A simplified surface albedo inverse model with MODIS data

Abstract

Land surface albedo, which is a fundamental component needed for determining the radiation balance of the Earth-atmosphere system, is the surface hemispherical reflectivity integrated over the solar spectrum. Briefly said, it is the ratio of the total upwelling irradiance to the total downward irradiance. Given the limit of experiment field and instrument equipment, it is very difficult to obtain a great deal of surface measured data. Thereof, in our research, from the point of view that surface albedos are the ratio of flux, surface albedos can be reversed with less calibrated field synchronous observed data. During the conversion process of surface albedos, the first is to obtain surface reflectance from remotely sensed data, which can be completed by running 6S atmospheric radiative transfer model; the second is to reverse surface narrowband albedos, which is fulfilled by MODTRAN model. The outputs of MODTRAN simulations are upwelling and downwelling irradiance. The sensor spectral response functions are integrated with these outputs to generate narrowband albedos; the third is to calculate surface broadband albedos, which is focused on determining the converting coefficients from narrowbands to broadbands under the linear relationship between narrowband albedos and broadband albedos. The spectral distribution of solar irradiance at the surface is set as the weighting function for converting the narrowband albedos to broadband albedos in the research. The conversion of broadband albedos is fulfilled finally. The simplified surface broadband albedo reverse model is tested for the west of Inner Mongolia using MODIS image data and synchronous observed surface spectral data based on the above conversion algorithm. The results show that the method is convenient and feasible under a specific surface/atmospheric condition.