New Measurements of Chlorophyll Fluorescence with GOME-2 and Comparisons with the Seasonal Cycle of GPP from Flux Towers

Abstract

GOSAT satellite fluorescence measurements, based on Fraunhofer line filling with lines free of atmospheric absorption, provide near global coverage on a monthly basis, but suffer from low precision and sparse spatial sampling. We describe a new method to retrieve global far-red fluorescence; we use hyper-spectral data with a simplified radiative transfer model to disentangle the spectral signatures of atmospheric absorption, surface reflectance, and fluorescence radiance. An empirically-based principal component analysis is employed to model and solve for the atmospheric absorption. The method is applied to data from the Global Ozone Monitoring Instrument 2 (GOME- 2). While not optimal for fluorescence measurements, GOME-2's high sampling and signal-to-noise ratio enable state-of-the-art fluorescence retrievals in the far-red chlorophyll emission feature. Near- global coverage is provided within a few days. We map far-red terrestrial fluorescence at higher spatio-temporal resolutions than previously published GOSAT data. This shows clearly for the first time a northward shift in fluorescence within the single month of May as the sun shifts northward during the boreal spring. We compare the GOME-2 fluorescence directly with a diverse set of North American tower-based GPP estimates. For several important vegetation types, the GOME-2 signal captures the spring onset and autumn shutoff of photosynthesis. In contrast, GPP from models that rely on satellite reflectance-based vegetation parameters overestimate the length of the growing season for some biomes. Satellite fluorescence measurements therefore show potential for improving seasonal aspects of model-based GPP estimates. Over 5 years of level 2 GOME-2 data are available at http://acdbext.gsfc.nasa.gov/People/Joiner/my_gifs/GOME_F/GOMEF.htm.