Effective Compositing Method to Produce Cloud-Free AVHRR Image

摘要

The Advanced Very High Resolution Radiometer (AVHRR) series of instruments have been frequently used for land cover change and global environment studies. The availability of more than 30 years' records have made important time-series studies possible. Removing cloud effects from AVHRR images is a critical task when using the data to monitor land cover changes. Considering that the maximum normalized difference vegetation index (NDVI) compositing method is not suitable for land cover types with low NDVI (e.g., water, snow, and bare ground), we propose a new compositing method to remove cloud effects while keeping information of all land cover types. The developed method includes three steps: 1) compositing daily AVHRR images using maximum brightness temperature in Channel 4; 2) replacing water pixels with pixels having maximum Channel 1/Channel 2 ratio; and 3) replacing vegetation pixels with pixels having maximum NDVI. The proposed method was tested on the Land Long-Term Data Record-AVHRR dataset in a region of 37$^{circ}{rm E}$ –180$^{circ}{rm E}$, 3$^{circ}{rm S}$–73 $^{circ}{rm N}$ in 1993 to generate monthly minimal cloud-effected images. After compositing, the average ratio of cloud-contaminated and invalid pixels is reduced from 64.8% to 22.0%. Compared with the other two methods (composite using maximum NDVI and composite using cloud masks), our method is shown to be more effective and convenient. Moreover, the compositing process takes consideration of the temporal profile of land surface, which is suited for long time global or continental land cover change studies. Composite images generated in different time periods are also evaluated to identify the proper compositing - eriod.