Retrieval of the Particle Size Distribution Using Coated Spheres Modeling

Tihomir S Kostadinov, California State University San Marcos, San Marcos, CA, United States, Ekaterina Kochetkova, University of Pennsylvania, Department of Earth and Environmental Science, Philadelphia, PA, United States, Stewart Bernard, Council for Scientific and Industrial Research, Earth Observation Research Group, Cape Town, South Africa, Xiaodong Zhang, University of Southern Mississippi, Marine Science, Stennis Space Center, MS, United States and Lisl Robertson-Lain, University of Cape Town, Dept. Oceanography, Cape Town, South Africa
The particle size distribution (PSD) is a key property of surface ocean waters that links marine ecosystem structure and function to ocean optics. The KSM09 ocean color algorithm (Kostadinov et al., 2009) retrieves the parameters of a power-law PSD from the spectral particulate backscattering coefficient. The PSD can then be used (under certain assumptions) to estimate the size-partitioned bio-volume (Kostadinov et al., 2010) and phytoplankton carbon (Kostadinov et al., 2016); however achieving realistic values for POC that validate well against in-situ measurements globally and are comparable to CMIP5 models requires the use of an empirical correction factor. KSM09 is constructed using homogeneous spheres Mie scattering model. However, the backscattering coefficient is influenced by the complex and heterogeneous composition of phytoplankton cells. Recent studies suggest that a coated sphere Mie scattering model is more suitable for reproducing oceanic backscattering coefficients. Here, we investigate the application of a coated spheres Mie scattering model as an update to the KSM09 PSD algorithm. Models with the absorbing chloroplast being represented by the coat vs. the core of the particle are both tested for realism. The coated spheres look-up tables (LUTs) are compared to the original KSM09 LUTs. The coated spheres model has more free parameters, namely the relative complex indices of refraction of the particle core and coat, coat thickness, and maximum particle diameter for a polydispersion. The uncertainty due to variability in these parameters is assessed via Monte Carlo simulation, as in KSM09. The size range to which the coated sphere model is most sensitive is also investigated. Finally, we discuss a compilation of in-situ data that will be used for validation and/or algorithm development, from the PSD level to POC and phytoplankton carbon.