Reconstruction of long-term direct solar irradiance data series using a model based on the Cloud Modification Factor

This paper develops a semi-empirical model to calculate direct solar shortwave irradiance on a horizontal surface for any cloudiness condition. The model is based on radiative transfer simulations combined with empirical relationships; it makes use of the global solar irradiance, experimentally measured in most of the radiometric stations, and the Cloud Modification Factor (CMF). A dataset of irradiance from eight Spanish locations is used to tune the model parameters and a part of the data is used to validate the model. By using long-term series of experimental global irradiance data, the corresponding long-term direct irradiances are reconstructed for each location using the proposed model. An evaluation of the model performance is carried out for different solar zenith angles. Following, monthly averages of the hourly data were calculated from the reconstructed values. A high correlation is found when compared with the corresponding measured values. To make use of the obtained results, a characterization of the typical climatic values of the solar direct irradiance in Spain is carried out. The results provide representative values of solar direct radiation and show the availability of this solar resource in the studied areas.