||In present study, the commercial marine S-Band and X-Band low grazing angle radar systems were utilized as the main tools in field experiments, and investigation of the influence of the surface waves was carried out.|
The first step in this study is the calibration of radars. Metallic balls with various sizes were launched in the observational area and used as the reflectors of radar detection. The External Calibration Approach (Gommenginger et al. 2000) was then applied to obtain the parameters of Radar System Transfer Function. In this study, the calibration of two different bands of the radar systems were achieved, respectively.
Secondly, field experiments including, typhoon Fanapi and a winter monsoon events, were carried out. Synchronized observations of gravity waves, air-sea fluxes, and dual-band Radar systems were implemented. The strategy of data analysis is twofold, i.e. the statistical analysis on the probability of long-term records of Radar Backscatter intensity, and the cross-correlation analysis on the dependency of Radar backscatter intensity to the wave elevation and slope. In the first part, the discussions were focused on the temporal averaged radar backscatter intensity to the wind speeds, significant wave heights, Mean Square Slope of gravity waves and the Roughness Parameters. The second part we emphasized on,the dependency of Radar backscatter intensity to the phase resolved water elevation and slope, which were obtained using zero-up crossing to separate individual wave form from the ADCP measured time series.
The result of present study shows that the probability of return signals (spike) are better described using Bimodal Gaussian distribution, especially for those whose footprints were greater than the lengths of surface wave. The Probability distribution can be regarded as the summation of two normal distributions with two individual means and standard deviations. The lower ones represent the wind induced wavelet effects, whereas, the higher peak represent the gravity waves. Meanwhile, this is the observation evidence of the two scale theory of low grazing angle microwave radar. Furthermore, the distribution of the high gray value (GN) and low gray value band on the radar image which affected by gravity waves features single maximum and minimum. The maximum value does not coincide with the wave crest; likewise minimum value does not coincide with wave trough. In this case, phase lag could be identified. Moreover, with increasing wave steepness, phase lag increases. It could be concluded that the low grazing angle radar is affected by the sheltering effect which should not be neglected in any circumstances.
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