The Bias Analysis of FY-2G Cloud Fraction in Summer and Winter

Evaluation of satellite retrieval cloud fraction is fundamental for good use in operational weather analyses application. Cloud fraction relative biases between FY-2G and Aqua/MODIS data are investigated in order to validate FY-2G cloud fraction. In order to understand the accuracy of FY-2G cloud fraction better, cases contain both clear and cloud pixels in the target area are selected, and 80 matched cases are analyzed. Results show that the cloud fraction of FY-2G has the same distribution pattern with Aqua/MODIS. The mean cloud fraction of FY-2G is 72.81%, and according to MODIS data it is 76.19%. Among 80 selected cases, 45 cases are in June and 35 cases are in December of 2015. In June, the mean cloud fraction of FY-2G and Aqua is 68.12% and 70.78%, respectively. In December, the mean cloud fraction of FY-2G and Aqua is 78.84% and 83.14%. FY-2G's cloud fraction is smaller than that of Aqua. For all cases, there are 79.15% pixels that their absolute relative bias between FY-2G and Aqua is smaller than 15%. In June, there are 77.78% pixels that the absolute relative bias between FY-2G and Aqua is smaller than 15%, while it is 81.24% in December. The cloud fraction correlation coefficient between FY-2G and Aqua is 0.74 through the year, 0.76 in June and 0.72 in December.During daytime, the mean cloud fraction of FY-2G and Aqua is 70.42% and 72.21%, respectively. There are 67.74% pixels that the absolute deviation between FY-2G and Aqua is smaller than 5%. The cloud fraction correlation coefficient between FY-2G and Aqua is 0.754. For night time cases, FY-2G mean cloud fraction is 75.59% and Aqua is 80.81%. The cloud fraction correlation coefficient between FY-2G and Aqua is 0.73. There are 72.34% pixels that their cloud fraction absolute deviation between FY-2G and Aqua is smaller than 5% during nighttime.Results show that the cloud fraction bias between FY-2G and Aqua is mainly caused by cloud detection accuracy. The cloud detection bias between FY-2G and Aqua mainly comes from different satellite observation ability and cloud detection algorithm. Compared with Aqua/MODIS data that has 36 channels with the lowest 0.01°×0.01° nadir spatial resolution, FY-2G has 5 channels with the highest 0.05°×0.05° spatial resolution. FY-2G's cloud detection easily makes mistakes when it has broken cloud, thinner cirrus or not all covered by cloud in the view. At the same time, different data processing methods within data match processing also cause bias between different kinds of satellite data.