Seventh Annual AMS Student Conference


A Statistical Analysis of Trade Wind Cumulus Properties Relevant to Warm Rain Formation

Amanda M. Sheffield, Purdue University, West Lafayette, IN; and S. Lasher-Trapp

Trade wind cumuli are small, shallow cumulus that are prevalent at low latitudes over the Earth's oceans. They are “warm” clouds, lacking ice, and when rain forms, it is by warm rain processes, i.e., condensation upon favorable nuclei in the atmosphere with subsequent raindrop formation by droplet collection. A better understanding of their properties and ability to precipitate is important for developing better conceptual models of the global energy balance, water cycle, and climate change.

The present study is a statistical analysis of characteristics of trade wind cumulus relevant to precipitation formation using 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. The goal is to assess ten days of cumulus sampling by a research aircraft 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 are selected by scrutinizing aircraft digital video and radar data. The statistical analysis includes percentiles of number concentration and size of cloud, drizzle and raindrops, liquid water content (including fraction of the adiabatic value), and updraft speed. These statistics are proving useful for understanding natural variability among the trade wind cumuli, and providing constraints for numerical modeling studies.

Preliminary results from the statistical analysis indicate some trends across the data set, but also substantial variability. Cloud liquid water content values fall very short of the adiabatic values, and increasingly so with altitude. On several flights, number concentration of cloud droplets increases with altitude, contrary to traditional theory. Drizzle and rain development was very limited on some days, while substantial on others. Days on which rain was more prevalent had stronger updraft speeds. Explaining these trends, finding other trends, and understanding their sources, are major goals of this study.

Poster Session 1, Student Conference General Poster Session
Sunday, 20 January 2008, 5:30 PM-7:00 PM, Exhibit Hall B

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