利用POLDER数据验证MODIS BRDF模型参数产品及Ross-Li模型

将MODIS BRDF模型参数产品(MCD43A1)模拟的近红外波段反射率与从POLDER-3/PARASOL BRDF全球数据集中筛选的9961个像元的BRDF观测数据进行对比，验证了MCD43A1所采用的RossThick-LiSparseR BRDF 模型(Ross-Li模型)拟合二向反射的能力。结果表明，Ross-Li模型总体上可以有效地模拟地物的二向反射，所有像元的近红外波段反射率模拟值与POLDER-3观测数据之间的R²达到0.943，RMSE为0.016，模拟反射率比POLDER-3数据总体偏低5.2%。但Ross-Li模型明显低估了热点反射率，热点模拟结果比POLDER-3数据平均偏低14%，模拟值的R2为0.824，RMSE为0.07。热点反射率模拟误差与地表覆盖类型有关，针叶林热点反射率模拟值偏低最严重，其次是阔叶林、草地与农田，灌木与裸地热点反射率模拟值偏低相对较小。通过修正Ross-Li模型中的体散射核，可以明显改善热点反射率的模拟效果(R²＝0.839，RMSE＝0.043)。Ross-Li模型对天底、暗点等特征方向反射率的模拟较为准确。Ross-Li模型的模拟精度随太阳天顶角和观测天顶角的增大而降低。对于农田与草地而言，Ross-Li模型的模拟精度随NDVI的增加而降低；但在森林与灌木覆盖条件下，当NDVI约为0.5时，Ross-Li模型的模拟效果最差。

Validation of MODIS BRDF model parameters product and the Ross-Li model with POLDER data

The ability of the RossThick-LiSparseR BRDF model (Ross-Li model) to simulate the bidirectional reflectance is assessed by comparing the refl ectance simulated using the Ross-Li model based on the MCD43A1 data with the observed POLDER-3 BRDF data for 9961 pixels selected from the global POLDER-3/PARASOL BRDF archive. It is found that the Ross- Li model is able to simulate the bidirectional refl ectance of ground objects effi ciently. The R² and root mean square error (RMSE) of simulated refl ectance are 0.934 and 0.016, respectively. On average, simulated refl ectance is 5.2% lower compared with the POLDER-3 data. However, the Ross-Li model underestimates the refl ectance by 14% for hotspots, with R2 equal to 0.824 and RMSE equal to 0.07. The errors in simulated refl ectance for hotspots depend on land cover types. Coniferous forests have the largest underestimates of simulated refl ectance for hotspots, followed by broadleaved forests, grasslands, croplands, shrubs and barren lands. The modifi cation for the volume kernel in the Ross-Li model can noticeably improve refl ectance simulation for hotspots (R²=0.839, RMSE=0.043).The refl ectance simulated by the Ross-Li model is relatively accurate at nadir and darkspot. The departure of simulated refl ectance from the POLDER-3 observations increases with the increases in solar zenith and view zenith angles. For grasslands and croplands, the accuracy of simulated refl ectance decreases with the increase in NDVI. However, for forests and shrubs, the discrepancy between refl ectance simulated by the Ross-Li model and the POLDER-3 observations maximizes when NDVI approximately equals 0.5.