Estimating Top-of-Atmosphere Daily Reflected Shortwave Radiation Flux over Land from MODIS Data

Abstract

High-spatial resolution data of the top-of-atmosphere (TOA) reflected shortwave radiation flux are needed to understand the effects of local-scale anthropogenic and natural processes on earth's radiation budget. A previous study developed an algorithm to estimate the TOA instantaneous shortwave component from polar-orbiting Moderate Resolution Imaging Spectroradiometer (MODIS) data. This paper presents a temporal scaling approach to predict daily values of TOA reflected shortwave radiation flux from temporally sparse MODIS observations. Radiative transfer simulation and statistical regression are used to establish the relationship between daily shortwave flux and MODIS spectral reflectance. A comparison between the Terra and Aqua combined MODIS computed data and the Clouds and the Earth's Radiant Energy System SYN1deg product of 1° regional daily shortwave fluxes have a bias of 3.8 Wm2 with a root-mean-square error (RMSE) of 13.3 Wm2 using data from 2009 over eight subsets across various latitudes. Comparing the regional monthly shortwave fluxes reduces the RMSE to 6.9 Wm2. The longer the averaging period the lower the uncertainty is self-explanatory.