SPARTA: Solar parameterization for the radiative transfer of the cloudless atmosphere

The high-performance SPARTA broadband clear-sky solar irradiance model is presented. It evaluates global (GHI), diffuse (DIF) and direct normal (DNI) irradiances from atmospheric transmittance functions developed using an advanced 2-band hybrid spectral integration scheme, calculates aerosol extinction using a universal and highly accurate aerosol transmittance scheme, and incorporates a versatile parameterization of the aerosol circumsolar solar irradiance. The model's performance is assessed against 1-min quality-assured measurements at three research-class radiometric ground stations spanning 15 years of data combined, and it is benchmarked against three high-performance radiative transfer models. The performance assessment proved that SPARTA was superior to the reference models for GHI, DIF and DNI at the three observational sites combined. SPARTA produced unbiased estimates of the three solar irradiance components (remarkably, it was the only model producing unbiased estimates of DIF) and yielded the smallest standard deviations of the model−observation differences (≈2 %, ≈10 % and ≈2 % for GHI, DIF and DNI, respectively). SPARTA was the only model capable of improving the estimation of DIF when using observed inputs of the aerosol scattering optical properties instead of modelled values. The results of the performance assessment proved that, provided accurate inputs to the model, SPARTA produces predictions with similar uncertainty than ground observations, especially if the ground sensors are not optimaly maintained or they are not A or B ISO-9060 class.