POSTER Session 4

Modeling Underwater Photosynthetically Available Radiation Profiles from Biogeochemical Argo Floats Using Multi-Spectral Irradiance Measurements

Biogeochemical (BGC) Argo floats currently measure downwelling planar irradiance (Ed) in three spectral bands centered on 380, 412, and 490 nm and photosynthetically available radiation (PAR) in the 400-700 nm range. Replacing the PAR band with a 555 nm spectral band is under consideration, with an expectation to accurately reconstruct PAR using other spectral bands. In this study, we propose a General Additive Model (GAM) to estimate PAR at any given depth (z) from just below the surface (z = 0 m) to 100 m using Ed at 380, 412, 490, and 555 nm. The model coefficients are functions of z, chlorophyll concentration (Chl), and particulate backscattering coefficient at 550 nm (bbp550), both of which are also measured by BGC Argo floats. Theoretical simulations conducted under various conditions, including different sun zenith angles and atmospheric, surface, and water conditions, indicate that without uncertainty in the measurements, PAR(z) is estimated with an RMS of 0.237 (4.2%) on average using only depth as an explanatory variable, and 0.087 (1.5%) using depth, Chl, and bbp550. After introducing typical uncertainties in Ed, Chl, and bbp550, the RMS increase to 0.412 (7.4%) and 0.327 (5.8%), respectively, with relatively larger uncertainties at greater depths due to increased light attenuation. Validation of the GAM using various in-situ hyperspectral Ed profiles demonstrates its robustness and ability to reproduce near-surface variability accurately. The proposed model benefits the BGC Argo program by expanding the Ed dataset within the photosynthetically active range and offering accurate PAR estimates across diverse environmental conditions.

Jing Tan, Scripps Institution of Oceanography, University of California San Diego, USA

Robert Frouin, Scripps Institution of Oceanography, University of California San Diego, USA

Edouard Leymarie, Laboratoire d’Océanographie de Villefranche, France

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