89th American Meteorological Society Annual Meeting

Sunday, 11 January 2009
An Investigation of Entrainment in Trade Wind Cumulus Clouds
Phoenix Convention Center
Amanda M. Sheffield, Purdue University, West Lafayette, IN; and S. Lasher-Trapp
Trade wind cumuli are small, shallow cumuli that are prevalent at low latitudes over the Earth's oceans. They are “warm” clouds, lacking ice, but have less liquid water than continental clouds of similar depth. Numerous explanations have been put forth regarding this trend, including losses due to precipitation and more active entrainment. A better understanding of this trend is important for developing better conceptual models of tropical convection, and thus the global energy balance, water cycle, and climate change.

The present study builds on a statistical analysis of characteristics of trade wind cumuli previously reported by the authors that used new data collected during the Rain in Cumulus over the Ocean (RICO) field campaign, held in December 2004 and January 2005 over the Caribbean Ocean. Ten days of cumulus sampling by a research aircraft was used to assess the variability of properties relevant to precipitation formation among the trade wind cumuli on a given day, and between different days. The emphasis is on smaller, not towering, cumulus, and cases were selected by scrutinizing aircraft digital video and radar data. These statistics were useful for understanding natural variability among the trade wind cumuli, and are used in the present study to evaluate the importance of entrainment in the evolution of these clouds.

Preliminary results from the statistical analysis indicate that precipitation appears not to be the reason in the substantial reduction in cloud water in the trade wind cumuli, in opposition to hypotheses presented in previous studies. Additional results on the effectiveness of entrainment in these clouds will be presented.

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