A New Wind Wave Spectrum Model And Its Application In Altimeter Wind Speed Retrieving

The wind wave spectrum models are widely used in various fields of ocean investigations, for example, ocean-atmosphere interaction, upper layer ocean dynamics, wave forcasting, ocean remote sensing and ocean engineering. Within the full wavenumber range, the wind wave spectrum can be divided into long wave spectrum (gravity spectrum) and short wave spectrum (gravity-capillary spectrum). They in particular play an important role in the study of the remote sensing theory concerning microwave scatterometers, radiometers and altimeters. In this thesis, a new wind wave spectrum model for full wavenumber range is proposed. Based on the new model, more accurate altimeter wind speeds are obtained.The spectrum model of the wind waves for deep water proposed by Liu et al. (2003) contains three parameters: the wind speed, the inverse wave age and the inverse spectral width. Based on the deep water wave model proposed by Liu et al. (2003), statistical relationships among the wind speed, inverse wave age and inverse spectral width are derived for field data and laboratory data, respectively. With the statistical relationships and equations given by Liu et al. (2003), a new wind wave spectrum model for deep water is proposed in this study. In the new model, the total spectral energy level, the location of the maximum spectral energy (represented by location of spectral peak), and the width character of energy distribution (represented by the spectral steepness) at high frequencies located to the right of spectral peak of wind waves are all determined by the two basic parameters, i.e., the wind speed and the inverse wave age.The statistical relationships also show that with the same wind speed and wave age, the steepness of the wind wave spectrum at high frequencies located to the right of the spectral peak for field case is different from that for laboratory case. This is the main difference between field wind wave spectrum and laboratory spectrum.With the inverse spectral width, the new model is more appropriate to describe the real wind wave status. Compared with measurements in the Black Sea, the model calculated zeroth spectral moment m0 and the spectral width are in both good agreements with measured data. Furthermore, the new model can match elevation spectrum data obtained by four-frequency microwave radar and other field measurements fairly well. The new model can not only better describe and explain the influence of the wind speed and wave age on the energy distribution of developing wind waves generated in open ocean, but also it plays a significant role in the study of oceanic microwave remote sensing, especially for understanding the uncertainty of retrieved ocean environment variables.Based on the specular reflection theory of electromagnetic waves at rough sea surface and the spectral model of wind waves with a wave age factor, the sea surface wind speeds are retrieved from the normalized radar backscatter cross section measured by TOPEX/Poseidon Ku-band altimeter using the mean square slope calculated from the spectral models of the wind waves and the gravity-capillary waves. The study shows the wave age factor has a significant influence on retrieving altimeter wind speeds. Compared with the empirical algorithm in operational use for altimeter winds retrieval, after the effects of wind age factor is included in the analytical algorithm, the root mean square difference and bias between retrieved altimeter wind speed from the analytical algorithm and buoy observations are 11% and 21% lower, respectively.Effects of the wave state on altimeter wind speed retrieval are also investigated based on the observations in the Gulf of Mexico. The comparison between measurements from the altimeter and buoys shows that the wave state has significant effects on the altimeter wind speed retrieval. After considering the effects of the wave state, the altimeter winds are retrieved more accurately with a new wind wave spectrum model. The wave age factor which denotes the wave state can be calculated from altimeter measured H s and U 10 (from algorithm in operational use for TOPEX/Poseidon altimeter). Thus, the analytical algorithm proposed in this study has a comprehensive applicability.