A cloud detection method based on a time series of MODIS surface reflectance images

The Moderate Resolution Imaging Spectroradiometer(MODIS)-Terra surface reflectance product(MOD09A1),with bands 1 to 7,is a gridded,eight-day composite product derived from the MODIS-Terra top of atmosphere reflectance swaths.It performs cloud detection and corrects for the effects of atmospheric gases and aerosols.The cloud mask(CM)algorithms for MODIS are based on empirical thresholds on spectral reflectance and brightness temperature.Since the spatial resolution of the thermal band is 1000 m,while that of MOD09A1 is 500 m,many undetected and false clouds are observed in MOD09A1.These errors always result in temporal and spatial inconsistencies in higher-level products.In this paper,a cloud detection algorithm(TSCD)based on a MOD09A1 time series is introduced.Time series cloud detection(TSCD)algorithm is based on the relative stability of ground reflectance and the sudden variations in reflectance that result from cloud cover.The algorithm first searches the clear-sky reference data,and then discriminates clouded and unclouded pixels by detecting a sudden change of reflectance in the blue wavelength and spectral correlation coefficient at the pixel level.Compared with cloud cover assessments obtained from MODIS’original CM,TSCD provides similar or better discrimination in most situations when the land surface changes slowly.