A new simple concept for ocean colour remote sensing using parallel polarisation radiance

Abstract

Ocean colour remote sensing has supported research on subjects ranging from marine ecosystems to climate change for almost 35 years. However, as the framework for ocean colour remote sensing is based on the radiation intensity at the top-of-atmosphere (TOA), the polarisation of the radiation, which contains additional information on atmospheric and water optical properties, has largely been neglected. In this study, we propose a new simple concept to ocean colour remote sensing that uses parallel polarisation radiance (PPR) instead of the traditional radiation intensity. We use vector radiative transfer simulation and polarimetric satellite sensing data to demonstrate that using PPR has two significant advantages in that it effectively diminishes the sun glint contamination and enhances the ocean colour signal at the TOA. This concept may open new doors for ocean colour remote sensing. We suggest that the next generation of ocean colour sensors should measure PPR to enhance observational capability.

Figure 1. The upward total radiance and PPR at the TOA simulated by PCOART.

The radiances were observed in the specular plane corresponding to the solar incident direction with units of mW/(cm²·μm·sr). The solar zenith angles were taken from 0° to 70° with a step of 1°. It should be noted that the discrete peaks (sun glint) were caused by the discrete solar zenith angles with a step of 1°, and the actual sun glint should be a continuous curve. (a) The total radiance at 443 nm, (b) PPR at 443 nm, (c) total radiance at 670 nm, (d) PPR at 670 nm.

Figure 2. The reflectance of the upward total radiance and PPR at 670 nm measured by POLDER at the 12th observing angles on 10 July 2003.

The reflectance is defined as R = πL/F₀ cos(θ₀), where L, F₀ and θ₀ are the total radiance (or PPR), extra-terrestrial solar irradiance and solar zenith angle, respectively. (a) Reflectance of total radiance, (b) reflectance of PPR, (c) reflectance of total radiance with solar zenith angle larger than 35°. The maps were generated by the Microsoft Visual C++ 6.0 and Adobe Photoshop CS softwares.

Figure 3. Comparison of the normalised ocean colour signal for the total radiance (solid lines) and the PPR (points) at the TOA for different Chla concentrations.

RAA is the relative azimuth angle between the solar incident and sensor viewing angles. (a) Comparison for 0° solar zenith angle (same for different azimuth angles), (b) comparison for 30° solar zenith angle, (c) comparison for 60° solar zenith angle.

Figure 4. Same as Fig. 3, but for different Tsm concentrations.

Figure 5. The percentage of the contribution of Q_w to I_p at the TOA for different levels of Chla and Tsm.

RAA is the relative azimuth angle between the solar incident and sensor viewing angles. (a) and (b) are the results for 30° solar zenith angle; (c) and (d) are the results for 60° solar zenith angle.

Figure 6. Comparison of the normalised water-leaving radiance (Lwn) retrieved by the total radiance and the PPR from POLDER data on 10 July 2003 under the 12th observing angle.

The red areas are the regions masked by sun glint. (a) Lwn at 443 nm retrieved by the total radiance, (b) Lwn at 443 nm retrieved by the PPR, (c) Lwn at 670 nm retrieved by the total radiance, (d) Lwn at 670 nm retrieved by the PPR. The maps were generated by the Microsoft Visual C++ 6.0 and Adobe Photoshop CS softwares.

Figure 7. Comparison of the normalised water-leaving radiance (unit of mW/(cm²·μm·sr)) retrieved by the total radiance and the PPR at the TOA from the 12 observing angles of the POLDER data on 10 July 2003.

The colour bars show the point densities. (a) Comparison at the 443 nm band, (b) comparison at the 670 nm band.