Cloud Microphysical Properties from Stellar Aureole Measurements

Atmospheric particles act to cool the Earth by reflecting incoming solar radiation if they are small (e.g., aerosols), and they can either warm or cool the earth through thermal absorption and emission if they are large (e.g., cirrus) depending upon their altitude. Climate change monitoring and modeling have improved significantly from advances attributable to the AERONET global network of ground-based sun photometers measuring the properties of aerosols. The climate impact of cirrus cloud particles is much less certain because they occur high in the atmosphere and are more difficult to monitor. Recent work pioneered by Visidyne has employed measurements of the solar aureole profiles caused by cirrus particles to retrieve their size distributions during the daytime. We propose a novel technique to extend this work to nighttime and to the larger, more thermally significant, particles. The basic idea is to determine the profile of aureole scattering patterns around stars at small angles (_1500 to 1_), and to invert this profile to determine the ice particle size distribution over the range D & apos; 50 mm to 10 mm, where D is the effective size of the ice particle. Such measurements are of intrinsic interest to cloud scientists and to climatologists, alike. Our approach utilizes only a good camera lens and a medium-quality astronomical CCD camera. We have carried out some preliminary observations to demonstrate how well, and under what conditions it will yield the desired results. Our approach has the advantages over in-situ measurements in that (i) it can be carried out on virtually any night when thin cirrus clouds are visible, (ii) it is relatively inexpensive to implement, and (iii) the measurements do not disturb the cloud environment or the particles themselves. Finally, we show how such stellar aureole measurements can be run autonomously to enhance existing, ground-based climate monitoring networks with instruments designed to measure stellar aureoles, thereby filling a gap in the information on cirrus clouds necessary for assessing and monitoring their climate impact. Our ultimate goal is long-term monitoring of cirrus ice-crystal size distributions as a function of altitude, season, and geographic latitude