Directional Reflectance Simulation And Characteristic Analysis Of Snow Based On Sentinel-2 Data

Snow cover is one of the most active natural elements on the earth surface and plays an important role in the global water cycle,radiation budget balance and carbon cycle.Snow has obvious anisotropy,and the accurate description of snow anisotropy is the key to understand various cycles.Modeling calculation and analysis of snow anisotropy using Bidirectional Reflectance Distribution Function(BRDF)is an important content of snow and ice research.In recent years,BRDF models of snow cover have been developed rapidly.However,due to different theoretical assumptions of the models,their application ranges are quite different.There are few BRDF models that can be applied to common single-angle images.In addition,the present discussion on the BRDF anisotropy system of snow cover is also insufficient.Therefore,based on the understanding of snow BRDF model system,this paper relies on Python data processing platform,POLDER multi-angle satellite and Sentinel-2 data to compare and analyze the Asymptotic Radiative Transfer(ART)model and RTLSRS(Ross Thick-Li Sparse R-Snow)model.Secondly,a new inversion parameter method is proposed to evaluate the inversion parameter errors of ART model.Finally,the model is applied to the common single-angle satellite to analyze the main characteristics of snow BRDF.It provides reference for distinguishing snow from other ground objects and extracting snow covered areas.The main findings of this study are as follows:(1)Snow model analysis based on POLDER satellite data shows that,from the perspective of model simulation conditions,ART model is more suitable for single-angle satellite,while RTLSRS model is limited in applicability.According to the simulation analysis results obtained from all angles,the fitting accuracy of ART model and RTLSRS model is low.The R~2of model fitting result is around 0.5,which is much lower than the fitting result of POLDER’s pure snow data of 0.9.According to the simulation results of the scattering direction in front of the main plane,the absolute and relative error values of the reflectance simulated by the RTLSRS model are better than those of the ART model,but the performance of the RTLSRS model is poor in the simulation of the maximum and minimum values.Therefore,the simulation accuracy of the ART model is higher than that of the RTLSRS model in the scattering direction in front of the main plane.(2)The analysis of snow parameter inversion of ART model and its influence on the calculation of bidirectional reflection shows that the values of snow grain size inversion deviation of ART model are mainly concentrated in the 60°range of the main plane and the geometric observation direction with the observation zenit Angle greater than 50°.By eliminating the reflectance data of this part,more accurate snow parameter can be obtained to simulate the reflectance.The short-wave infrared band data can be used to obtain more information of snow particle size parameters,and the visible band can be used to obtain more information of snow pollutants.The difference analysis between the existing method and the least square inversion d shows that,on the whole,the d result obtained by the existing method is small,and the difference between the existing method and the least square method d is less than 4%.When the difference changes from 0 to 50%,the reflectance simulation R~2 varies between 0.610-0.607,and the change is less than 1%.RMSE and Bias gradually increase,and the Bias change reaches 0.1,indicating that with the increase of the difference,the error of bidirectional reflection fitting becomes larger and larger.(3)The BRDF simulation of snow based on Sentinel-2 data found that both models had large underestimation of the high reflectance value,the ART model was more significant,and the RTLSRS model had the best stability.The analysis of BRDF characteristics of ice and snow in different directions shows that,on the whole,the simulation results in the front scattering direction are larger than those in the vertical main plane,and the simulation results in the 45°forward scattering direction are larger than those in the 45°back scattering direction.The analysis of BRDF distribution characteristics of snow and ice under different conditions shows that with the increase of zenith Angle,the reflectance increases in all directions on the whole.With the increase of band,the reflectance of ART model and RTLSRS model decreases.With the change of time,the BRDF distribution of snow in ART model and RTLSRS model changes little.However,there is a great difference in the overall distribution.The RTLSRS model’s main plane is larger than that of the vertical main plane,while the ART model obtains the maximum value in the scattering direction in front of the main plane,followed by the vertical main plane direction,and the minimum value in the backscattering direction of the main plane.