A unified radiative transfer simulator for hyperspectral radiometers and polarimeters

NASA's Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission will carry a hyperspectral imaging radiometer (ocean color instrument, hereafter referred to as OCI), and two Multi-Angle Polarimeters (MAP), i.e., the Hyper-Angular Rainbow Polarimeter 2 (HARP2) and Spectro-Polarimeter for Planetary Exploration 1 (SPEXone). These instruments will provide a wealth of co-located data from the hyperspectral imager and multiangle and multi-wavelength polarimeters, which will enable the earth science research and applications community to address new science questions and develop new applications in areas such as aerosol radiative forcing, oil spill monitoring over ocean and in-land waters, harmful algal blooms, and ocean biogeochemistry. To fully maximize the value of the PACE data, there is a need to develop a radiative transfer forward model that can properly model the physical processes of light and matter interaction. In this paper we present a radiative transfer simulator for the PACE instruments that is able to emulate the synthetic data for OCI, HARP2, and SPEXone in a unified theoretical framework. The core of the PACE simulator is based on a monochromatic radiative transfer model for atmosphere-ocean or atmosphere-land systems which is based on the successive order of scattering (RTSOS) method. The gas absorption by H2O, CO2, O2, CH4, O3, and NO2 is accounted for by an absorption cross section look-up table based on high-resolution transmission molecular absorption database (HITRAN). Inelastic scattering in ocean waters is modeled properly, including Raman scattering by pure ocean waters, fluorescence by colored dissolved organic matter, and fluorescence by chlorophyll. The spherical shell effects of the Earth's atmosphere are taken into account through an improved pseudospherical shell (IPSS) algorithm, which is both efficient and accurate (<2%) for large solar and viewing zenith angles. We will describe the theoretical framework of the simulator and present a number of applications, including the development of a new model for instantaneous photosynthetically available radiation (IPAR), analysis of fluorescence line-height, and a sensitivity study of ocean water absorption in the ultraviolet.