National Aeronautics and Space Administration

The analysis of two years of Collection 3 and five years of Collection 4 Terra MODIS LAI and FPAR data sets is presented in this article with the goal of understanding product quality with respect to version (Collection 3 versus 4), algorithm (main versus back-up), snow (snow-free versus snow on the ground) and cloud (cloud-free versus cloudy) conditions. Retrievals from the main radiative transfer algorithm increased from 55% in Collection 3 to 67% in Collection 4 due to algorithm refinements and improved inputs. Anomalously high LAI/FPAR values observed in Collection 3 product in some vegetation types were corrected in Collection 4. The problem of reflectance saturation and too few main algorithm retrievals in broadleaf forests persisted in Collection 4. The spurious seasonality in needleleaf LAI/FPAR fields was traced to fewer reliable input data and retrievals during the boreal winter period. About 97% of the snow covered pixels were processed by the back-up NDVI-based algorithm. Similarly, a majority of retrievals under cloudy conditions were obtained from the back-up algorithm. For these reasons, the users are advised to consult the quality flags accompanying the LAI and FPAR product.