Simplified Clear-Sky radiative transfer Model (SCSM)

The code includes relevant functions for clear-sky radiative transfer calculations over ocean under low atmospheric aerosol load.

The clear-sky radiance over ocean in the visible range (here at 807 nm) depends on a narrow set of parameters and can be estimated by simplified 1D radiative transfer calculations. Knowing the extraterrestrial irradiance emitted by the sun and entering the atmosphere, the radiative transfer code describes the radiance at any location (x, y, z) and for any direction defined by zenith and azimuthal angles. In a clear-sky atmosphere with small solar and viewing zenith angles we can use 1D plane-parallel radiative transfer to estimate the radiance observable at the top of atmosphere or from a sensor in space.

The clear-sky radiance reaching a sensor in space is a combination of three main components: (1) the direct sun ray reflected at the ocean surface and (2) the hemispheric diffuse radiance reflected at the surface towards the sensor. On the way from TOA to the surface and back to the sensor the radiation is attenuation following Lambert-Beer and depending on the atmospheric optical thickness and the cosine of the sensor or view zenith angle. In addition, there is component (3), the diffuse light from single-scattering events happening within the atmosphere.

Several modular python functions describe the components and simplified radiative transfer calculations needed to derive the direct and diffuse components of solar radiation reaching the ground, as well as the amount reflected by an ocean surface. How the light is reflected at the surface into the view direction of a sensor is characterized by the bi-directional reflection function which depends on the surface wind speed and the generated ocean wave slope distribution following the Cox and Munk parameterization. For the diffuse downwelling radiance we make use of a look-up table included in this package (LUT_diffuse_radiance.nc). The diffuse upwelling radiance is approximated by integrating over single-scattering events including the Henyey Greenstein approximation for the scattering phase function.

A complete description of the simplified clear-sky model is provided within a paper manuscript intended for submission to Atmospheric Chemistry and Physics (ACP) under Mieslinger et al., 2021.