Microphysical and radar observations of seeded and nonseeded continental cumulus clouds

Controlled cloud seeding experiments were conducted near Bethlehem, South Africa, during the summer of 1984-85. The experimental unit was the semi-isolated cumulus congestus cloud. Microphysical measurements were obtained by three instrumented aircraft flying in stacked formation. Radar observations were made by a 5-cm wavelength radar performing volume scans at 5-min intervals. Strict cloud selection criteria, based on the measurements made during an initial inspection penetration, assured the treatment of clouds in their developing stages as their tops rose up through the -10 degrees C level. A total of 60 clouds were chosen and treated. When the test cloud data were stratified acc ording to cloud diameter, a positive association existed between seeding and the number of clouds that echoed. Wider clouds responded more favourably to seeding, presumably because they were affected less by entrainment. from Authors