An Isolated Cumulus Cloud Modification Project

Abstract

A randomized seeding experiment was conducted in Flagstaff, Ariz., in July and August of 1967 wherein the heights, rainfall amounts and rainfall durations of isolated cumuli were measured. Twenty-one clouds were studied, 10 seeded and 11 not seeded, on 11 days. On nine days there were paired seed and no-seed events. The choice of the test clouds and much of the analysis was performed with the aid of a simple numerical model of cumulus dynamics and microphysics. The project aims were to show distinct effects of seeding on cloud-top height, rainfall and duration; to verify and/or improve the model; and to use the model to interpret the importance of the seeding effect. For the 21 test clouds, the average increases in radar tops, precipitation and duration were 5900 ft, 2.00 mm and 10 min, respectively, with student's t-test significance values being 96, 92 and 81%, respectively. On the nine days of paired seed and no-seed events, the t-test significance exceeded 98% for the three variables quoted above. On every one of the nine days, the seeded clouds showed increased height, rainfall and duration. Using the model after it was calibrated against the 11 control clouds, it was found that the seeding increased the 10 cloud-top heights by an average of 6000 ft, with a maximum of 21,500 ft. The student t-test significance value for this increase was 96%. Using the calibrated model on all of the 21 test clouds showed how much benefit could have been derived from seeding all of the clouds. The average potential increase in cloud-top height was 6500 ft. This represents a 52% increase assuming bases at 13,000 ft. Comparison with observations showed an rms prediction error of 3341 ft. The corresponding rainfall and duration increases were +2.86 mm (up from 2.99 mm) and 7 min (up from 11 min), respectively. These hypothetical increases are comparable to the observed values.