Exploitation of hyperspectral satellite data for the detection of fluorescence originating from biological sources

Astrid.Bracher [ at ] awi.de


Oxygenic photosynthesis is responsible for virtually all of the biochemical production of organic matter, and regulates atmospheric carbon dioxide and oxygen concentrations, which profoundly affects both climate and biogeochemical cycles. Observations of biomass, health and productivity of ocean and land ecosystems are crucial for monitoring changes in the Earth system. Here, space-borne hyperspectral data, mainly of the SCIAMACHY instrument (onboard ENVISAT), have been investigated in terms of application to globalobservations of marine and terrestrial primary producers. This study focuses primarily on retrieving inelastic processes from the natural waters: CDOM and chlorophyll (chl) a fluorescence. Originally, the chl a fluorescence retrieval was developed in its red peak and for the ocean application only. However, it was further extended to the far-red peak of chl a fluorescence, and subsequently applied to terrestrial scenes. All retrievals are based on the Differential Optical Absorption Spectroscopy (DOAS), and involve fitting spectral features of the filling in of the Fraunhofer lines by fluorescence processes. The reference spectra of chl a and CDOM fluorescence, used in the DOAS fits, were calculated with the ocean-atmosphere coupled radiative transfer model SCIATRAN. Furthermore, a simple algorithm to retrieve a chl proxy of terrestrial vegetation was developed. The retrievals were developed with the use of simulated radiances, and subsequently applied to SCIAMACHY data. Although the original aim of the SCIAMACHY instrument was to monitor atmospheric composition, its unique spectral characteristics (namely broad spectral range from 240 nm to 2380 nm, and high resolution of 0.2 nm to 1.5 nm) have enabled previously other novel retrievals to be developed. These included observations of inelastic processes (rotational and vibrational Raman scattering), and marine phytoplank- ton. In this thesis, the chl a fluorescence and chl proxy retrievals were applied to eight years of the SCIAMACHY data (2004-2011). In addition to presenting yearly composites and monthly climatologies of the obtained results, monthly averages were applied to study the seasonality of both, marine phytoplankton and terrestrial vegetation. Modeling studies of CDOM fluorescence, followed by preliminary retrievals applied to SCIAMACHY data, have not been successful in retrieving CDOM fluorescence from hyperspectral satellite data. On the other hand, the obtained chl a fluorescence results showed good spatial agreement with other datasets. Marine observations of the red peak of chl a fluorescence captured successfully the phytoplankton seasonal cycles and interannual variability for two studied regions: a subregion of the Indian Ocean near Madagascar, and the equatorial Pacific. Good agreement with multispectral ocean color products (MODIS nFLH and MODIS Chl a) was obtained. Response of phytoplankton to climate fluctuations, as expressed by Multivariate ENSO Index, was observed for the equatorial Pacific. In case of land observations, all retrieved parameters (red and far-red chl a fluorescence, and chl proxy) followed the seasonal cycles of vegetation for five regions representing different biomes worldwide (croplands in the North America, evergreen needleaf forest in Euroasia, evergreen broadleaf forest in Central Africa, woody savannas in Central Africa and savannas in Southern Africa). However, the three SCIAMACHY datasets did not show exactly the same seasonal pattern, and their relationship varied over time and among biomes. This proves that the retrieved parameters do not carry the same information on vegetation, and hence suggests that they all should be used simultaneously for observations of vegetation dynamics. The calculated ratio and the difference of the two peaks of chl a fluorescence, followed the increase of chl content and canopy development, which supports previous findings by in situ measurements and models.The red and far-red chl a fluorescence and chl proxy algorithms have enabled simultaneous retrievals of multiple parameters of marine phytoplankton and terrestrial vegetation. While the application of the SCIAMACHY results is still constrained (mainly by the noisiness of the results and spatio-temporal resolution of the satellite measurements), the developed retrievals and their successful application to studies of ocean and land phenology have advanced the prospects of observations of phytoplankton and terrestrial vegetation with hyperspectral satellite instruments.

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Wolanin, A. (2015): Exploitation of hyperspectral satellite data for the detection of fluorescence originating from biological sources , PhD thesis, University Bremen.

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