Significant Wave Height Measurements Research Articles

A review of methods of remote sensing of sea-surface conditions by HF radar and design considerations for narrow-beam systems

Present achievements in HF ground-wave radar remote mapping of surface currents and wave characteristics are reviewed, and the features of the different systems that are in use are described. Particular emphasis is given to the problems remaining to ensure effective mapping of nondirectional wave parameters and wave directional spectra over a sector of sea extending up to a range of 150-200 km from a coastal or island site. As an example of current progress, measurements in the United Kingdom of first-moment wave period by single-site radar have achieved measurements to within 15 percent of buoy data over a range from 4 to 9 s. Measurements of significant wave height gave standard deviations of 15 and 20 percent for wave heights less than and greater than 2 m, respectively. In the latter case, a bias of 40 percent was found, which later work suggests is reducible to about 14 percent, using a model-fitting technique. To achieve directional wave spectrum measurement with a single radar requires some 40-dB ratio between the Bragg line return and the noise floor of the Doppler spectra. A two-site radar has been shown to yield advantages for directional spectrum measurement. The design of steerable narrow-beam radars in the 6-25-MHz band to achieve such performance is discussed. Suitable transmitted waveforms to achieve good rejection of interference and to be acceptable in the HF band are put forward. The operational experience in the United Kingdom, obtained with a particular FM interrupted continuous-wave radar system associated with a steerable array, is reported. Results of simulating the effect of the antenna radiation pattern on the observed Doppler spectra are illustrated. The principal effects are on the weaker Bragg line and the second-order continuum around it. This can result in errors in deduced surface wind direction and directional wave spectrum. Finally, some useful results on propagation monitoring of a skywave radar with the aid of a range-Doppler graphical output are reported.

Swell in the Pacific Ocean observed by SEASAT radar altimeter

Abstract The first synoptic observations of ocean swell generation, propagation, and dissipation have been obtained from SEASAT radar altimeter measurements. The sequential significant wave height and surface wind speed measurements have been deduced from altimeter observations; by assuming that the Pierson and Moskowitz fully developed wave model applies, a model for determining minimum swell is used to generate three‐day average swell maps of the Pacific Ocean for the period June to October 1978, the SEASAT lifetime. The generation of numerous swell events was observed with the altimeter in the high‐latitude regions of the South Pacific Ocean (south of 40°S latitude), which was acting as a continuous swell source. Regional differences in the northward propagation and dissipation patterns of different swell events have been analyzed. In their northward propagation the swell families generated in the Ross Sea neighborhood can impinge the coasts of North America with significantly more energy than the swel...

An empirical determination of the effects of sea state bias on SEASAT altimetry

A simple linear empirical model has been developed to allow the user of SEASAT altimetry data to correct the altitude measurements for sea state bias effects. This bias is the result of two separate effects: one relating to instrumental effects and the other relating to a bias in the electromagnetic mean sea level relative to the geometric mean sea level. The electromagnetic bias is caused by the fact that the troughs of the ocean waves reflect the altimeter signal more strongly than the crests, thus causing the altimeter‐determined mean sea level to shift toward the wave troughs. An additional, but independent, instrument‐related bias is introduced because height corrections applied in the ground processor are not sufficient to compensate for simplifying assumptions made for the processor aboard SEASAT. This altitude bias also is predominately wave height dependent and directed toward the wave trough. The sum of these two effects is referred to as sea state bias. After applying appropriate corrections to the altimetry data, an empirical model for the sea state bias is obtained by differencing significant wave height and height measurements from coincident ground tracks. Ignoring small temporal variations in surface topography, the height differences on a given track should be zero; consequently, any residual height difference is attributed to sea state bias. The height differences are minimized by solving for the coefficient of a linear relationship between height differences and wave height differences that minimize the height differences. It is concluded that, for the altimeter data released by the Jet Propulsion Laboratory, 7% of the value of significant wave height should be subtracted from the SEASAT height measurements to correct for sea state bias. This correction reduced biases in the SEASAT data by more than 50% in the 36 cases examined here.

Waveheight and wind speed measurements from the SEASAT radar altimeter

Comparisons between the SEASAT radar altimeter inferred estimates of significant waveheight (SWH) and wind speed with buoy measurements for near overflights are presented. Details of the ‘calibration’ of SEASAT's measurement of σ0(0°) against those made by the GEOS‐3 radar altimeter are presented. The ‘onboard’ estimate of SWH is shown to be biased high by 0.5 m for SWH > 2 m. Comparisons of buoy measurements of SWH with altimeter waveform data processed by using the Fedor algorithm for 51 cases show a mean difference of 0.07 m with a standard deviation of 0.29 m over the range of 0.5–5 m SWH. The SEASAT inferred estimates of σ0(0°) are biased high by 1.6 dB relative to GEOS‐3 data. For 87 corrected comparisons with buoy measurements of wind speed, the mean difference is −0.25 m/s, and the standard deviation of the difference is 1.6 m/s over the range of 1–10 m/s wind speed. The numbers are very similar to GEOS‐3 data comparisons and thus provide a second corroboration of the waveheight and wind speed measurement capability of a short pulse spaceborne radar altimeter.

A comparison of SEASAT 1 altimeter measurements of wave height with measurements made by a pitch‐roll buoy

Measurements of significant wave height made by a pitch‐roll buoy during the Joint Air‐Sea Interaction Experiment (JASIN) are compared with measurements made at the same time by the SEASAT 1 radar altimeter as processed by the Jet Propulsion Laboratory SEASAT algorithms. Comparisons are made for eight occasions during which the significant wave height lay between 0.7 and 2.3 m. The radar altimeter is found to underestimate slightly the wave heights when compared with the pitch‐roll buoy. The mean ratio of the measurements is 0.96±0.04, and the standard deviation of individual comparisons about this mean is 0.10.

High-Frequency Skywave Radar Measurements of Hurricane Anita

We tracked and monitored hurricane Anita over a 5-day period by using the SRI-operated Wide Aperture Research Facility (WARF) high-frequency skywave radar. The WARF-derived positions for Anita agreed to within +/- 19 kilometers of the coincident temporal positions along the National Hurricane Center's smooth track. Hurricane Anita passed near the open ocean-moored buoy EB-71 of the National Oceanic and Atmospheric Administration, and measurements of wind direction, wind speed, and significant wave height made during this period at the WARF and in situ at the buoy showed agreement of 7 degrees , 0.4 meter per second, and 0.5 meter, respectively. The WARF estimates of longshore coastal surface currents showed good agreement with measurements made at a moored current meter.

On the Remote Detection of Swell by Satellite Radar Altimeter

A technique is developed and discussed which utilizes only significant wave height and wind speed measurements available from the GEOS-3 radar altimeter in real time to locate swell dominated regions of the earth's oceans. It is shown that reasonable agreement exists between the GEOS-3 map of the North Atlantic for 24 February 1976 and a map produced from hindcasted values of sea and swell obtained from the Special Projects Branch of the National Weather Service. Only data presently available from existing satellite radar altimeters are needed to map those areas of the oceans dominated by swell. It is concluded that the example given indicates that the decision boundary developed in this study to distinguish swell-dominated seas is a realistic choice.

Long-term wave height distributions at seven stations around the British Isles

Long-term instrumental measurements of significant wave height and mean zero-crossing period at 7 stations are analysed. The marginal distribution of significant heights is well described by a Weibull law. The long-term distribution of individual wave heights is calculated from the joint distribution of significant wave height and mean wave period. It is found to be nearly exponential.