A spatially downscaled sun-induced fluorescence global product for enhanced monitoring of vegetation productivity

Creators: Duveiller, Gregory; Filipponi, Federico; Walther, Sophia; Köhler, Philipp; Frankenberg, Christian; Guanter, Luis; Cescatti, Alessandro

Abstract

Sun-induced chlorophyll fluorescence (SIF) retrieved from satellite spectrometers can be a highly valuable proxy for photosynthesis. The SIF signal is very small and notoriously difficult to measure, requiring sub-nanometre spectral-resolution measurements, which to date are only available from atmospheric spectrometers sampling at low spatial resolution. For example, the widely used SIF dataset derived from the GOME-2 mission is typically provided in 0.5∘ composites. This paper presents a new SIF dataset based on GOME-2 satellite observations with an enhanced spatial resolution of 0.05∘ and an 8 d time step covering the period 2007–2018. It leverages on a proven methodology that relies on using a light-use efficiency (LUE) modelling approach to establish a semi-empirical relationship between SIF and various explanatory variables derived from remote sensing at higher spatial resolution. An optimal set of explanatory variables is selected based on an independent validation with OCO-2 SIF observations, which are only sparsely available but have a high accuracy and spatial resolution. After bias correction, the resulting downscaled SIF data show high spatio-temporal agreement with the first SIF retrievals from the new TROPOMI mission, opening the path towards establishing a surrogate archive for this promising new dataset. We foresee this new SIF dataset becoming a valuable asset for Earth system science in general and for monitoring vegetation productivity in particular. The dataset is available at https://doi.org/10.2905/21935FFC-B797-4BEE-94DA-8FEC85B3F9E1 (Duveiller et al., 2019).

Additional Information

© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Published by Copernicus Publications. Received: 12 July 2019 – Discussion started: 19 August 2019. Revised: 1 April 2020 – Accepted: 7 April 2020 – Published: 13 May 2020. We thank Joanna Joiner for valuable discussions regarding her GOME-2 SIF retrieval and how to use it in the context of this study. G Most of the heavy calculations were done using JEODPP (Soille et al., 2018). The figures where realised using the ggplot2 R package with ancillary spatial vectors from Natural Earth. Code availability: Code associated with this study, including the script to downscale SIF and the scripts to generate the figures in this paper, is available here: https://doi.org/10.5281/zenodo.3753521 (Duveiller and Filipponi, 2020). Data availability: The dataset of daily corrected downscaled SIF described in this document has been labelled Version 2.0 to differentiate it from Version 1.0, which is the original dataset described in Duveiller and Cescatti (2016). These are all available in the following repository: Duveiller et al. (2019) – Downscaled-GOME2-SIF, European Commission, Joint Research Centre (JRC) (dataset), https://doi.org/10.2905/21935FFC-B797-4BEE-94DA-8FEC85B3F9E1, PID – http://data.europa.eu/89h/21935ffc-b797-4bee-94da-8fec85b3f9e1 (last access: 11 May 2020). The files of the Version 2.0 dataset that are described here are grouped by year in distinct NetCDF files for each of the two GOME-2 retrievals (JJ and PK). The product is distributed in an equirectangular projection with a pixel size of 0.05∘. The temporal coverage spans from 2007 to 2018. The temporal sampling of the product is 8 d. However, every record is based on SIF input data retrieved over a 16 d moving window. This results in a certain amount of temporal auto-correlation, as the 16 d window moves every 8 d, leaving an overlap of 8 d in each successive record. The day reported in the NetCDF file corresponds to the ninth day of the 16 d retrieval period to match the MODIS convention used in the MCD43C4 product. Along with the dataset, we also provide in a separate file the slope and intercept values at the pixel level to allow users to rescale the downscaled PK GOME-2 values to TROPOMI estimates. Supporting data to this paper are available in this extra repository: https://doi.org/10.5281/zenodo.3753024 (Duveiller et al., 2020). This includes data to generate the figures in this paper as well as OCO-2 and TROPOMI gridded datasets that had to be generated specifically for this study. Author contributions: GD, FF and AC designed the study. SW, PK and CF prepared and provided input data and guidance on how to use it. FF and GD processed the data. GD analysed the data and made the figures. GD wrote the text, with contributions from all the authors. The authors declare that they have no conflict of interest. Disclaimer: The data are provided as is, with no warranties. Review statement: This paper was edited by Yuyu Zhou and reviewed by Min Chen and two anonymous referees.

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