CFOSAT散射计海面后向散射系数误差及影响

Analysis of Sea Surface Backscatter Coefficient Errors and Its Effects for the CFOSAT Scatterometer

仪器噪声(K_(pc))和地球物理噪声(K_(pg))是影响星载微波散射计后向散射系数(σ^(0))测量精度以及海面风场反演精度的关键因素.针对中法海洋卫星(CFOSAT)散射计(CSCAT),详细分析了K_(pc)和K_(pg)随海面风速、入射角、风单元网格分辨率以及离岸距离等因素变化的特征.结果表明,低风速条件下海面风场变异性较大,K_(pg)为σ^(0)测量误差的主导因素,而且风单元网格越大、海面风场的不均匀性越强,K_(pg)也就越大,而在高风速条件下,海面风场变异性较小,K_(pc)与K_(pg)贡献相当.此外,σ^(0)测量误差总体上随着离岸距离的减小而增大,表明近海岸区域的K_(pg)为散射计观测不可忽略的影响因子.研究结果揭示了星载微波散射计σ^(0)测量误差的影响因素,对CSCAT近海岸风场反演具有重要的指导意义.

Noise is a key factor that affects the accuracy of spaceborne scatterometer backscatter coef-ficient(σ^(0)),as well as the retrieved sea surface wind quality.In general,the scatterometerσ^(0)measure-ment error is attributed to both instrumental noise and geophysical noise,which are expressed in terms of normalized standard deviation(K_(p)).In this paper,the instrumental noise(K_(pc))and the geophysical noise(K_(pg))are analyzed as a function of sea surface wind speed,incidence angle,spatial resolution and offshore distance for the China-France Oceanography Satellite Scatterometer.The result shows that the variability of sea surface wind field is large under low wind conditions,so the geophysical noise domi-nates the measurement error of radar backscatters.Notably,the larger the grid size of Wind Vector Cell(WVC),the more inhomogeneous the sea surface wind,such that the K_(pg)value increases as the WVC size,but at the same time,the larger the wind cell grid and the larger the number of independent obser-vation samples,the smaller the K_(pc).While under high wind conditions,the variability of sea surface wind is small,and the contribution of instrument noise and geophysical noise is similar.Regarding the sensitivity of measurement error to the incidence Angle,K_(pc)and K_(pg)show a minimum value at the inci-dence angle of 40°,which is consistent with the antenna gain pattern.Finally,the relationship between the backscatter measurement error and the offshore distance is studied in order to clarify the feasibility of near shore wind field inversion for the CSCAT.The results show that the observation error generally increases as the offshore distance decreases,indicating that the near-shore geophysical noise is non-negli-gible for the scatterometer measurements approaching to the coastal line.In summary,the results pre-sented in this paper reveal the influence factors of scatterometerσ^(0)measurements,which are relevant for better understanding the wind inversion and quality control of CSCAT,notably near the coastal areas.