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. The aim of this work is to describe the features of and tovalidate a simple, fast, accurate, and physically based spectral radiativetransfer model in the solar wavelength range under clear skies. The model,named SSolar-GOA (the first “S” stands for “spectral”), was developed toevaluate the instantaneous values of spectral solar irradiances at ground level orat a given altitude of the atmosphere. The model requirements are designed based onthe simplicity of the analytical expressions for the transmittance functionsin order to be easily replicated and applied by a wide community of usersfor many different applications (atmospheric and environmental researchstudies, satellite remote sensing, solar energy, agronomy and forestry, ecology,and others). Although spectral, the model runs quickly and has sufficientaccuracy for the evaluation of solar irradiances with a spectral resolutionof 1–10 nm. The model assumes a single mixed molecule–aerosol scatteringlayer where the original Ambartsumian method of “adding layers” in aone-dimensional medium is applied, obtaining a parameterized expression forthe total transmittance of scattering. Absorption by the differentatmospheric gases follows “band model” parameterized expressions. Theinput parameters must be realistic and easily available since the spectralaerosol optical depth (AOD) is the main driver of the model. The validationof the SSolar-GOA model has been carried out through comparison withsimulated irradiance data from the libRadtran package and with direct and globalspectra measured by spectroradiometers. Thousands of spectra under clearskies have been compared for different atmospheric conditions and solarzenith angles (SZA). The SSolar-GOA is validated by a quantitativecomparison with libRadtran, showing that it underestimates direct normal,global, and diffuse spectral components with relative differences of+1 % (RMSE % = 4.6–8), +3 % (RMSE % = 5.3–8), and 8 %(RMSE % = 9.3–9.6), respectively, when the SZA varies from6 to 60∘. Compared with the measuredirradiance data of the LI-1800 and ASD spectroradiometers, the relativedifferences of direct normal and global components are within the overallexperimental error, about ±2 %–12 % (RMSE % = 5–8.3), withunderestimated or overestimated values. The diffuse component presents thehighest degree of relative difference that can reach ±20 %–30 % andRMSE of 25 %–50 %. The relative differences depend strongly on the spectralsolar region analysed and the SZA, but the high values of RMSE are due tothe artifice generated by the different spectral resolution of theabsorption coefficients of both models. Model approach errors combined withcalibration instrument errors may explain the observed differences. TheSSolar-GOA v1.0 is implemented in Python and open-source licensing.

SSolar-GOA v1.0: a simple, fast, and accurate Spectral SOLAR radiative transfer model for clear skies