Retrieving Properties of Thin Clouds from Solar Aureole Measurements

Abstract

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.

Disk and circumsolar radiances in the presence of ice clouds

“…3.4). As expected based on previous research (e.g., DeVore et al, 2009;Segal-Rosenheimer et al, 2013) and the phase functions in Fig. 4, the ice crystal size has systematic effects on radiances in the vicinity of sun.…”

“…The impacts on near-forward scattered radiation due to ice crystal roughness and habit have also been considered (DeVore et al, 2012;Segal-Rosenheimer et al, 2013;Reinhardt et al, 2014). However, the present study extends the knowledge gained in previous research (DeVore et al, 2009(DeVore et al, , 2012Segal-Rosenheimer et al, 2013;Wilbert et al, 2013;Reinhardt, 2013;Reinhardt et al, 2014) as we simulate in detail the angular distribution of phase function and radiances instead of circumsolar irradiances integrated over some angular range (as in Reinhardt et al, 2014). The potential applications of these results include remote sensing of optical thickness, ice cloud properties based on the radiance field near the sun, and the design of concentrating solar energy applications.…”

“…SAM measures the disk and circumsolar radiances with a very high dynamic range and produces the disk and aureole radiances as a function of angle from the center of the sun out to 8 • with an angular resolution of 0.0148 • . DeVore et al (2009) demonstrated the ability of SAM measurements to derive the effective radius and optical thickness of ice clouds and DeVore et al (2012) used MODIS retrievals of thin cirrus to calculate solar disk and aureole measurements that were compared with SAM measurements. Reinhardt et al (2014) have also developed a method to determine circumsolar radiation from satellite observations.…”

Solar Resource for Space and Terrestrial Applications

“…It is hoped that the needed refi nements will result from the deployment of a recently introduced instrument, SAM [56] . A typical result of SAM measurements appears in Figure 19.9 , showing radiance profi les at 670 nm under four different sky conditions, characterized by their optical depth 11 between 0.046 and 0.578 (at 670 nm).…”

The Truncated Geometric Approximation and the Size Distribution of Small Atmospheric Particles

“…Recognizing the key role that diffraction plays in forming a solar aureole, DeVore et al (2009) approximated diffraction using a rectangular function, which led to a simple, robust algorithm relating the size distribution of large particles to the shape of the aureole radiance profile. An analogous situation exists with the shape of solar extinction spectra-for example, as seen through aerosol layers.…”