Precipitation Development in Natural and Seeded Cumulus Clouds in Southern Africa

Abstract

The development of precipitation was studied in southern Africa in 23 clouds, 12 unseeded and 11 seeded, from 11 days during the Bethlehem Precipitation Research Project. Surface and upper air data were used to describe the environmental conditions while aircraft and radar data were used to determine the ice water budget in the clouds. Two relatively simple cloud models were used to help identify seedable situations. They were a one-dimensional steady state model with bulk microphysical parameterization and a one-dimensional time-dependent model with detailed microphysics. The data were divided into three sets based on the main airmasses affecting the area: maritime tropical (mT), continental tropical (cT), and maritime polar (mP). The smaller clouds on the mT days, with tops warmer than ? 20°C, were the most likely candidate for precipitation enhancement from both the microphysical and dynamic seeding viewpoints. There the time-dependent model calculated a precipitation efficiency increase from 2% to 15% due to seeding. For clouds in the cT air the rapid natural onset of ice suggested that they were not seedable microphysically. Clouds in the mP air were determined to be not seedable because they were either very efficient microphysically or their lifetimes were too short. The developed procedures give insight into the chances for rain enhancement in a given meteorological situation. To treat these results in a statistically significant manner would require a much larger sample of cases.