Comparison of Model Estimated and Measured Direct-Normal Solar Irradiance. Halthore R. N., Schwartz, S. E., Michalsky, J. J., Anderson, G. P., Ferrare R. A., Holben B. N., and ten Brink H. M. J. Geophys. Res. 102, 29991-30002 (1997).

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 sunlight 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, wherein measured atmospheric composition and aerosol optical thickness were input to a radiative transfer model, MODTRAN-3, to estimate DNSI, which is then compared with measured values obtained with a normal incidence pyrheliometer. For 36 independent comparisons, the agreement between measured and model estimated values falls within the combined uncertainties in the measurement (0.2%) and model calculation (2%), albeit with a slight average model overestimate, (0.72 0.81)%; for a DNSI of 839 W m-2, this corresponds to 6.0 6.8 W m-2. The agreement is nearly independent of airmass and water-vapor path abundance. These results thus establish the accuracy of the current knowledge of the solar spectrum and atmospheric extinction as represented in MODTRAN-3. An important consequence is that atmospheric absorption of short-wave radiation is accurately parametrized in the model to within the above uncertainties.

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