Visual Cloud Histories Related to First Radar Echo Formation in Northeast Colorado Cumulus

Abstract

Using quantitative analysis of time-lapse motion pictures from aircraft and a sensitive meteorological radar, the cloud top history is related to the early radar echo development in 12 vigorous, summer, convective cloud turrets in northeastern Colorado. At a threshold of about 5 dB(Z), the first echoes appear typically 5?10 min after the cloud top passes the -?20°C level. The first echo either appears at cloud top or reaches the top very quickly. It sometimes appears at a well-defined height, but sometimes nearly simultaneously over an altitude range of 3 km or more. Radar echo at 5 dB(Z) typically fills the visual cloud 5?10 min after first echo. In terms of overall cloud lifetime there is plenty of time for the particles responsible for the first echo to form by the ice process. A detailed model of the rates of ice particle formation by vapor growth followed by riming gives a 5 dB(Z) radar echo within 7?10 min at concentrations as low as 1 m?3, at most temperatures between ?10 and ?20°C and in cloud conditions realistic for northeast Colorado. The natural echo development may often result from the transport of embryonic ice particles into regions with vigorous updraft and high liquid water content where growth by accretion is rapid, rather than from growth entirely within the vigorous updrafts, for which the time may often be insufficient.