Warm rain processes account for a significant fraction of precipitation that falls in the tropics. Despite fifty years of research on this topic, a quantitative understanding of the production of warm rain has remained elusive. It is commonly stated that traditional condensation and coalescence theory cannot produce rain in the observed time of 20 min. Several ideas have been proposed to explain ways in which drops >50 micron diameter can be formed more quickly to initiate the coalescence process, while others describe ways that smaller drops might coalescence more readily than currently thought.
One potential mechanism for producing the large drops needed to initiate warm rain is the entrainment of dry air from outside the cloud and its subsequent mixing within the cloud. Laboratory results have suggested that some droplets in the cloud may not be affected by entrainment, while others completely evaporate, so subsequent growth can be shared among fewer droplets, making them larger. Recent work by the PI and her collaborators has demonstrated that large drops can be produced as a result of entrainment and mixing. Results from this modeling framework will be tested against and constrained by observations, and then be extended to coalescence processes in order to understand the influence of entrainment on warm rain formation.
Moreover, to fully understand and test this mechanism, more information is required about the process of entrainment itself. Past observational and numerical studies have presented conflicting results about the dominance of a larger thermal circulation within the cloud, or smaller entraining eddies at the cloud edges. A goal of the research is to gain insight into the cloud motions that are important for cumulus entrainment by analyzing new observations in conjunction with high-resolution numerical modeling.
Ultragiant particles ingested by cumulus are another potential mechanism to produce the large drops needed for precipitation initiation. These particles are much larger than traditional cloud condensation nuclei, and are thus capable of initiating coalescence without any prior growth by condensation. The greatest question about this mechanism is whether or not enough of these particles exist in the atmosphere to significantly influence warm rain production in a cumulus cloud. A final goal of the research is to make reliable measurements of local concentrations of ultragiant particles in the atmosphere, and compare their relative importance to warm rain formation with that from the large drops formed by entrainment and mixing.
Data will be collected in support of these objectives during the Rain in Cumulus over the Ocean (RICO) field campaign. The objective of RICO is to characterize and understand the properties of trade wind cumulus at all scales, with particular emphasis on precipitation. Aircraft and radar observations of the clouds will be analyzed and used with detailed Lagrangian microphysical calculations performed within high-resolution cloud simulations to meet the project objectives.
The proposed work seeks to fill holes in current understanding of cumulus entrainment, ultragiant particles, and their roles in warm rain formation. It is unique in that the roles of ultragiant particles and large drops formed by entrainment and mixing will be compared within similar modeling frameworks, constrained by crucial observations collected during the RICO project. Acquiring new knowledge on these topics will be beneficial for developing new parameterizations for convection and precipitation in weather and climate forecast models, thus affecting broader scientific endeavors.
