Comparison of Model Estimated and Measured Direct-Normal Solar Irradiance

Direct-normal solar irradiance (DNSI), the total energy in the solar spectrum incident in unit time on a unit area at the earth's surface perpendicular to the direction to the Sun, depends only on atmospheric extinction of solar energy without regard to the details of the extinction - whether absorption or scattering. Here we report a set of closure experiments performed in north-central Oklahoma in April 1996, under cloud-free conditions, wherein measured atmospheric composition and aerosol optical thickness are input to a radiative transfer model, MODTRAN-3, to estimate DNSI, which is then compared with measured values obtained with normal incidence pyrheliometers and absolute cavity radiometers. Uncertainty in aerosol optical thickness (AOT) dominates the uncertainty in DNSI calculation. AOT measured by an independently calibrated sunphotometer and a rotating Direct-Normal Solar Irradiance - A Closure Experiment, Halthore et al. 2 shadow-band radiometer agree to within the uncerta..