AbstractsAstronomy & Space Science

Global retrievals of terrestrial chlorophyll fluorescence from spaceborne spectrometers

by Philipp Köhler

Institution: Freie Universität Berlin
Year: 2016
Posted: 02/05/2017
Record ID: 2066542
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000102738


This thesis is based on research articles (either published in or submitted to peer-reviewed journals), contributing to the assessment of global vegetation photosynthesis through the monitoring of sun-induced chlorophyll fluorescence (SIF) from space which is the aim of the GlobFluo project funded by the Emmy Noether Programme of the Deutsche Forschungsgemeinschaft (GU 1276/1-1). SIF is a weak electromagnetic signal emitted by the photosynthetic machinery of active vegetation in the visible and near-infrared spectral regions (650 – 800~nm). In this thesis, two fundamentally different algorithms were developed (physically- and statistically-based) to decouple the SIF signal from the about 100-times-more-intense reflected solar radiation in the measured top-of-atmosphere (TOA) radiance spectrum. The physically-based approach has been applied to data acquired from JAXA's Greenhouse Gases Observing Satellite (GOSAT) between June 2009 – August 2011. The statistically-based retrieval has been employed to derive SIF from data of two different satellite instruments: (i) ESA's SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) between August 2002 – March 2012, and (ii) EUMETSAT's Global Ozone Monitoring Experiment-2 (GOME-2) between January 2007 – April 2015. Comparisons of these three new global SIF data sets with each other as well as with existing data sets reveal highly consistent spatiotemporal patterns. However, differences arise when comparing absolute values which is presently less critical since the absolute value is not applied in current research. Furthermore, the influence of potentially confounding factors on the retrieval has been analyzed. It could be proven that results from both retrieval approaches are relatively robust against cloud contamination. In contrast, the South Atlantic Anomaly (SAA), a region with an anomalously reduced strength in the Earth‘s magnetic field, leads to an increased retrieval noise in large parts of the South American continent. Therefore, a particularly stringent filtering ensured that the impact of the SAA on GOME-2 SIF measurements in a case study in the Amazon rainforests is as low as possible. Regarding the seasonality of SIF in the Amazon, two often disregarded aspects have been identified to be particularly important for the interpretation of SIF: (i) an essential influence of the satellite overpass time due to the response of SIF to instantaneous photosynthetically active radiation (PAR), and (ii) a significant impact of the sun-surface-sensor geometry due to anisotropic SIF emission characteristics. Given the intrinsic influence of canopy structure on SIF measurements, combining SIF with appropriate vegetation parameters (ideally retrievable from the same sensor) may be one key aspect to relate SIF to canopy photosynthesis. In particular, the canopy scattering coefficient (CSC), resulting from the concept of canopy spectral invariants, has been identified as a promising candidate for this purpose when observing densely vegetated areas. Therefore,…