Estimation of net surface shortwave radiation from MODIS data

摘要

Net surface shortwave radiation (NSSR) is a key quantity for the estimation of surface energy budget and is used in various land-surface models. In this article, two different methodologies, including three empirical algorithms and one advanced simplified theoretical algorithm for estimating instantaneous NSSR from Moderate Resolution Imaging Spectroradiometer (MODIS) data are explored and summarized. An advanced simplified theoretical algorithm is developed by combining two simplified radiative-transfer models with various MODIS atmosphere and land products. To comprehensively evaluate these algorithms, ground measurements from seven stations widely distributed in different climatic regions of China are used. The results indicate that under clear-sky conditions, the three empirical algorithms present appreciable difference in accuracy, while under cloudy skies, they produce similar, but not very good, predictions. Compared with these empirical methods, the simplified theoretical algorithm we adopt can significantly improve accuracy. The root mean square difference (RMSD) yielded by this algorithm is approximately 54 Wm-2 under clear skies and 83 Wm-2 under cloudy skies, respectively. Since the utility of instantaneous NSSR estimates is limited compared to that of the daily average value, a simple scheme to acquire the daily average NSSR is established, which is based on instantaneous estimations from two satellite MODIS sensors (Terra: AM and Aqua: PM), and the daily average NSSR over the Beijing area is also mapped.