Robert C. Beal

Comparison of SAR-derived wind speed with model predictions and ocean buoy measurements

As part of the Alaska synthetic aperture radar (SAR) Demonstration Project in 1999 and 2000, wide-swath RADARSAT SAR imagery has been acquired on a regular basis in the Gulf of Alaska and the Bering Sea. During 1998 and 1999, similar data were acquired off the East Coast of the United States as part of the StormWatch Project. The radar cross section measurements from these images were combined with wind direction estimates from the Navy Operational Global Atmospheric Prediction System model to produce high-resolution maps of the surface wind speed. For this study, 2862 SAR image frames were collected and examined. Averaged wind estimates from this data base have been systematically compared with corresponding wind speed estimates from buoy measurements and model predictions, and very good agreement has been found. The standard deviation between the buoy wind speed and the SAR estimates is 1.76 m/s. Details of the SAR wind extraction procedure are discussed, along with implications of the comparisons on the C-band polarization ratio.

The influence of the marine atmospheric boundary layer on ERS 1 synthetic aperture radar imagery of the Gulf stream

In September 1991, the ERS 1 synthetic aperture radar (SAR) collected a set of four colocated images over the western Gulf Stream (GS). The SAR images were supplemented by satellite infrared imagery and measurements of the marine atmospheric boundary layer (MABL) from both a pair of surface buoys and a suite of instrument on the Ukrainian research vessel R/V Vernadsky, which between September 15 and 19 made six transects across the GS northern boundary within the SAR swaths. The influence of wind variations within the MABL was evident in each of the four SAR images. Evidence for thermal wind effects is seen in the changing character of the imagery as the mean wind across the sea surface temperature boundary changed direction from pass to pass. Both in situ data and analytical models suggest that much of the day-to-day SAR image variability was the result of changes in the character of the MABL, especially evident near the northern GS boundary where the sign of the atmospheric stability fluctuated over short spatial scales. Because many of the governing parameters of the MABL rapidly decorrelate, significant changes in the SAR imagery occurred during the 3 days between passes. Nevertheless, at least some manifestation of the GS was evident in the SAR imagery on each of the four passes.

Seasat Synthetic Aperture Radar: Ocean Wave Detection Capabilities

A preliminary assessment has been made of the capability of the Seasat synthetic aperture radar to detect ocean waves. Comparison with surface and aircraft measurements from five passes of the satellite over the Gulf of Alaska indicates agreement to within about – 15 percent in wavelength and about � 25� in wave direction. These results apply to waves 100 to 250 meters in length propagating in a direction predominantly across the satellite track, in sea states with significant wave height (H⅓) in a range of 2 to 3.5 meters.

A Comparison of SIR-B Directional Ocean Wave Spectra with Aircraft Scanning Radar Spectra.

Directional ocean wave spectra derived from Shuttle Imaging Radar-B (SIR-B) L-band imagery collected off the coast of southern Chile on 11 and 12 October 1984 were compared with independent spectral estimates from two airborne scanning radars. In sea states with significant wave heights ranging from 3 to 5 meters, the SIR-B-derived spectra at 18� and 25� off nadir yielded reasonable estimates of wavelengths, directions, and spectral shapes for all wave systems encountered, including a purely azimuth-traveling system. A SIR-B image intensity variance spectrum containing predominantly range-traveling waves closely resembles an independent aircraft estimate of the slope variance spectrum. The prediction of a U.S. Navy global spectral ocean wave model on 11 October 1984 exhibited no significant bias in dominant wave number but contained a directional bias of about 30�espect to the mean of the aircraft and spacecraft estimates.

Combined estimates improve high‐resolution coastal wind mapping

The operational meteorological community and the numerical weather prediction community share a common need for high-resolution estimates of the near-surface wind field in data-sparse regions of the globe, such as the coastal zones of Alaska and the Pacific Northwest. Wind estimates over coastal waters using conventional multiple-antenna scatterometers or passive microwave sensors are difficult to obtain because the large footprint associated with these sensors results in significant contamination from land. Since synthetic aperture radar (SAR) can provide high-resolution imagery of the surface virtually up to the shoreline, “SAR scatterometry” represents a potentially significant new technique for measuring ocean-surface wind fields at resolutions more than an order of magnitude finer than is now possible with any other spaceborne technique.

Comparison of SIR-C SAR wavenumber spectra with WAM model predictions

During April and October of 1994, the Space Radar Laboratory (SRL) flew on the space shuttle Endeavour at the relatively low altitude of 215 km. Using horizontal polarization, C-band signal from the spaceborne imaging radar (SIR-C), an onboard processor, designed and fabricated by the Johns Hopkins University Applied Physics Laboratory, formed synthetic aperture radar (SAR) images and computed over 100,000 corresponding image spectra. The low altitude, small look angle (23°–25°) and the use of horizontal polarization minimized both the loss of azimuth resolution caused by ocean surface motion and the relative contribution of the hydrodynamic component of the modulation transfer function. As a result, the SIR-C SAR was able to image azimuth-traveling waves with minimal distortion. After using linear inversions to convert image spectra to wave height-variance spectra, the distributions of wavenumber and propagation direction from the processor-derived spectra were consistent with wave model (WAM) predictions. Although the SAR wave height-variance spectra underestimated significant wave height (SWH) at the higher SWHs, this error is compensated for by a simple linear correction. We collected 57 pairs of crossover spectra where the ground track pairs were nearly orthogonal. The crossovers were separated by 6 hours. Crossover comparisons show that the retrieved spectral parameters are independent of wave propagation direction. At this altitude and configuration, the SAR range and azimuth responses are nearly equal. The real-time processing of spaceborne SAR data to produce accurate estimates of wavenumber, propagation direction, and SWH is clearly feasible with the orbital and instrument geometry of SIR-C.

Spaceborne imaging radar - Monitoring of ocean waves

A well-organized, very low energy ocean swell system off the East Coast of the United States was tracked with the Seasat synthetic aperture radar from deep water, across the continental shelf, and into shallow, water. The results indicate that spaceborne imaging radar may be used to accurately measure ocean wavelength and direction, even in coastal areas and in the presence of a mixed ocean.

Real-time observations of Southern Ocean wave fields from the Shuttle Imaging Radar

During the April and October 1994 flights of the space shuttle Endeavour, C-band synthetic-aperture radar (SAR) signal was acquired and processed in real-time into ocean wave images and two-dimensional image spectra by an on-board processor. The image spectra were transmitted to the ground as they were acquired. During 20 days of observations, spanning the two missions, over 100000 spectra were collected. These data provide the first truly synoptic directional measurements of the Southern Ocean wave field. With additional processing, estimates of ocean wavelength and propagation direction were extracted from the spectra. During the October mission, daily comparisons of these estimates with wave model predictions were made available at a World Wide Web site. The authors describe the computations performed by the processor and the real-time data flow and ground processing. In addition, they summarize and interpret some gross characteristics of the processor-observed real-time wave field from the April and October missions. >

Measuring ocean waves from Space : 1978 to 1988

Abstract Global estimates of directional ocean wave spectra from space can help to spur improvements in the accuracy of existing ocean wave forecasts. The results of three ocean wave experiments conducted since 1978 indicate that a low altitude (∼300km) free-flying SAR could accurately monitor the performance of global wave models, and thus could help to assess the impact of both scatterometer-aided wind fields and improved wave model physics. The first opportunity to demonstrate such a potential should occur around the mid-1990s, when the European ERS scatterometer and the US Shuttle Space Radar Laboratory should be operating simultaneously.

Seasat detection of waves, currents and inlet discharge

Abstract A new era of remote sensing for coastal and oceanographic monitoring was born on 26 June 1978 with the launch of Seasat. Duck-X was a 2 month experiment conducted during August to October 1978 off the east coast of the U.S.A. for the validation of the Seasat synthetic aperture radar (SAR), During this field experiment, various oceanographic phenomena were monitored. Ground truth observations of these phenomena have been correlated with Seasat SAR imagery. The ground truth sensors included airborne photographic and radar imagery, meteorological satellite imagery, land based radars, and conventional wave gauges. This paper focuses on ocean surface waves, ocean currents and coastal inlet discharge Specifically, the direction and length of the principal ocean wave trains are compared for the periods of Seasat overflight of the Duck-X area. During these overflights significant wave heights were 1.5 m and less and the maximum wave period was 14 s. The current correlations concentrate on the western bou...