Comparison of cloudless sky parameterizations of solar irradiance at various Spanish midlatitude locations

We have developed a comparison among several cloudless sky parameterization schemes of the solar irradiance incoming at surface level. The data set was recorded at various Spanish mid-latitude radiometric stations, covering different climatic regimes. It consists of hourly values of the relevant quantities covering periods greater than one year at each radiometric station. Diffuse irradiance data have been corrected for shadowband effect using a model developed by the authors and successfully tested at various radiometric stations. At a first stage, the models were run using ail the available local information. After that, we have run the models that present the better performance and, considering that under some circumstances not all the input parameters are available replacing some parameters by their monthly values. As the study reveals, the Gueymard and the Iqbal C model, which require the availability of appropriate information on aerosols, perform best. The influence of precipitable water is of second order thus allowing for the use of estimates based on monthly mean values obtained from climatological records. On the other hand, the study performed on the influence of solar elevation angle on models performance reveals that the worst results correspond to lower solar elevations. This could be a result of limitations in the transmittance parameterization for these elevations.