rangefinder mounted on the foremast of the R/V Sikuliaq. Although time series measurements on a stationary vessel are thought to be accurate, an underway vessel introduces a Doppler shift to the observed wave spectrum. This Doppler shift is a function of the wavenumber vector and the velocity vector of the vessel. Of all the possible relative angles between wave direction and vessel heading, there are two main scenarios: 1) vessel steaming into waves and 2) vessel steaming with waves. Previous studies have considered only a subset of cases, and all were in scenario 1. This was likely t...

On the nature of the frontal zone of the Choctawhatchee Bay plume in the Gulf of Mexico

River plumes often feature turbulent processes in the frontal zone and interfacial region at base of the plume, which ultimately impact spreading and mixing rates with the ambient coastal ocean. The degree to which these processes govern overall plume mixing is yet to be quantified with microstructure observations. A field campaign was conducted in a river plume in the northeast Gulf of Mexico in December 2013, in order to assess mixing processes that could potentially impact transport and dispersion of surface material near coastal regions. Current velocity, density, and Turbulent Kinetic Energy Values, e, were obtained using an Acoustic Doppler Current Profiler (ADCP), a Conductivity Temperature Depth (CTD) profiler, a Vertical Microstructure Profiler (VMP), and two Acoustic Doppler Velocimeters (ADVs). The frontal region contained e values on the order of 10−5 m2 s−3, which were markedly larger than in the ambient water beneath (O 10−9 m2 s−3). An energetic wake of moderate e values (O 10−6 m2 s−3) was observed trailing the frontal edge. The interfacial region of an interior section of the plume featured opposing horizontal velocities and a e value on the order of 10−6 m2 s−3. A simplified mixing budget was used under significant assumptions to compare contributions from wind, tides, and frontal regions of the plume. The results from this order of magnitude analysis indicated that frontal processes (59%) dominated in overall mixing. This emphasizes the importance of adequate parameterization of river plume frontal processes in coastal predictive models.

Study of the Far Wake of a Large Ship

AbstractA large dataset of high-resolution photographic images of far wakes of a volunteer observing ship (Royal Caribbean’s Explorer of the Seas) has been acquired under various meteorological conditions and ship operation modes. This work presents the description of instrumentation, methodology, and the results of the experiment. Environmental and ship operation factors that affect appearance and geometric properties of ship wakes in photographic and satellite-based radar images have been analyzed. The photo imagery reveals an asymmetry of the wake depending on wind direction relative to the ship course. In addition, a good agreement between the averaged shape of the wakes measured from the photographic images and a few available satellite-based radar images of the wake of the same ship has been found.

VHF radar measurements of flow patterns in bays and estuaries

In late April through early May 1994, the University of Miami Ocean Surface Current Radar (OSCR) system was deployed to measure currents over a portion of Biscayne Bay, FL. The purpose of this exploratory deployment of the OSCR system was to evaluate its capability to map currents in an environment having low amplitude Bragg resonant waves and containing numerous shoals and small islands. The results indicate that, with the exception of very shallow shoal regions, high quality vector current maps can be obtained. The validity of these maps is demonstrated by comparisons with known tidal flows in Biscayne Bay.

Arctic Sea Ice Drift Measured by Shipboard Marine Radar

This study presents Arctic sea ice drift fields measured by shipboard marine X-band radar (MR). The measurements are based on the maximum cross correlation between two sequential MR backscatter images separated 1 min in time, a method that is commonly used to estimate sea ice drift from satellite products. The advantage of MR is that images in close temporal proximity are readily available. A typical MR antenna rotation period is 1–2 s, whereas satellite revisit times can be on the order of days. The technique is applied to 4 weeks of measurements taken from R/V Sikuliaq in the Beaufort Sea in the fall of 2015. The resulting sea ice velocity fields have 500 m and up to 5 min resolution, covering a maximum range of 4 km. The MR velocity fields are validated using the GPS-tracked motion of Surface Wave Instrument Float with Tracking (SWIFT) drifters, wave buoys, and R/V Sikuliaq during ice stations. The comparison between MR and reference sea ice drift measurements yields root-mean-square errors from 0.8 to 5.6 cm s. The MR sea ice velocity fields near the ice edge reveal strong horizontal gradients and peak speeds> 1 m s. The observed submesoscale sea ice drift processes include an eddy with 6 km diameter and vorticities <–2 (normalized by the Coriolis frequency) as well as converging and diverging flow with normalized divergences <–2 and >1, respectively. The sea ice drift speed correlates only weakly with the wind speed (r 5 0.34), which presents a challenge to conventional wisdom.

Water surface slope spectra in nearshore and river mouth environments

With the ever-growing interest in satellite remote sensing, direct observations of short wave characteristics are needed along coastal margins. These zones are characterized by a diversity of physical processes that can affect sea surface topography. Here we present connections made between ocean wave spectral shape and wind forcing in coastal waters using polarimetric slope sensing and eddy covariance methods; this is based on data collected in the vicinity of the mouth of the Columbia River (MCR) on the Oregon-Washington border. These results provide insights into the behavior of short waves in coastal environments under variable wind forcing; this characterization of wave spectra is an important step towards improving the use of radar remote sensing to sample these dynamic coastal waters. High wavenumber spectral peaks are found to appear for U 10 > 6 m/s but vanish for τ > 0.1 N/m2, indicating a stark difference between how wind speed and wind stress are related to the short-scale structure of the ocean surface. Near-capillary regime spectral shape is found to be less steep than in past observations and to show no discernable sensitivity to wind forcing.