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Published March 15, 2020 | Supplemental Material
Journal Article Open

A framework for harmonizing multiple satellite instruments to generate a long-term global high spatial-resolution solar-induced chlorophyll fluorescence (SIF)


Several decade-long satellite retrievals of solar-induced chlorophyll fluorescence (SIF) have become available during the past few years, but understanding the long-term dynamics of SIF and elucidating its co-variation with historical gross primary production (GPP) remains a challenge. Part of the challenge is due to the lack of direct comparability among these SIF products as they are derived from various satellite platforms with different retrieval methods, instruments characteristics, overpass time, and viewing-illumination geometries. This study presents a framework that circumvents these discrepancies and allows the harmonization of SIF products from multiple instruments to achieve long-term coverage. We demonstrate this framework by fusing SIF retrievals from SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) and Global Ozone Monitoring Experiment 2 (GOME-2) onboard MetOp-A developed at German Research Center for Geosciences (GFZ). We first downscale both original SIF datasets from their native resolutions to 0.05° (SIF_(GOME2_005) and SIF_(SCIA_005) respectively) using machine learning (ML) algorithms imposed with regionalization constraints to account for the varying relationships between predictors and SIF in space and time. We then apply the cumulative distribution function (CDF) matching technique to correct the offset between SIF_(GOME2_005) and SIF_(SCIA_005) inherited from the original instrumental discrepancies to generate a harmonized SIF time series from 2002 to present (SIF_(005)). Finally, we quantify the uncertainty of SIF_(005). SIF_(005) is validated with 1) the original retrievals to ensure the spatial and temporal variabilities are preserved, 2) airborne SIF derived from the Chlorophyll Fluorescence Imaging Spectrometer (CFIS, R² = 0.73), and 3) ground-based SIF measurements at a subalpine coniferous forest (R² = 0.91). The SIF_(yield) derived from SIF_(005) has high seasonal consistency with the ground measurements (R² = 0.93), suggesting that the harmonized product SIF_(005) carries physiological information beyond the absorbed photosynthetically active radiation. Additionally, SIF_(005) has a good capability for large-scale stress monitoring as demonstrated with several major historical drought and heatwave events. The framework developed in this study sets the stage for future development of even more advanced SIF products from all SIF-capable satellite platforms once issues related to inter-sensor calibration are resolved and SIF physiology is better understood.

Additional Information

© 2020 Elsevier Inc. Received 26 August 2019, Revised 3 December 2019, Accepted 8 January 2020, Available online 20 January 2020. J. Wen and Y. Sun are funded by the USAID Feed the Future program 7200AA18CA00014. Y. Sun, N.C. Parazoo, and T. S. Magney acknowledge the support from Earth Sciences Division MEaSUREs program. P. Köhler is funded by the Earth Science U.S. Participating Investigator (Grant Number: NNX15AH95G). G. Duveiller is funded at the European Commission Joint Research Centre by the Copernicus-2 administrative agreement (nr. 5054) supported by DG-GROW. T. S. Magney is supported by the OCO-2 Science Team at the Jet Propulsion Laboratory, California Institute of Technology. G. Hooker is funded with National Science Foundation DMS-1712554 and TRIPODS 1740882. L. Yu is funded by the 2018 China Scholarship Council (CSC)-IBM Future Data Scientist Scholarship Program. C. Y. Chang is supported by USDA-NIFA postdoctoral fellowship (Grant Number: 2018-67012-27985). We thank C. Frankenberg for providing CFIS SIF retrievals and the helpful discussion on analyzing and interpreting the PhotoSpec SIF measured at Niwot Ridge. Data used in this study: MODIS products are from https://lpdaac.usgs.gov/; MERRA-2 PAR product is from: https://disc.gsfc.nasa.gov/; GOME-2 and SCIAMACHY SIF are from ftp://fluo.gps.caltech.edu/data/Philipp/; airborne CFIS SIF is from ftp://fluo.gps.caltech.edu/data/CFIS/; tower SIF measurements is from: https://data.caltech.edu/records/1231; SIF_(LUE) is from https://jeodpp.jrc.ec.europa.eu. The harmonized long-term record of SIF_(005) can be downloaded from https://cornell.box.com/s/gkp4moy4grvqsus1q5oz7u5lc30i7o41. Author contribution statement: J. Wen: Conceptualization, Methodology, Software, Formal analysis, Writing - Original Draft, Writing - Review & Editing, Visualization P. Köhler: Data Curation, Writing - Review & Editing G. Duveiller: Data Curation, Writing - Review & Editing N. C. Parazoo: Writing - Review & Editing T. S. Magney: Formal analysis, Investigation, Data Curation, Writing - Review & Editing G. Hooker: Software, Formal analysis L. Yu: Formal analysis, Data Curation C. Y. Chang: Writing - Review & Editing Y. Sun: Conceptualization, Methodology, Formal analysis, Resources, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration, Funding acquisition The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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