Angular normalization of GOME-2 Sun-induced chlorophyll fluorescence observation as a better proxy of vegetation productivity

Sun-induced chlorophyll fluorescence (SIF) has been regarded as a promising proxy for gross primary productivity (GPP) over land. Considerable uncertainties in GPP estimation using remotely sensed SIF exist due to variations in the Sun-satellite view observation geometry that could induce unwanted variations in SIF observation. In this study, we normalize the far-red Global Ozone Monitoring Experiment-2 SIF observations on sunny days to hot spot direction (SIFh) to represent sunlit leaves and compute a weighted sum of SIF (SIFt) from sunlit and shaded leaves to represent the canopy. We found that SIFh is better correlated with sunlit GPP simulated by a process-based ecosystem model and SIFt is better correlated with the simulated total GPP than the original SIF observations. The coefficient of determination (R-2) are increased by 0.040.03, and 0.070.04 on a global average using SIFh and SIFt, respectively. The most significant increases of the R-2 (0.090.04 for SIFt and 0.050.03 for SIFh) appear in deciduous broadleaf forests.