Process-based modeling of solar-induced chlorophyll fluorescence with VISIT-SIF version 1.0

Abstract. Satellite retrievals of solar-induced chlorophyll fluorescence (SIF) can provide opportunities to improve our understanding of terrestrial ecosystem dynamics and the carbon cycle at the global scale. Here, we present a new biogeochemical process-based carbon and nitrogen cycle model for representing SIF retrievals (VISIT-SIF version 1.0: Vegetation Integrative SImulator for Trace gases and SIF) acquired by the Greenhouse gases Observing SATellite (GOSAT) with an hourly time step and a spatial resolution of approximately 0.31 × 0.31°. The implementation of radiation transfer models (RTMs) helps to address the interaction of chlorophyll fluorescence with vegetation and atmosphere. However, the computation of RTMs becomes more time-consuming, which can make it impractical in applications of satellite observations with larger data volumes. This study resolves this issue by parameterizing the radiative transfer processes and the geometric relationships. This approach enables the ease of implementation of VISIT-SIF for simulating satellite SIF retrievals even for the satellites having off-nadir observation angles. With an initial 7 years of data (2009–2015), our model simulations showed a consistent global mean value of 0.51 ± 0.39, with GOSAT SIF retrievals of 0.46 ± 0.42 mW m−2 sr−1 nm−1; the root mean squared error was 0.29 mW m−2 sr−1 nm−1. We also found that the mean seasonal variability in the simulated SIFs was mostly consistent with the GOSAT SIF retrievals at the subcontinental scale. However, the simulated results indicated less sensitivity to water stress in the late dry season in arid and semiarid regions relative to that of the GOSAT SIF retrievals, which is consistent with the findings of previous studies using multiple biogeochemical process-based models. This comparison suggested that there is a critical need to improve our knowledge of SIF variability and biophysical processes in such regions.