Growth of Tropical Cumulus Congestus Clouds

Cumulus congestus is an important mode of tropical convection. The Atmospheric Radiation Measurement (ARM) site at Nauru Island is one of three sites in the Tropical Western Pacific (TWP) region being used to study the growth of these clouds. By using ground-based observations, including millimeter-wavelength cloud radar, the structure and characteristics of cumulus congestus clouds that passed over the site were obtained. The snapshot observations captured by the radar portrayed the diurnal cycle of cloud growth through reflectivity and mean doppler velocity values. Many factors needed to be accounted for when analyzing the structure of the cloud tops, including the fall velocity associated with hydrometeors. As an important mode of tropical convection, understanding the life cycle phase of cumulus congestus clouds has important implications on the local atmospheric energy balance in regards to determining the amount of solar energy that is reflected as well as how much infrared energy is absorbed or emitted between the Earth's surface and the atmosphere. Cloud growth can be limited by either dry air entrainment above the boundary layer or from a loss in buoyancy as air parcels encounter weak stability. Determining the vertical velocity of the cloud tops will differentiate between terminal cumulus congestus clouds, which will have lost buoyancy and ceased growth, and transient cumulus congestus clouds, which will have continued to ascend to greater altitudes and gain buoyancy. From this analysis, only a small portion of these tropical clouds continued to ascend to higher heights.