Comparison between MODIS and GOES PAR data products

Solar radiation in the visible domain, commonly quantified as the Photosynthetically Available Radiation (PAR), drives phytoplankton photosynthesis and is an important parameter for models of marine net primary productivity (NPP). For basin scale applications, daily PAR product is now available from various ocean color satellites including MODIS. This daily PAR product, however, is estimated from roughly once per day measurement at a time around local solar noon. Because of the frequent movement and blockage by clouds, such daily PAR product is subject to various uncertainties on different time scales; and it is necessary to characterize this uncertainty before its application for the estimation of NPP. The Geostationary Operational Environmental Satellite system (GOES) provides hourly solar radiation product (24 images per day) for North America. The integration of the GOES hourly measurements thus enables a more realistic estimation of daily PAR. Here, both GOES iPAR and daily PAR products are used to evaluate the corresponding MODIS products, with an objective to characterize the uncertainties in the latter products. Preliminary results show that MODIS daily PAR is generally lower than GOES daily PAR when all data are used in the comparison, although MODIS iPAR and daily PAR show excellent agreement with GOES iPAR and daily PAR, respectively, for cloud-free locations. The impact of the PAR product on basin-scale estimation of NPP is further evaluated.