Upscaling GOME-2 SIF from clear-sky instantaneous observations to all-sky sums using the APAR-based approach

Solar-induced chlorophyll fluorescence (SIF) is closely linked to photosynthesis, and provides new opportunities for detecting global Gross Primary Production (GPP). Instantaneous satellite SIF products are usually available only for clear-sky condition, which means that there is a temporal inconsistency in the direct link between these and continuous carbon flux records.

To address this issue, we designed an APAR-based method to upscale the GOME-2 SIF (developed by Joiner et al., 2016) from instantaneous clear-sky observations to all-sky sums, and hereby derived a SIF product for all-sky conditions (ASSIF) from GOME-2 at 8-day and monthly intervals during 2007 and 2018. The SIF all-sky upscaling method was generated with several explanatory variables, including instantaneous GOME-2 soundings, MODIS vegetation indices, and the photosynthetically active radiation (PAR) from the Modern-Era Retrospective Analysis for Research and Applications Version 2 (Merra-2).