Quantitative Evaluation of Leaf Inclination Angle Distribution on Leaf Area Index Retrieval of Coniferous Canopies

Both leaf inclination angle distribution(LAD)and leaf area index(LAI)dominate optical remote sensing signals.The G-function,which is a function of LAD and remote sensing geometry,is often set to 0.5 in the LAI retrieval of coniferous canopies even though this assumption is only valid for spherical LAD.Large uncertainties are thus introduced.However,because numerous tiny leaves grow on conifers,it is nearly impossible to quantitatively evaluate such uncertainties in LAI retrieval.In this study,we proposed a method to characterize the possible change of G-function of coniferous canopies as well as its effect on LAI retrieval.Specifically,a Multi-Directional Imager(MDI)was developed to capture stereo images of the branches,and the needles were reconstructed.The accuracy of the inclination angles calculated from the reconstructed needles was high.Moreover,we analyzed whether a spherical distribution is a valid assumption for coniferous canopies by calculating the possible range of the G-function from the measured LADs of branches of Larch and Spruce and the true G-functions of other species from some existing inventory data and threedimensional(3D)tree models.Results show that the constant G assumption introduces large errors in LAI retrieval,which could be as large as 53%in the zenithal viewing direction used by spaceborne LiDAR.As a result,accurate LAD estimation is recommended.In the absence of such data,our results show that a viewing zenith angle between 45 and 65 degrees is a good choice,at which the errors of LAI retrieval caused by the spherical assumption will be less than 10%for coniferous canopies.

A Practical Approach to Improve the MODIS MCD43A Products in Snow-Covered Areas

The MODerate Resolution Imaging Spectroradiometer(MODIS)MCD43A products have been extensively applied in the remote sensing field,but recent researchers have demonstrated that these products still had the potential to be further improved by using the latest development of the kernel-driven model[RossThick-LiSparseReciprocal-Snow(RTLSRS)]in snow-covered areas,since the MCD43A product algorithm[RossThick-LiSparseReciprocal(RTLSR)]needed to be improved for the accurate simulation of snow bidirectional reflectance distribution function(BRDF)signatures.In this paper,we proposed a practical approach to improve the MCD43A products,which used the Polarization and Directionality of the Earth's Reflectance(POLDER)observations and random forest algorithm to establish the relationship between the BRDF parameters(MCD43A1)estimated by the RTLSR and RTLSRS models.We applied this relationship to correct the MCD43A1 product and retrieve the corresponding albedo(MCD43A3)and nadir reflectance(MCD43A4).The results obtained highlight several aspects:(a)The proposed approach can perform well in correcting BRDF parameters[root mean square error(RMSE)=~0.04].(b)The corrected BRDF parameters were then used to retrieve snow albedo,which matched up quite well with the results of the RTLSRS model.(c)Finally,the snow albedo retrieved by the proposed approach was assessed using ground-based albedo observations.Results indicated that the retrieved snow albedo showed a higher accuracy as compared to the station measurements(RMSE=0.055,bias=0.005),which was better than the results of the MODIS albedo product(RMSE=0.064,bias=-0.018),especially at large angles.These results demonstrated that this proposed approach presented the potential to further improve the MCD43A products in snow-covered areas.