Research on Cloud Fraction Inversion Algorithm of Environmental Trace Gas Monitoring Instrument

The trace gas retrieval algorithms rely on information on cloud properties for each ground pixel. Such information is important for the development of EMI cloud products because clouds have a significant impact on the photon path. The photon path strongly affects the information on trace gases contained in the satellite observations. In this study we discuss a cloud algorithm based on O2-O2 absorption at 477 nm using EMI measurements. Firstly, the O2-O2 slant column densities (SCDs) were retrieved from the range of 460-490nm using the differential optical absorption spectroscopy (DOAS) technique, meanwhile the stripes of O2-O2 SCDs were corrected. Secondly, the different values of solar zenith angle (SZA), viewing zenith angle (VZA), relative azimuth angle (RAA), surface altitude and surface albedo were set as input parameters to the radiative transfer model SCIATRAN to establish lookup tables of O2-O2 SCDs and continuum reflectance. To acquire lookup tables of cloud fraction with O2-O2 SCDs and continuum as nodes we inverted the above-mentioned lookup tables. Finally, the EMI measured values of O2-O2 SCDs and continuum reflectance used to obtain effective cloud fraction. The results with the OMI cloud products were compared to validate EMI cloud algorithm. The high and low values of cloud fraction distribute at the same locations. What’s more, the trends of EMI and OMI cloud fraction frequency percent are in good agreement, the cloud frequency distribution shows a decreasing trend from low to high cloud fraction, and when cloud fraction at 0 and 1, both of their frequency percent are very high. The OMI and EMI cloud fraction agree well with each other and have a high correlation coefficient (R) of 0.82.

痕量气体反演算法依赖每个地面像元（ground pixel）的云属性信息。此类信息对于EMI云产品的研发至关重要，因为云对光子路径具有显著影响，而光子路径又会极大影响卫星观测中包含的痕量气体信息。本研究探讨了一种基于477 nm处O₂-O₂吸收的云反演算法，该算法利用EMI测量数据。
首先，采用差分光学吸收光谱法（differential optical absorption spectroscopy, DOAS），从460~490 nm波段反演得到O₂-O₂斜柱密度（slant column densities, SCDs），同时对O₂-O₂斜柱密度的条纹进行校正。其次，将太阳天顶角（solar zenith angle, SZA）、观测天顶角（viewing zenith angle, VZA）、相对方位角（relative azimuth angle, RAA）、地表海拔与地表反照率的不同取值作为辐射传输模型SCIATRAN的输入参数，构建O₂-O₂斜柱密度与连续谱反射率的查找表（lookup tables）。为构建以O₂-O₂斜柱密度与连续谱反射率为节点的云量查找表，我们对前述查找表进行了反演。最终，利用EMI测量得到的O₂-O₂斜柱密度与连续谱反射率数据，计算得到有效云量。
为验证本研究提出的EMI云反演算法，将反演结果与OMI云产品进行对比。二者的云量高值与低值分布区域完全一致。进一步分析表明，EMI与OMI的云量频率百分比变化趋势高度吻合：云量频率分布随云量升高呈递减趋势，且当云量为0或1时，二者的频率百分比均处于较高水平。OMI与EMI的云量结果一致性良好，相关系数（correlation coefficient, R）高达0.82。