Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part II: Marine Stratocumulus Observations

Abstract

A multispectral scanning radiometer has been used to obtain measurements of the reflection function of marine stratocumulus clouds at 0.75, 1.65 and 2.16 ?m. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE), conducted off the coast of southern California during July 1987. Multispectral images of the reflection function were used to derived the optical thickness and effective particle radius of stratiform cloud layers on four days. In addition to the radiation measurements, in situ microphysical measurements were obtained from the University of Washington Convair C-131A aircraft. In this paper we compare remote sensing results with in situ observations, which show a good spatial correlation for both optical thicknesses and effective radius. These comparisons further show systematic differences between remote sensing and in situ values, with a tendency for remote sensing to overestimate the effective radius by ?2?3 ?m, independent of particle radius. The optical thickness, in contrast, is somewhat overestimated for small optical thickness and underestimated for large optical thicknesses. An introduction of enhanced gaseous absorption at a wavelength of 2.16 ?m successfully explains some of these observed discrepancies. Marginal probability density functions of optical thickness, liquid water path and effective radius have been derived from our remote sensing results. The joint probability density function of liquid water path and effective radius shows that the effective radius increases as the liquid water path increases for optically thin clouds, in contrast to optically thick clouds for which the effective radius decreases with increasing liquid water path.