Abstract
Abstract. Retrieving spectral wave parameters such as the peak wave direction and wave period from marine radar backscatter intensity is very well developed. However, the retrieval of significant wave height is difficult because the radar image spectrum (a backscatter intensity variance spectrum) has to be transferred to a wave spectrum (a surface elevation variance spectrum) using a modulation transfer function (MTF) which requires extensive calibration for each individual radar setup. In contrast to the backscatter intensity, the Doppler velocity measured by a coherent radar is induced by the radial velocity (or line-of-sight velocity) of the surface scattering and its periodic component is mainly the contribution of surface waves. Therefore, the variance of the Doppler velocity can be utilized to retrieve the significant wave height. Analyzing approximately 100 days of Doppler velocity measurements of a coherent-on-receive radar operating at X-band with vertical polarization in transmit and receive, a simple relation was derived and validated to retrieve significant wave heights. Comparison to wave measurements of a wave rider buoy as well as an acoustic wave and current profiler resulted in a root mean square error of 0.24 m with a bias of 0.08 m. Furthermore, the different sources of error are discussed and investigated.
Highlights
Ocean surface waves are one of the most important maritime parameters that are frequently monitored for purposes of coastal protection, shipping as well as offshore industry operations
The aim of this study is to find a simple relation between the radar-retrieved radial Doppler velocities and the significant wave height of the sea state
For validation of the methodology the resulting radar-retrieved significant wave heights are compared to results of a directional wave rider as well as of a bottom mounted Acoustic Wave and Current Profiler (AWAC)
Summary
Ocean surface waves are one of the most important maritime parameters that are frequently monitored for purposes of coastal protection, shipping as well as offshore industry operations. The radar backscatter at moderate incidence angles (20 to 80◦) is primarily caused by Bragg scattering, a scattering mechanism where the electromagnetic waves couple to small-scale surface roughness (∼ 3 cm for X-bands) that is aligned with the look direction of the radar. Ocean surface waves are imaged by marine radars because they modulate the small-scale surface roughness. At grazing incidence shadowing modulation becomes of major importance, and it is caused by the very low radar backscatter coming from diffraction in the geometrically shadowed areas of the waves (Barrick, 1995; Plant and Farqueson, 2012)
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