Remote Sensing Of Ocean Swell And Some Other Coastal Processes By HF Radar

Nearshore marine environment contains many complex processes, but the lack of high-resolution data over a large area during a long time is often the primary obstacle to further research. High-frequency (HF) radar is a mean of remote sensing which obtains continuous near-real time sea surface information over a large area. Thus the study of inversion of marine paremeters from HF radar data is very meaningful. This thesis makes use of a 13-month-long dataset collected by two phased array HF radar to look at the characteristics of the sea echo signals, study the data processing and inversion method, compute sea surface parameters and evaluate the accuracy of radar inversion.The thesis refers to the ground wave HF radar, whose radio waves contact with ocean by Bragg resonance scattering. The development history and applications of HF radar is introduced. The basic theory of electromagnetic wave is reviewed. The principles of inversion of sea surface current, wind direction and swell parameters are described. The feasibility of the swell parameter inversion is investigated.Based on theoretical analysis and statistical studies of a large number of samples, the thesis proposes a series of methods on raw signal processing and quality control, including the determination of the noise level, data averaging in space and time, the proper positions of spectral peaks, the peak width threshold, etc. Respecting the characteristics of different physical processes, inversion of current and wind uses spectra collected every 20 min; inversion of swell parameters uses the one-hour averaged spectra. The statistics of qualified spectra for swell parameter calculation are presented for both stations. A set of efficient, with a reduced computational cost, automatic computing programs are developed to do the processing and derive marine parameters.Radial current velocities are derived from single radar station and current vector fields are obtained by combination of both stations. One-year mean flow field in the Iroise is given, together with the computation of vorticity and divergence.Relative wind direction with respect to radar look direction is measured through Bragg peaks ratio. Different empirical models are employed to derive radar-inverted relative wind direction. Results show reasonable agreement with model estimations. Different directional distribution models are used to measure the spreading factor for the Iroise Sea.The thesis focuses on the study of swell parameters. Results are assessed by buoy and wave model (WAVEWATCH III) data. The validations show that the accuracy of swell frequency is very good, the accuracy of swell significant waveheight is reasonably nice, and the accuracy of relative swell direction is bad. Consistency of measurements by both radar stations is verified by comparing the two, which supports the use of double samples to do the inversion. Use of two radars only been further improves the accuracy but also solves the ambiguity of relative swell direction from single station and gives the absolute wave to a certain precision. The thesis proposes a constant relative direction method to derive swell significant waveheight, based on the studies of radar integral equation and the inverted results of relative swell direction. This proposal is demonstrated to improve the agreement of radar inversion and buoy/model provided significant waveheight and increases significantly the number of samples.The thesis investigates the accuracy of swell parameters from HF radar. Random errors of radar observations are quantified. Comparing the differences between different observation methods gives estimations of the contribution of radar intrinsic uncertainty and other factors.