Optimal Measurement of Surface Shortwave Irradiance Using Current Instrumentation--The ARM Experience

Sortwave (solar) measurements of surface irradiance for clear sky conditions disagree with a number of different models. Betts et al (1993) used the European Center for Medium-range Forecasts (ECMWF) shortwave model to calculate surface irradiances that were 5-10% higher than measurements. Wild et al. (1995) used a different formulation of the ECMWF shortwave model, but found that the modeloverpredicted clear-sky shortwave an average of 3%. Ding and Wang (1996) used data from the Atmospheric Radiation Measurement (ARM) program and found that the GENESIS GCM shortwave model, likewise, overpredicted clear-sky irradiance by about 4%. Kato et al. (1996) compared four observations of total horizontal irradiance made at the Southern Great Plains (SGP) ARM site near Lamont, Oklahoma,with model calculations using the delta two-stream model of Toon et al. (1989). Generally, the models overpredicted surface shortwave irradiance for clear skies except in the unrealistic case of highly absorbing soot aerosol loading. However, the measurements disagree among themselves by as much as the higher of the measurements disagree with realistic aerosol models. To help resolve themeasurement dilemma, reference instruments were deployed in 1996 April at the SGP ARM site central facility very near the standard shortwave measurements. The rest of the paper describes the experiment undertaken to ascertain total horizontal shortwave irradiance at the surface, including, a separation of the direct normal and diffuse horizontal components. Results and a discussion of sameconcludes the paper.