Abstract. The aim of this work is to describe the features and to validate a simple, fast, accurate and physical-based spectral radiative transfer model in the solar wavelength range under clear skies. The model, named SSolar-GOA (the first “S” stands for “Spectral”), was developed to evaluate the instantaneous values of spectral solar irradiances at ground level. The model data output are well suited to work at a spectral resolution of 1–10 nm, are adapted to commercial spectroradiometers or filter radiometers, and are addressed to a wide community of users for many different applications (atmospheric and environmental research studies, remote sensing, solar energy, agronomy/forestry, ecology, etc.). The model requirements are designed based on the simplicity of the analytical expressions for the transmittance functions in order to be easily replicated and applied by users. Although spectral, the model runs quickly and has sufficient accuracy. The model assumes a single mixed molecule-aerosol scattering layer where the original Ambartsumian method of “adding layers" in a one-dimensional medium is applied, obtaining a parameterized expression for the total transmittance of scattering. Absorption by the different atmospheric gases follows “band model” parameterized expressions. Both processes are applied to a single atmospheric homogeneous layer as necessary approaches for developing a simple model under the consideration of non-interaction. Besides, the input parameters must be realistic and easily available since the spectral aerosol optical depth (AOD) is the main driver of the model. The validation of the SSolar-GOA model has been carried out through extensive comparison with simulated irradiance data from the LibRadtran package and with direct/global spectra measured by spectroradiometers. Thousands of spectra under clear skies have been compared for different atmospheric conditions and solar zenithal angles (SZA). From the results of the comparison with LibRadtran, the SSolar-GOA model shows a high performance for the entire solar spectral range for direct, global, and diffuse spectral components with relative differences of +1 %, +3 %, and 8 %, respectively, and our model always gives an underestimation. Compared with the measured irradiance data of the Licor1800 and ASD spectroradiometers, the relative differences of direct and global components are within the overall experimental error (about ±2–12 %) with underestimated or overestimated values. The diffuse component presents the highest degree of difference which can reach ±20–30 %. Obviously, the relative differences depend strongly on the spectral solar region analysed and the SZA. Model approach errors combined with calibration instrument errors may explain the observed differences.
The Palaeoclimate and Terrestrial Exoplanet Radiative Transfer Model Intercomparison Project (PALAEOTRIP): experimental design and protocols
