A Comparison of MODIS-Derived Cloud Amount with Surface Observations at Five SURFRAD Sites
Wednesday, 17 December 2014
Clouds determine the amount of the solar radiation incident at the surface. Accurately quantifying cloud amount (or cloud fraction) is of great importance but difficult to accomplish. Satellite and surface observe clouds in different field of views (FOVs), the inconformity of different FOVs may make large discrepancies when compare satellite and surface derived cloud fraction. From the view of surface incident surface solar radiation, this paper compares Moderate Resolution Imaging Spectroradiometer (MODIS) Level-2 cloud fraction data with two surface radiation budget network (SURFRAD) surface cloud amount dataset at five sites. The correlation coefficients between MODIS and surface cloud fraction are about 0.80-0.91, varied with different SURFRAD sites. There are considerable cases when MODIS observe large cloud fraction bias compared with surface data. The result shows that the variances between MODIS and surface cloud fraction datasets are more apparent when small convective or broken clouds exist in the view. The magnitude of the discrepancy between MODIS and surface derived cloud fraction depends on the sensor zenith angle (SZA) of the satellite. On the average, compared to surface cloud fraction data, MODIS observe larger cloud fraction at SZA >40 º and smaller cloud fraction at SZA <20º. When comparing long-term average of MODIS with surface datasets, Aqua/MODIS observes higher yearly mean cloud fraction. That may be due to that convective clouds are more well developed in the afternoon when Aqua observing.