Abstract
Radar populations of seedable clouds observed between 4.57 km (15,000 ft) and 7.62 km (25,000 ft) each hour were gathered near Miami and Tampa during various months through the year. They show that numbers of tops in this range vary greatly from year to year and one location to another. The maximum was 503 tops in July 1971 over 4396 km2 near Miami; the minimum was zero in the identical area from 16 to 30 April 1970. Diurnal variations are also shown.
The one-dimensional Lagrangian cumulus model EMB 68K was used to calculate potential top growth due to seeding supercooled cumuli with silver iodide by the dynamic method. This computation was made for all Miami and Tampa soundings between 16 April and 31 May, 1969. Given cloud base at 610 m, radii were found that reached heights of 6.10 and 7.62 km unseeded; then the seedability was obtained for these radii with the numerical model. From the numerous statistics shown for these radii, it was determined, for example, that seedability is larger when clouds in the seedable height range are observed within 6 hr of sounding time compared to similar time periods without any seedable clouds.
The relationship between seedability and rainfall change based on a group of experimental clouds over South Florida was explored. Although not sharply definitive, a linear regression was determined. With this relation, it is possible to combine observed clouds and observed seedabilities to compute rainfall changes due to seeding. Calculations are made with a set of specific restrictions for simplification. Suggestions are given regarding improvements and further study, since this method of estimating dynamic seeding results without aircraft awaits refinements in both theoretical and observational understanding of cumuli before reliable quantitative results are obtained.
