On the Modification Potential of Illinois Summertime Convective Clouds, with Comparisons to Florida and FACE Observations

Abstract

The potential for enhancing rain output over central Illinois through modification of summertime convective clouds was investigated by use of model predictions of cumulus growth, as well as direct, internal measurements of cloud physical characteristics. No clouds were seeded in Illinois. Comparisons were made with results gathered during seeding experiments on single convective clouds in Florida and also during the Florida Area Cumulus Experiment (FACE). Predictions of the top heights of unseeded and seeded Illinois clouds (and the predicted, post-seeding height increase, the ?seedability?) were made using a one-dimensional cumulus model and 10 summers of regularly-gathered radiosonde data from Rantoul and Peoria. Seedability (which in Florida is correlated with enhanced rain volume) was small (<2 km) or zero in Illinois much more frequently than in Florida, and was largest, in the mean, in July and smallest in June; it varied markedly from morning to evening, from month to month within a summer, and from summer to summer. Measurements made during two July days in 1977, in cumuli over central Illinois, described the natural evolution of the ice-water budget and the life history of the updraft. In-cloud microphysical characteristics, near the ?10°C level (?6 km) in Illinois, in convective clouds that were developing in moist air in advance of a weak cold front, were equivalent to those characteristics encountered at the same penetration level of Florida cumuli of similar size and depth. Clouds that were penetrated in the dry air behind the cold front had microphysical structures that differed greatly from those in clouds developing in tropical maritime air mass conditions.