Meteorological Radiation Model (MRM v6.1): Improvements in diffuse radiation estimates and a new approach for implementation of cloud products

This work reviews techniques and models for solar radiation simulations and communicates further improvements performed in the Meteorological Radiation Model (MRM) developed at the National Observatory of Athens. The new version (MRM v6.1) considers a forward to-the-ground scattering as a function of solar altitude for the diffuse radiation estimates, while concurrently uses two running codes i.e., with sunshine duration as input (MRM v6.1a) and with cloud products (MRM v6.lb). The new scattering function leads to increase in diffuse radiation, especially for low zenith angles, and to better simulations with the measured diffuse (RMSE=36.3% and MBE=6.5%, against RMSE=42.0% and MBE=22.3% for the latest MRM v6). These changes lead also to better simulations of global radiation (RMSE=8.7% against RMSE=9.5% for MRM v6), while the direct radiation is not affected. The accuracy in the simulations increases significantly for clear-sky conditions, while it shows a small dependence on aerosol amount and solar altitude. Furthermore, in the sub-version MRM v6.1b the calculations for the cloud transmittance have been modified to allow for the inclusion of cloud products as inputs in case of non-availability of sunshine duration data. In this study, MRM v6.lb uses MERRA retrievals of cloud optical depth and cloud fraction for calculations of the cloud transmittance; these parameters usually lead to significant uncertainties in the simulations of the hourly direct and global radiations, especially for large cloud fractions. This indicates the need for high spatial and temporal resolution satellite data of cloudiness for accurate estimations of solar radiation under cloudy conditions and highlights the incapability of radiative transfer models on such simulations. However, on monthly basis both MRM v6.1a and v6.1b provide high accuracy in solar radiation estimates, thus rendering MRM v6.1 as a powerful tool for solar energy applications.