Observation of global, seasonal cycle of regional-scale chlorophyll fluorescence from space using GOSAT

The measurement of solar-induced chlorophyll fluorescence from space is challenging because its signal (typically 1-5% in the near-IR) must be differentiated from the much larger reflectance effect. Fluorescence has been detected from ground- and airborne-based instrumentation by exploiting the fact that it is a proportionally larger fraction of the total radiance within dark lines and bands of the atmospheric spectrum. These dark features include solar Fraunhofer lines or oxygen bands such as the oxygen-B at 687 nm and oxygen-A near 760 nm. The oxygen bands, primarily the A band, have been used more extensively with with recent ground-, aircraft-, and space-based spectrometers that have spectral resolutions of tenths of a nanometer and larger (individual lines not resolved), because these bands appear wider and deeper and align more closely with the peaks of fluorescence. Here, instead of using oxygen-A band absorption, we make use of unique high spectral resolution measurements from the Japanese Greenhouse gases Observing SATellite (GOSAT). With GOSAT, we observe filling- in of the potassium (K) I solar Fraunhofer line near 770 nm. Though not designed for this purpose, we demonstrate that GOSAT may be used to retrieve fluorescence at a regional scale. We show the first (to our knowledge) global maps of fluorescence derived from GOSAT for July and December of 2009 (see below). We also compare the derived fluorescence with the enhanced vegetation index (EVI) from MODIS for several different geographical regions.