The onset of precipitation in warm cumulus clouds: An observational case-study.
Abstract
Data collected with a ground radar and an instrumented aircraft during the Small Cumulus Microphysics Study (SCMS) are analysed to examine the physical processes that control the onset of precipitation. On 10 August 1995, the Météo-France Merlin-IV successively sampled the core of the top cell in three convective clouds, following the cells from their initial stage of young vigorous ascending turrets up to their decaying stage. Simultaneous range-height indicator (RHI) scans were also performed by the National Center for Atmospheric Research CP2 radar. Although the three clouds showed similar values of the droplet number concentrations and were able to generate precipitation embryos, the first two collapsed after reaching an altitude of 3 km above sea level, without producing any precipitation, while the third one reached a higher level of 4 km and produced significant precipitation. This case-study illustrates how sensitive the onset of precipitation is to cloud dynamics, revealing that in a conditionally unstable atmosphere hardly noticeable changes in cloud thermodynamics and microphysics may lead to major changes in cloud vertical development and precipitation. Copyright © 2010 Royal Meteorological Society