Growth of Tropical Cumulus Congestus Clouds

The Department of Energy's Atmospheric Radiation Measurement climate research facility site at Nauru Island is one of three sites in the tropical western Pacific region. Vertically developed convective clouds frequently observed at these sites are forced by some of the warmest sea surface temperatures on Earth. Clouds play an important role in the local energy balance because they reflect incoming solar radiation as well as absorb and emit terrestrial infrared radiation, the net impact of which can have either a warming or cooling effect on the Earth. How deep these clouds grow is the major determining factor for their radiative impact. To determine how these clouds grow, snapshot observations from a zenith pointing cloud radar were analyzed. It is unknown at the time that the cloud passes over the radar whether it has reached its maximum height or if it will continue to grow. In order to establish whether or not these clouds continue to grow, the vertical air velocity of the cloud top is determined in order to quantify the fraction of upward or downward vertical motion that is present. From this analysis, 39 of the 50 cases from 1999 – 2002 reveal upward vertical motion near the cloud top. This information helps us to interpret analysis of cloud growth and decay processes, most notably mixing of environmental air and cloudy air, helping us to better understand the cloud lifecycle and putting vertically pointing measurements into context.