A preliminary assessment of a detailed two stream short-wave narrow-band model using spectral radiation measurements

A data bank of measurements of global, direct and diffuse solar spectral irradiances at ground level for clear skies has been compiled for Valencia (Spain) dating back to December 1992. The measurements were made with a commercial Li-cor 1800 spectroradiometer with a range of 300-1100 nm and a spectral resolution of 6 nm. A preliminary comparative assessment has been carried out between the experimental data and model data. The chosen model was a detailed narrow-band model (208 spectral intervals from 0.2 to 4 mu m) developed at the ''Laboratoire d'Optique Atmospherique (LOA)'' of the University of Lille (France). This plane-parallel multilayer model uses a two-flux method to solve the radiative transfer equation and an exponential sum-fitting procedure to solve the absorption-scattering problem for finite spectral intervals. For this first comparative assessment we focused our attention on the capability of the LOA model to predict irradiance data (direct, global and diffuse) using four values of visibility (40, 23, 14 and 5 km) for two aerosol models (maritime and continental) in the boundary layer. These first results show the low sensitivity of global irradiance to different turbidity conditions. Conversely, the spectral direct and diffuse irradiances were highly influenced by the chosen aerosol model taking into account the visibility values. The spectral distribution of predicted global and direct irradiances are in relatively good agreement with the observed values. The diffuse data show larger discrepancies, which are in part due to the nature of the measurement process itself. However, the observed differences can be partially explained by taking into account the associated errors of the measured data, the elapsed time between the measured spectra and the prediction power of the LOA model. (C) 1997 Elsevier Science Ltd.