Our project is focused on refining a new semi-analytic algorithm (SAA) developed from a theoretical radiative transfer equation and prepared for hyperspectral inversion under the PACE Science Team 2. This algorithm is called ZTT, which notes the developmental history of the algorithm (Z for Zaneveld 1995, and TT for Twardowski and Tonizzo, 2018). The ZTT form is different from the eloquent expression relating remote sensing reflectance to inherent optical properties (IOPs) introduced by Gordon et al. (1988), which has been a mainstay in inversion schemes for the last several decades. An advantage of ZTT over the Gordon form is the native incorporation of the scattering phase function, which allows for a determination of the complete backward directional light field. Currently, this is a separate calculation in NASA's SeaDAS processing system for SAA models using the Gordon form.
There are several aspects of the current ZTT algorithm that can be upgraded leading to improved performance for IOP retrievals. ZTT has numerous parameters that were originally developed from abstract theoretical optical descriptors, as constants or functions of measurable quantities with constrained ranges. In practice, these parameters have spectral variability. Our proposed refinement of ZTT will include re-development of these parameters with new computational techniques and expected availability of hyperspectral optical data sets. Since the ZTT algorithm has explicit use of phase functions for scattering, polarimetry data from PACE can be directly utilized for comparative analysis, and serve as a check to both ZTT and polarimeter products. Ultimately, these data will aid in the refinement activities, with potential for new products related to polarized states of the upwelling light field. We will explore this as a proposed activity, synergizing the utilization of PACE data from the Ocean Color Imager (OCI) and polarimetry data from HARP2 and SPEXone.
The broad objectives of the proposed work are:
- To support integration and evaluation of the ZTT bio-optical inversion algorithm for retrieval of optical properties of optically active water constituents with hyperspectral ocean color data from PACE.
- To refine the ZTT algorithm through improved parameters.
- To evaluate new test products from ZTT that take unique advantage of PACE OCI and polarimetry data.
