Height and Temperature of First Echoes in Unseeded and Seeded Convective Clouds in South Dakota

Abstract

The height and temperature of first radar echoes observed on 49 test cases in a randomized cloud seeding project have been related to seeding treatment and to five cloud parameters, namely, cloud base height, cloud base temperature, maximum radar echo height, and the height and speed of maximum updraft as computed with a cloud model. First echo height above cloud base in unseeded clouds averaged 11,000 ft and was strongly correlated with maximum updraft speed. First echo height in clouds seeded with silver iodide averaged 6900 ft. It was most closely correlated with cloud base temperature, suggesting that first echoes tend to appear at some fixed temperature. First echoes in salt seeded clouds appeared on an average 5300 ft above cloud base and showed no significant correlation with any of the cloud parameters mentioned. It is concluded that both salt and silver iodide seeding can cause first echoes to appear closer to cloud base in unseeded clouds, with salt probably having a more pronounced effect in summertime convective clouds.