Modeled and measured surface insolations are compared with the purpose of evaluating the ability of a radiative transfer model to predict the amount of solar radiation reaching the surface under clear-sky conditions. Model uncertainties are estimated by performing sensitivity studies for variations in aerosol optical depth, aerosol optical properties, water vapor profiles, ozone content, solar irradiance at the top of the atmosphere, and surface albedo. In this fashion, a range of possible calculated values is determined and compared to observations. Experimental errors are evaluated by comparison with independent, simultaneous measurements performed using two World Radiation Reference instrument arrays that were operational for a limited period during SUCCESS. Assuming a mineral aerosol, it is found that there is agreement between calculated and measured fluxes, with differences approximately equal to and within one standard deviation. Such agreement improves further if a layer containing a small amount of carbonaceous aerosol is added. The presence of carbonaceous aerosol is likely because occasional biomass burning activities took place during SUCCESS in the area around the experimental site (the clouds and radiation test bed operated by the Department of Energy in Oklahoma).

We have extended the interpretations made in two prior studies of the aircraft shortwave radiation measurements that were obtained as part of the Atmospheric Radiation Measurements (ARM) Enhanced Shortwave Experiments (ARESE). These extended interpretations use the 500 nm (10 nm bandwidth) measurements to minimize sampling errors in the broadband measurements. It is indicated that the clouds present during this experiment absorb more shortwave radiation than predicted by clear skies and thus by theoretical models, that at least some (* 20wavelengths less than 680 nm, and that the observed cloud absorption does not appear to be an artifact of sampling errors nor of instrument calibration errors.

We describe a comprehensive set of surface radiative flux measurements made during the Fall 1995 ARM Enhanced Shortwave Experiment (ARESE) at a variety of locations in northern Oklahoma. Accurate measurements of solar spectral broadband (0.224 to 3.91 micron and 0.68 to 3.3 micron) and narrow spectral bandpass (10 nm) downwelling irradiances were made over atmospheric conditions ranging from "pristine" clear to heavy overcast. Precise instrument calibrations are used during analyses in deriving absolute downwelling fluxes accurate to about 1% for the broadband and 3% for the spectral instruments. Measured instrument angular responses are incorporated in these analyses to minimize deviations from ideal cosine responses. All calibrated irradiance measurements are available through the ARM ARESE archive via anonymous FTP at ftp.arm.gov and on the World Wide Web at www.arm.gov.

A variety of broadband and spectral surface irradiance measurements from three instrument platforms show significant agreement with one another during three clear sky days (October 11, 15, and 18, 1995) during the ARM Enhanced Shortwave Experiment (ARESE). This agreement is only possible when carefully considering absolute and angular calibration issues and applying the recommended correction procedures to the raw signals. By removing systematic measurement errors associated with the various instruments, it is possible to achieve consistencies in the various broadband data sets good to about 1-2% accuracy. It is through these careful comparisons that the measurement techniques currently used in determining downwelling surface flux radiances reflect the highest degree of accuracy and certainty.