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

Abstract. In this paper, we propose a three-stream ocean radiative transfer (RT) module as an extension of the NEMO ocean modelling framework. This module solves the subsurface irradiance field in 1D water columns, discriminating between two downward streams, direct and scattered, and a backscattered upward stream. The module solves 33 wavebands ranging between 250 and 4000 nm, with a finer 25 nm resolution 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. An optional feedback towards NEMO is presented, enabling the use of irradiances to compute temperature in the hydrodynamics. The module also includes a stochastic version in which the inherent optical properties of the main optically active components of seawater can be perturbed. This mode is meant to account for uncertainty in the modelling of marine optics. This module is can be plugged to any NEMO configuration, with the computation of optical properties either driven by a biogeochemical model or directly forced into the RT module.

We apply this module in a test case for the Black Sea, coupled with the physical-biogeochemical framework NEMO 4.2.0-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 truthfully to observations, both in situ and from remote-sensing. We also find that using irradiances to compute the temperature and PAR in the model maintains consistency in the calculation of physical and biogeochemical variables in the model, such as temperature or chlorophyll concentration, while enabling additional capabilities in the model in the simulation of radiometric quantities.