CFOSAT散射计海面后向散射系数误差及影响
doi: 10.11728/cjss2024.02.2023-0144 cstr: 32142.14.cjss2024.02.2023-0144
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董楹：女, 1999年3月出生于江苏省南通市. 现为南京信息工程大学海洋科学学院研究生, 主要研究方向为海洋微波遥感. E-mail: 20211237012@nuist.edu.cn
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Analysis of Sea Surface Backscatter Coefficient Errors and Its Effects for the CFOSAT Scatterometer
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摘要: 仪器噪声(Kpc)和地球物理噪声(Kpg)是影响星载微波散射计后向散射系数(σ0)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Kpc和Kpg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, Kpg为σ0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, Kpg也就越大, 而在高风速条件下, 海面风场变异性较小, Kpc与Kpg贡献相当. 此外, σ0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Kpg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σ0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.Abstract: Noise is a key factor that affects the accuracy of spaceborne scatterometer backscatter coefficient (σ0), as well as the retrieved sea surface wind quality. In general, the scatterometer σ0 measurement error is attributed to both instrumental noise and geophysical noise, which are expressed in terms of normalized standard deviation (Kp). In this paper, the instrumental noise (Kpc) and the geophysical noise (Kpg) 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 dominates 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 Kpg value increases as the WVC size, but at the same time, the larger the wind cell grid and the larger the number of independent observation samples, the smaller the Kpc. 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, Kpc and Kpg show a minimum value at the incidence 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-negligible for the scatterometer measurements approaching to the coastal line. In summary, the results presented in this paper reveal the influence factors of scatterometer σ0 measurements, which are relevant for better understanding the wind inversion and quality control of CSCAT, notably near the coastal areas.
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图 1 信噪比随入射角和风速的变化
Figure 1. Expected SNR as a function of wind speed and incidence angle
图 4 CSCAT 不同空间分辨率Kpc和Kpg随风速的变化
Figure 4. Kpc and Kpg vary with wind speed at CSCAT different spatial resolutions
图 5 CSCAT 25 km分辨率WVC σ0测量误差随入射角和风速的变化
Figure 5. WVC σ0 measurement error varies with incidence angle and wind speed at CSCAT 25 km resolution
图 6 CSCAT σ0测量误差随网格分辨率的变化
Figure 6. CSCAT σ0 measurement error varies with grid resolution
图 8 CSCAT 12.5 km分辨率风场数据与ECMWF背景风场数据的标准差(a)和偏差(b)随离岸距离的变化
Figure 8. Variation of standard deviation (a) and bias (b) between CSCAT 12.5 km resolution wind field data and ECMWF background wind field data with offshore distance
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