AMT4CO2Flux scientists have made a significant breakthrough in the field of ocean colour remote sensing with the development of a revised spectral optimisation approach to accurately estimate remote-sensing reflectance (Rrs) using the above-water method (AWA).
This innovative approach, detailed in a recently published scientific paper, allows for the removal of surface-reflected radiance, enhancing the precision and reliability of Rrs measurements.
The determination of Rrs is vital for satellite-based ocean colour measurements which provide valuable information about the composition and health of marine ecosystems. While various in field methods exist for measuring Rrs, the AWA has gained popularity due to its ease of implementation and suitability for use in a variety of water types and condition.
The research team conducted radiative simulations and field measurements, which revealed that the effective sea-surface skylight reflectance (ρ) exhibits spectral variability. To address this, they developed a novel wavelength-dependent model for sea-surface skylight reflectance (ρ,) enabling the removal of surface-reflected radiance. The model was then integrated into a spectral optimisation approach for the determination of Rrs, resulting in exceptional agreement between in situ measurements of Rrs using the AWA-derived and skylight-blocked approach (SBA).
Impressively, the coefficient of determination between the two approaches exceeded 0.92, indicating a strong correlation. These results were also achieved in challenging conditions with high wind and wave activity, showcasing the robustness and versatility of the revised approach.
Importantly, the researchers demonstrated the effectiveness of the optimisation approach and the new ρ model across various water types, expanding its applicability for different scenarios. They emphasised the potential of this method for improving the quality of current and historical above-water in situ measurements of Rrs.
The implications of this research are far-reaching, as it opens new avenues for enhancing ocean colour data quality, satellite algorithm development, and satellite product validation. The revised approach will improve remote-sensing reflectance measurements and ultimately ocean colour satellite imagery which is an essential climate variable. These data are important for evaluating the health and state of the marine environment and contribute to our understanding of the Earth's oceans.
One noteworthy advantage of this approach is its potential to salvage previously rendered unusable spectra when processed with traditional schemes. This breakthrough opens up a wealth of possibilities for reanalysing and utilising vast amounts of data that were once considered unreliable or discarded due to limitations of existing methodologies.
As scientists continue to refine and apply this approach to ocean colour research, we can anticipate more accurate and comprehensive assessments of our marine ecosystems, facilitating informed decision-making for sustainable ocean management and conservation efforts.
Gavin Tilstone, co-author and Principal Investigator of the AMT4CO2Flux project said:
Current approaches that account for sea-surface skylight reflectance in remote sensing reflectance measurements assume that they are spectrally constant at specific viewing directions. This is not the case and so this new method forges a new method to account for this. Application of the method will improve remote sensing reflectance measurements at sea, reduce the uncertainties associated with them and will potentially improve the quality of ocean colour satellite imagery. This will ultimately benefit ocean colour research, that has a wide range of applications from the assessment of climate change trends, monitoring ocean health and the cycling of carbon from the atmosphere to phytoplankton.
Lin, J., Z. Lee, G.H. Tilstone, X.W. Liu, J., M. Ondrusek, and S. Groom. 2023. Revised spectral optimization approach to remove surface-reflected radiance for the estimation of remote-sensing reflectance from the above-water method. Optics Express 31 22964-22981. doi:10.1364/OE.486981