Using scattering calculations to compare MODIS retrievals of thin cirrus optical properties with SAM solar disk and aureole radiance measurements

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.