Global Cross-Comparison of Suomi NPP VIIRS Vegetation Index EDR with Aqua MODIS

The Visible Infrared Imaging Radiometer Suite (VIIRS) sensor series is slated to continue the highly calibrated data stream initiated with Earth Observing System (EOS) Moderate Resolution Imaging Spectroradiometer (MODIS). A number of geophysical products, termed Environmental Data Records (EDRs), are being produced from VIIRS data, including Vegetation Index (VI) EDR. VIIRS VI EDR is a daily, 375 m resolution product and includes the "Top-of-the-Atmosphere (TOA)" Normalized Difference Vegetation Index (NDVI) and the "Top-of-Canopy (TOC)" Enhanced Vegetation Index (EVI). The TOC NDVI is being added to the product. In this study, we cross-compared VI EDR from the first VIIRS sensor onboard the Suomi National Polar-orbiting Partnership (NPP) satellite platform with the Aqua MODIS counterparts in global scale with the aim of developing a thorough understanding of radiometric compatibility between the two VI datasets. VIIRS VI products from April 2014 through June 2014 were obtained along with MODIS daily products. They were all reprojected and spatially-aggregated into a 4 km sinusoidal grid while screening for cloud and aerosol contaminations using quality flags. We then masked VIIRS-MODIS observation pairs for near-identical observation geometry: (1) view zenith angle (VZA) < 7.5 degrees, (2) 20 degrees < VZA < 27.5 degrees, (3) 40 degrees < VZA < 47.5 degrees, and (4) 55 degrees < VZA < 62.5 degrees. Their mean differences (MDs) were computed for each VZA bin and for all bins at once for quantitative evaluation. VIIRS and MODIS TOA NDVI had a very small overall MD of 0.005 NDVI units, whereas TOC EVI had a fairly large overall MD of -0.04 EVI units. These systematic differences were consistent across the 2-month period examined in this study. TOC NDVI, on the other hand, had largely fluctuating MD across this period, ranging from 0.005 to 0.01 NDVI units, an indication of inconsistent atmospheric correction or cloud mask results. With respect to VZA, inconsistent MDs were always obtained for the 55-62.5 degree VZA bin for all the three indices. Overall, VIIRS and MODIS VI products are subject to systematic differences, which are however consistent for VZ < 50 degrees. Future studies should examine the performance of VIIRS atmospheric corrections and compatibility of VIIRS and MODIS observations at large VZA.