Comparison of differences between MODIS 250Â m and 1Â km cloud masks

The spatial resolution of remote sensing instruments installed onboard satellites is one of the key factors for accurate estimations of cloud amount. In general terms, the larger the instantaneous field of view (IFOV), the greater the overestimation of cloud amount - assuming that data are collected with exactly the same methodology, and processed with exactly the same algorithms. While most meteorological imagers collect data at a spatial resolution of 1 km, the Moderate Resolution Imaging Spectroradiometer (MODIS) offers cloud amount estimates at both 1 km (the standard product) and 250 m (additional, high-resolution products). However, these datasets are produced using different methodological approaches, which impacts the quality and reliability of the product. This study compared 250 m data with 1 km data over elevated terrain with complex orography. Results showed significant discrepancies between the datasets, with 250 m data reporting mean seasonal (June-August) cloud amount 15.8% lower, than 1 km dataset. This was not related to the presence of snow, or to the increased spatial resolution of the cloud mask. On the other hand, both 1 km and 250 m data described similar spatial variability in mean monthly cloud amount (correlation coefficients of 0.85-0.98, p < 0.01).