An Algorithm for a Whole-Sky Camera with Low Color Depth and Relationships Between Cloud Fractions Derived from the Sky Camera and MODIS Observations

抄録

Whole-sky images taken during April 2012 at Sendai, Japan by a sky camera with a fish-eye lens were used to validate cloud fractions derived from data obtained by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra satellite. We developed an algorithm to estimate the cloud fractions from the sky camera JPEG data with 8-bit low color depth, and applied this algorithm to have the cloudiness as seen from the ground. The existence of cirrus, thin clouds, and broken clouds yielded large ambiguities in a comparison between these cloud fractions. How wide a view of the whole-sky image taken at an observatory is suitable for the validation of the MODIS cloud fractions was also examined, and the results indicate that cloud homogeneities strongly affect the size of the area in which the cloud fractions given as an indicator at the site. The cloud fractions from the MODIS with average radii of 0.04° (about 3km), 0.08° (≈6km), 0.2° (≈15km), 0.3° (≈20km), and 0.4° (≈30km) around the site were compared with the image-based cloud fractions at the site. Consequently, the cloud fractions observed at the site had values that were generally closer to the average of the MODIS cloud fractions with a radius of 0.08° (about 6km). Throughout this study, we assumed that the subsidiary whole-sky pictures taken at adjacent sites encircling the main observatory at a radius of about 5 ~ 7km would be very helpful for more accurately validating the cloud fractions derived from satellite observations.