This paper describes a newly designed Sun and Aureole Measurement (SAM) aureolegraph and the first results obtained with this instrument. SAM measurements of solar aureoles produced by cirrus and cumulus clouds were taken at the Atmospheric Radiation Measurement Program (ARM) Central Facility in Oklahoma during field experiments conducted in June 2007 and compared with simultaneous measurements from a variety of other ground-based instruments. A theoretical relationship between the slope of the aureole profile and the size distribution of spherical cloud particles is based on approximating scattering as due solely to diffraction, which in turn is approximated using a rectangle function. When the particle size distribution is expressed as a power-law function of radius, the aureole radiance as a function of angle from the center of the solar disk also follows a power law, with the sum of the two powers being 25. This result also holds if diffraction is modeled with an Airy function. The diffraction approximation is applied to SAM measurements with optical depths &2 to derive the effective radii of cloud particles and particle size distributions between ;2.5 and ;25 mm. The SAM results yielded information on cloud properties complementary to that obtained with ARM Central Facility instrumentation. A network of automated SAM units [similar to the Aerosol Robotic Network (AERONET) system] would provide a practical means to gain fundamental new information on the global statistical properties of thin (optical depth & 10) clouds, thereby providing unique information on the effects of such clouds upon the earth's energy budget.

Scattering calculations for optically thin cirrus clouds based on retrievals of optical thickness and effective particle size from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua are compared with solar disk radiance and aureole measurements from Visidyne's Sun and Aureole Measurement (SAM) instrument. In the four examples presented, the calculated solar disk radiance differs from the measurement by a factor ranging from 2 to 8. The shape of the aureole radiance profile also differs and indicates that either the effective particle size retrieved by the MODIS daytime cirrus algorithm is too small or there is another problem with the phase functions underlying the MODIS algorithm.

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Two methods of determining instrumental scattering for correcting aureolegraph measurements of particulate solar scattering are presented. One involves subtracting measurements made with and without an external occluding ball and the other is a modification of the Langley Plot method and involves extrapolating aureolegraph measurements collected through a large range of solar zenith angles. Examples of internal scattering correction determinations using the latter method show similar power-law dependencies on scattering, but vary by roughly a factor of 8 and suggest that changing aerosol conditions during the determinations render this method problematic. Examples of corrections of scattering profiles using the former method are presented for a range of atmospheric particulate layers from aerosols to cumulus and cirrus clouds.