Improving daily output of global to direct solar irradiance models with ground measurements

A comparison of the DirIndex model for computing direct normal solar irradiance from global horizontal using different clear sky transmittance models is presented for four ground stations that belong to Baseline Surface Radiation Network (BSRN) and Aerosol RObotic NETwork (AERONET) networks. The results of DirInt model, which does not include any clear sky transmittance contribution, are also shown. The input for the different clear sky models selected (European Solar Radiation Atlas (ESRA), SOLar Irradiance Scheme (SOLIS) simplified, and Reference Evaluation of Solar Transmittance, 2 bands (REST2)) was generated from the original aerosol optical depth and water vapour measurements provided by AERONET. The results show different trends in the performance of the DirIndex model combined with the clear sky methods. An attempt to correct the trends to the proper one is finally proposed here and the improvement achieved is shown.