Three-stream modelling of radiative transfer for the simulation of Black Sea biogeochemistry in a NEMO framework

In this paper, we propose a three-stream ocean radiative transfer module as an extension of the Nucleus for European Modelling of the Ocean (NEMO). This module solves in-water irradiance fields in 1D water columns, discriminating between two downward streams, direct and scattered, and a backscattered upward stream. The module solves 33 wavebands between 250 and 4000 nm, with a resolution of 25 nm in the visible range. The sea surface reflectance is also calculated as a model output, based on the ratio between the upward and downward irradiances at the air-sea interface. We also use a feedback loop towards the computation of temperature in the NEMO model, made optional with this module. It also includes a stochastic version in which the inherent optical properties of the major optically active components of seawater can be perturbed. This mode is meant to account for uncertainty in the modelling of marine optics. This module can be plugged into any NEMO configuration, with the computation of optical properties driven either by a coupled biogeochemical model or directly forced into the radiative transfer module. We apply this module in a test case for the Black Sea, within the NEMO framework and coupled to the Biogeochemical Model for Hypoxic and Benthic Influenced areas (BAMHBI). We find that substituting the existing radiative transfer scheme with our model unlocks the ability to simulate radiometric variables that can be compared more directly to observations, both in situ and from remote-sensing. We also find that using irradiances to compute the temperature and scalar irradiance that is available to phytoplankton in the coupled model maintains consistency in the calculation of physical and biogeochemical variables. The simulation of variables such as temperature or chlorophyll concentration is maintained, while enabling additional capabilities in the model with the simulation of radiometric quantities.