Inversion of the asymmetry factor for desert areas of China

Abstract

Deserts, which have high surface albedo and wide area, are important components of the earth system. It is very important for the research of surface radiation and energy balance to understand the anisotropic scattering of desert areas. The emergence and development of multi-angle remote sensing made possible the inversion of the anisotropic scattering of desert areas at the regional or global scale. Firstly, this paper explored the accuracy of the inversion of asymmetry factor using the Hapke model and the simulated single-and multi-phase MISR data. Based on the results, the asymmetry factor of representative surface of desert areas in northwestern China was retrieved. The values of the asymmetry factor retrieved from MISR data were compared with the values retrieved from the laboratory data. The results showed that the single-phase MISR data could be used for the inversion of asymmetry parameter of desert areas. The sign of the asymmetry parameter for the laboratory measurements was positive, which suggests that the surface of laboratory samples is forward scattering. The sign of the asymmetry parameter for MISR data was negative; that is, it is backscattering. The values of the asymmetry parameters retrieved from MISR data were related to the character of the land surface. At Loulan, where the surface was smoother than other sites, retrieved values exhibited the largest negative values of asymmetry factor, suggesting the strongest backscattering. The sand dune area of the Kumtag Desert, which has the greatest roughness, had only slightly negative asymmetry factor values. These findings indicated that at the sensor scale, a rough surface (e.g., dunes) does not necessarily mean more backscattering than a smooth surface. This finding has significant implications for empirical methods (e.g., using the normalized index of backward-scattered radiance minus forward-scattered radiance as an indicator of surface roughness), which should be used carefully for analyzing surface roughness from the remote sensing data. MISR, Hapke model, asymmetry parameter, inversion, Kumtag Desert, multi-angle remote sensing Citation:Deserts of the world have high surface albedo and wide area. Moreover, deserts affect global surface radiation and energy balance by means of the emission of mineral materials to the atmosphere and ocean. Hence, deserts play an important role in the evolution of the earth system [1][2][3][4]. The surface scattering of the desert areas is anisotropic. Numerous researches for anisotropic reflection of the desert and soils have been done due to its importance [5][6][7][8][9][10][11][12][13][14]. Irons and Nolin reviewed the research progress from the view of laboratory, field, and remote sensing [15,16]. Currently there are four kinds of models describing the bidirectional reflectance distribution function (BRDF): radiative transfer model [17][18][19][20][21], geometric optical model [22][23][24], computer simulation model [25], and empirical model [26]. The bidirectional reflectance model derived by Hapke rela...

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