S5POC+I

S5POC: Exploitation of Sentinel-5-P for Ocean Colour Products

The S5POC project within ESA's Sentinel-5p+ Innovation activity developed five novel ocean colour products for the TROPOMI instrument on the Sentinel-5 Precursor satellite. S5POC, lead by Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), was carried out with the University Bremen (UB), FC.ID representing University Lisbon (ULis) and University of Victoria (UVic), from July 2019 to June 2022. Despite TROPOMI’s lower signal-to-noise ratio than OLCI (~500nm about 1000 opposed to >1500), retrievals were successfully developed for test areas in the global ocean, including very dark waters in the subtropical Atlantic. Quality of TROPOMI products is even improved in comparison to SCIAMACHY (Oelker et al. 2022) which mainly can be attributed to the better spatial resolution and coverage of TROPOMI data matching spatial and temporal dynamics in the ocean.

TROPOMI algorithm to retrieve three spectral (UVAB, UVA, short- blue) diffuse attenuation of underwater light (K_d) products in the open ocean was developed (Oelker et al. 2022) following Dinter et al. (2015). Based on Differential Optical Absorption Spectros-copy the vibrational Raman scattering (VRS) is retrieved and then combined with coupled ocean-atmosphere RTM to derive K_d in the corresponding excitation range. TROPOMI spectral K_d are the first direct satellite observations of UV light in the ocean, while common ocean colour sensors observe the backscattering in the VIS and only for 490 nm operational products are available. VRS signals are well detected in TROPOMI data (fit errors  <15%), K_d retrievals exhibit low sensitivity to parametrization of oceanic and atmospheric effects and show good agreement to K_d obtained from in-situ underwater light spectra and similar OLCI and OC-CCI KD490 products. The gap-filling TROPOMI K_d products meet user needs for global climate and biogeochemical modelling which require spectral information on shortwave light penetration for improving estimates of the ocean’s heat budget, primary productivity, photochemical reaction rates of climatically important compounds, and the UV dose rates as an indicator for damaging effects on aquatic organisms.

Limited by TROPOMI’s wavelength coverage, only CHL of PFTs, diatoms and prokaryotes could be retrieved following Bracher et al. (2009). Although these PFTs do not cover all major groups of the world’s ocean and since 2020 there are operational global and regional CMEMS PFT CHL products, covering the whole phytoplankton community, TROPOMI PFT products are gap filling because of lower uncertainties, even in coastal waters and should be used in synergy with global OLCI-PFT products (Xi et al. 2021), to enable improvements to the data quality of OLCI products and downscaling TROPOMI products to 300 m. High quality satellite PFT data are key for all processes in the ocean considering biogeochemical fluxes and food web dynamics and its exchange with atmosphere, thereby key for assessing ocean’s feedback to climate change and ocean economic and protection management.

Next steps are the implementation of TROPOMI K_d and PFT algorithms for operational global processing, including pixel per pixel uncertainties, production of global time series for the entire S5P’s life time and thorough global validation also including in-situ multi-platform automated data sets. Then, the generic algorithms shall be adapted for Copernicus S4 and S5 sensors, combined or used in synergy with multispectral satellite (e.g. OLCI) and multiplatform in-situ hyperspectral data to enhance the temporal and spatial sampling, secure the longterm data set and fill the gaps for assessing the spectral underwater radiation and four dimensional tracking of phytoplankton groups in the global ocean.

Project duration: July 2019 to June 2022.

Funding: ESRIN/ESA within the Sentinel-5-P+Innovation project.

Project Partners:

Institute of Environmental Physics (IUP), University of Bremen

FCiências.ID - Associação para a Investigação e Desenvolvimento de Ciência (FC.ID)

University of Victoria (UVic)

References:

Bracher A. et al. (2009) Quantitative observation of cyanobacteria and diatoms from space using PhytoDOAS on SCIAMACHY data. Biogeosciences 6: 751-764.

Dinter T. et al. (2015) Retrieval of light availability in ocean waters utilizing signatures of vibrational Raman scattering in hyper-spectral satellite measurements. Ocean Science, 11: 373-389.

Oelker J., Losa, S. N., Richter A., Bracher A. (2022) TROPOMI-retrieved underwater light attenuation in three spectral regions in the ultraviolet to blue. Frontiers in Marine Science 9: 787992. doi: 10.3389/fmars.2022.787992.

Xi H., Losa S.N., Mangin A., Garnesson P., Bretagnon M., Demaria J., Soppa M.A., d’Andon O.H.F., Bracher A. (2021) Global chlorophyll a concentrations of phytoplankton functional types with detailed uncertainty assessment using multi-sensor ocean color and sea surface temperature satellite products. Journal Geoph. Res.-Oceans 126: e2020JC017127, doi: 10.1029/2020JC017127