[ACG53-05] Analysis of differences between NDVI-based indices of Himawari 8 AHI and Aqua MODIS using Rayleigh-corrected reflectances
Keywords:Himawari 8, MODIS, NDVI, FVC
A major issue in comparing GEO and LEO products is that the ray-matched condition, in which the illumination and viewing angle condition is nearly identical, does not appear in middle to high latitude regions. The mismatch of the angular condition and effects of surface anisotropy, i.e., bidirectional reflectance distribution function (BRDF), cause biases in reflectances of sensors that propagate into downstream products such as the Normalized Difference Vegetation Index (NDVI). In addition, NDVI data may differ between sensors due to differences in their spectral response functions.
In this study, we scrutinize an index related to greenness, specifically, the fraction of vegetation cover (FVC), and examine its consistency between AHI and Aqua MODIS (near-nadir viewing) for land surfaces in mid-latitude regions. The FVC is referred to as an “NDVI-based index” as it is a function of NDVI and can be less than 0 or greater than 1. Previously, we developed an algorithm that, using top-of-atmosphere (TOA) reflectances as input data, makes the output (NDVI-based index) consistent between sensors in order to mitigate effects of viewing zenith angle (nadir and off-nadir) and spectral response functions. However, the spatial heterogeneity of the atmospheric layer arising from Rayleigh scattering, aerosol loading, ozone and water vapor contents added uncertainties when comparing NDVI-based indices.
Here, as the next step in our studies, we use Rayleigh-corrected (RAC) reflectances to evaluate differences between NDVI-based indices from AHI and Aqua MODIS. Results indicate that NDVI-based indices using RAC reflectances show more consistent results relative to NDVI using RAC reflectances. Additionally, absolute differences between RAC NDVI-based indices of two sensors are similar to or slightly smaller than that between TOA NDVI-based indices. Finally, the benefit of NDVI-based index for transforming NDVIs between sensors is introduced for comparing/integrating observation data obtained using AHI and MODIS for monitoring terrestrial vegetation.