Measurement and Modeling of Shortwave Irradiance Components in Cloud-Free Atmospheres

Atmosphere scatters and absorbs incident solar radiation modifying its spectral content and decreasing its intensity at the surface. It is very useful to classify the earth-atmospheric solar radiation into several components direct solar surface irradiance (Edirect), diffuse-sky downward surface irradiance (Ediffuse), total surface irradiance, and upwelling flux at the surface and at the topof-the atmosphere. Edirect depends only on the extinction properties of the atmosphere without regard to details of extinction, namely scattering or absorption; furthermore it can be accurately measured to high accuracy (0.3%) with the aid of an active cavity radiometer (ACR). Ediffuse has relatively larger uncertainties both in its measurement using shaded pyranometers and in model estimates, owing to the difficulty in accurately characterizing pyranometers and in measuring model inputs such as surface reflectance, aerosol single scattering albedo, and phase function. Radiative transfer model simulations of the above surface radiation components in cloud-free skies using measured atmospheric properties show that while Edirect estimates are closer to measurements, Ediffuse is overestimated by an amount larger than the combined uncertainties in model inputs and measurements, illustrating a fundamental gap in our understanding of the magnitude of atmospheric absorption in cloud-free skies. The excess continuum type absorption required to reduce the Ediffuse model overestimate (~3-8% absorptance) would significantly impact climate prediction and remote sensing. It is not clear at present what the source for this continuum absorption is. Here issues related to measurements and modeling of the surface irradiance components are discussed.