2016-11-03: Spaceborne Vegetation Fluorescence Time Series Evaluated with Flux Tower Measurements
A global, monthly averaged time series of Sun-induced Fluorescence (SiF), spanning January 2007 to June 2015, was derived from Metop-A Global Ozone Monitoring Experiment 2 (GOME-2) spectral measurements. Far-red SiF was retrieved using the filling-in of deep solar Fraunhofer lines and atmospheric absorption bands based on the general methodology described by Joiner et al, AMT, 2013. A Principal Component (PC) analysis of spectra over non-vegetated areas was performed to describe the effects of atmospheric absorption. Our implementation (SiF KNMI) is an independent algorithm and differs from the latest implementation of Joiner et al, AMT, 2013 (SiF NASA, v26), because we used desert reference areas for determining PCs (as opposed to cloudy ocean and some desert) and a wider fit window that covers water vapour and oxygen absorption bands (as opposed to only Fraunhofer lines). As a consequence, more PCs were needed (35 as opposed to 12). The two time series (SiF KNMI and SiF NASA, v26) correlate well (overall R of 0.78) except for tropical rain forests. Sensitivity experiments suggest the strong impact of the water vapour absorption band on retrieved SiF values. Furthermore, we evaluated the SiF time series with Gross Primary Productivity (GPP) derived from twelve flux towers in Australia. Correlations for individual towers range from 0.37 to 0.84. They are particularly high for managed biome types. In the de-seasonalized Australian SiF time series, the break of the Millennium Drought during local summer of 2010/2011 is clearly observed.
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Flow diagram of the GOME-2 SiF retrieval algorithm. One year of fluorescence-free reference pixels is selected from the GOME-2 Level-1b data as input for the Principal Component (PC) algorithm. The 35 most significant PCs are used to build a cost function and minimization of the cost function for each pixel yields 41 fit parameters (one fluorescence parameter, five surface albedo polynomial coefficients and 35 PC coefficients). The peak fluorescence at 737 nm is then averaged monthly in 0.5° by 0.5° latitude-longitude grid cells resulting in the final Level-3 product.
Global map of monthly mean SiF values at 737 nm for June 2013.