Diffuse-Sky Downward Irradiance (DFDI) At Surface In Cloud Free Atmospheres - A Closure Experiment.

Halthore R. N. (a), Nemesure S. (a), Schwartz S. E. (a), Imre D. G. (a), Berk A. (b), Dutton E. G. (c), and Bergin M. H. (d) , Boulder CO; (e) CIRES, Boulder CO.

(a) Brookhaven National Laboratory; (b) Spectral Sciences Corp.; (c) NOAA CMDL, Boulder CO; (d) CIRES, Boulder CO

8th Atmospheric Radiation Measurement (ARM) Science Team Meeting, Tucson, AZ, Mar. 23-26, 1998.

We have compared the measurement and calculation of DiFfuse Downward Irradiance (DFDI) at the surface for cloud-free atmospheres at the ARM SGP site and elsewhere. The models (MODTRAN 2 stream and 8 stream, and 6S) consistently over-predict DFDI compared to the measurements for measured or representative values of atmospheric and aerosol optical properties. In a previous study (Halthore et al., 1997) we had shown that the direct-normal solar irradiance was correctly computed in the models. Taken together these findings admit two possible explanations either of which can be invoked to close the gap between model results and measurements: (i) the optical properties of aerosols are highly unusual- specifically the aerosol single scattering albedo much lower than typical and the aerosol back scattering coefficient is much higher normal or (ii) there is an unknown absorption in the atmosphere which has eluded detection until now. Both conclusions have significant implications for shortwave radiative transfer which will be discussed.

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

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