Simplified physically based retrieval of sun-induced chlorophyll fluorescence from GOSAT data

Abstract

First global retrievals of sun-induced chlorophyll fluorescence (F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> ) have been achieved in the last years by using data from the Fourier transform spectrometer (FTS) onboard the Japanese Greenhouse Gases Observing Satellite (GOSAT). The high spectral resolution (approximately 0.025 nm) of the FTS enables measurements of F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> by the evaluation of the in-filling of solar Fraunhofer lines around 755 nm. This study presents a new F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> retrieval algorithm (GARLiC, for GOSAT retrieval of chlorophyll fluorescence) and compares its results to F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> from two previously implemented approaches. GARLiC is intended to simplify some of the assumptions of existing retrieval approaches without a loss in retrieval accuracy. We show that GARLiC F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> retrievals are comparable to the previously used methods. We also assess the effect of clouds on F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> retrieval from GOSAT data through the analysis of the effect of different cloud filter thresholds on F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> time series. Our results show a low sensitivity of Fraunhofer-line-based F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> retrievals to cloud contamination.