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The dual-channel dynamic threshold method quantitative calculation and 6S mode automatic cloud detection the apparent reflectivity method GMS-5 and FY2B image

Author: LiuXi
Tutor: DuBingYu；XuJianMin
School: Nanjing Institute of Meteorology
Course: Meteorology
Keywords: Cloud detection Dynamic threshold Apparent reflectance
CLC: P412
Type: Master's thesis
Year: 2004
Downloads: 325
Quote: 3
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Abstract

The cloud is composed of water droplets suspended in the atmosphere, and (or) ice particulate visible aggregates. Become an important research tool for cloud analysis of satellite data with its own advantages. Cloud detection, in order to obtain more accurate cloud parameters from satellite data First Clouds like contract knowledge out. This article describes two cloud detection algorithm: a dual-channel dynamic threshold method and the apparent reflectivity method. The dual-channel dynamic threshold method based on the visible cloud images and infrared cloud, cloud and surface features of its range. 32 × 32 pixel pixel array histogram statistics, the threshold calculated the difference between the cloud and surface features Ranges. Like cloud determining in accordance with the determined threshold value, for each pixel array individually. If a pixel in the infrared and visible channels sentenced knowledge into the cloud, the pixel cloud pixel. Used only when the lower solar elevation angle, poor lighting conditions, the the nephogram tone very dark or nighttime visible channel data is missing, the infrared channel sentenced knowledge cloud. Dual-channel dynamic threshold method GMS5 cloud images and impurity astigmatism and in addition the stray after FY2B cloud images cloud detection in low latitudes during the day, combined with the visible channel and infrared channels, the algorithm has better cloud distinguishing results. However, when the infrared channel only in the middle and low latitude night, and high latitudes, cloud the accuracy of detection is reduced. Lower surface temperature, the presence of snow, these factors also makes the high latitudes cloud detection accuracy decreases. Cloud detection results of of impurity removal astigmatism after FY2B nephogram better than the result of impurity astigmatism. The apparent reflectance method first cloud images to be detected near the seven days to 10 days of the cloud clear sky synthesis, DAE table made by check 6S mode value of the clear sky synthesis inversion of surface reflectance 6S mode and then check made Another DAE the table surface reflectance apparent reflectance forward. The apparent reflectivity multiplied by a different coefficient threshold, the apparent reflectance is greater than the threshold value to be detected nephogram pixel for cloud pixels. Used for cloud detection the apparent reflectivity method GMS5 nephogram clear sky synthesis map a regional cloud cover is easy to be detected in the the nephogram the corresponding region of cloud misinterpreted as a surface, an area of ??clear sky synthesis Figure not covered with snow be detected cloud map of the region covered with snow, you will surely snow misinterpreted as a cloud. MODIS cloud detection results of the dual-channel dynamic threshold method and the apparent reflectivity method authenticity test. The correct rate of these two methods in eastern China and on the east of the sea in about 80% higher false positive rate of clear skies cloud of false positives is relatively low, that is, the apparent reflectivity method and dual-channel dynamic threshold method determination of the clear zone is more stringent. Apparent on land the reflectivity method skies false positive rate was significantly lower than the dual-channel dynamic threshold method, the error rate is roughly the same in the cloud. Marine apparent reflectivity method skies false positive rate of less than dual-channel dynamic threshold method, but the gap is smaller, cloud the false positive rate is roughly the same. This shows that the apparent reflectivity method discriminant raise the level of the clear zone land. Of this research work shows that, when no historical data, do not know the region a priori the climate value and radiation values, dual-channel dynamic threshold method and the apparent reflectivity method are likely to get a reasonable result.

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