基于高斯曲线拟合-FFT方法的南海风场反演

海面风场是海洋学的基本参量,获取海面风场对了解海洋的物理过程以及海洋与大气之间的相互作用至关重要。宽阔的海域面积及复杂的海面状况通常使南海海面上的风场信息很难被及时获取。ENVISAT ASAR是一种全天候全天时监测海面的微波雷达传感器,可实时获取海面风场数据。本文基于已有ASAR数据对南海海面风场进行反演实验,首先将结合高斯曲线拟合的FFT风向反演方法应用于南海风向反演,并参考Cross-Calibrated Multi-Platform （CCMP）风场数据去除180o方向模糊获得海面风向。然后,将高斯曲线拟合-FFT风向与传统的峰值-FFT风向进行对比,最后将准确率较高的高斯曲线拟合-FFT风向分别输入CMOD4模型和CMOD5模型获得海面风速大小。实验结果与CCMP参考数据的比较结果表明,在风条纹不明显的情况下,利用结合高斯曲线的FFT风向反演方法和CMOD4模型风速反演方法可有效地进行南海海面风场反演。该成果对利用SAR数据实时获取南海大面积海面风场信息,尤其是观测点缺乏海域的风场信息,具有重要的指导意义。

Wind Field Retrieval of South China Sea Based on Gaussian-FFT Method

Wind speed is a basic parameter of oceanography. It plays an important role in the interaction between ocean and atmosphere. Therefore, it is significant and necessary to obtain the wind datasets over the sea surface. However, due to the large area and complex condition, it is usually difficult to get the wind field data of South China to satisfy different demands in time. Conventional approaches, such as placing observation stations or buoys, are not only expensive but also dependent on the weather condition. Therefore it is urgently necessary to find other ways to get the wind datasets timely. ENSISAT ASAR, an all-weather and all-time microwave radar sensor, could collect the real-time and dynamic information over sea surface, which provides a new approach for researchers to acquire wind field datasets over sea surface, especially for the waters with complicated conditions, such as South China Sea. In this paper, the Gaussian-FFT method is firstly applied to retrieve the wind field of South China Sea based on ASAR image. At first, the FFT spectrum of ASAR image is acquired with the FFT algorithm. Secondly, a “cigar-shaped” two-dimensional (2-D) Gaussian function is fitted to the FFT spectrum to find the direction of wind streaks and further to obtain the wind direction which is perpendicular to it. In this experiment, the wind direction acquired from the ASAR image by the Gaussian-FFT algorithm also has a 180 ambiguity in direction. To resolve the 180 ambiguity, CCMP wind field datasets are taken into consideration to act as the wind field references. Besides, the wind direction computed with the Gaussian-FFT method is compared with the wind direction obtained by the Peak-FFT method. Then, the optimal wind direction (Gaussian-FFT wind direction) is input into the CMOD4 and CMOD5 models to compute the wind speed values respectively. Through comparing the wind field retrieval results with the CCMP datasets, we proved that it is valid to retrieve wind direction from ASAR image with Gaussian-FFT algorithm and it is achievable to obtain wind speed value over South China Sea with CMOD4 model. The approach used to obtain the wind field in this paper is of great significance to provide guidance to the wind field inversion in other waters of South China Sea, especially in areas that are lack of field observations. In addition, it is also critical for other researches whose specialties are related to oceanography, as this approach could offer vital wind parameters to these researches.