Consistency tests applied to the measurement of total, direct, and diffuseshortwave radiation at the surface

Motivated by recent studies suggesting that the clear-sky atmosphere absorb s more shortwave (solar) radiation than the theoretical models, we have per formed two consistency tests on the data used in several of these studies. These data consist of broadband measurements of shortwave irradiance to the surface (total, direct, and diffuse) taken in Oklahoma. In the absence of aerosols, Rayleigh scattering is the sole source of diffuse radiation and t hus without any unknown source of atmospheric SW absorption, the measured d iffuse irradiance should not be less than that produced by a model incorpor ating both Rayleigh scattering and conventional atmospheric absorption. The measurements of broadband diffuse irradiance, however, exhibit considerabl e sub-Rayleigh behavior. On the other hand, measurements of the diffuse irr adiance in narrow spectral bands, centered at 415, 500, and 608 nm, indicat e no sub-Rayleigh behavior, suggesting that exhibited by the broadband meas urements is probably unrealistic. Related to this is the finding that the t otal surface SW irradiance, when evaluated as the sum of the direct-beam ir radiance (pyrheliometer) and the diffuse irradiance (shaded pyranometer), d iffers considerably and diurnally from the single measurement of the total irradiance by the unshaded pyranometer under conditions in which possible c osine response errors of the unshaded pyranometer have been minimized. This indicates that the pyranometer daytime offsets differ from each other, sug gesting their daytime offsets likewise differ from their nighttime offsets, which are nearly identical. We emphasize that these conclusions apply sole ly to the data for Oklahoma, and they are focused upon obtaining a better u nderstanding of the clear-sky absorption problem that analyses of these dat a have raised.