Explaining the production of warm rain in shallow trade-wind cumulus clouds

Despite numerous years of research, we are still unable to quantify the production of warm rain. The Rain in Cumulus Over the Ocean (RICO) field campaign, which took place during the period from November 2004 to January 2005, off the coast of Antigua in the Caribbean, set out to address this issue. The main goal of the project was to make new measurements of the onset of precipitation in cumulus clouds. Several observational platforms were used including the National Center for Atmospheric Research (NCAR) dual-polarization and dual-wavelength radar S-PolKa, three research aircraft, and a research vessel. There was also a ground-based aerosol site located on Antigua.

Our research focuses on quantifying the evolution of the droplet size distribution (DSD) in-cloud, by exploring various mechanisms that may be responsible for the initiation of precipitation. Two key questions are being addressed. Firstly, can the sub-cloud aerosol size distribution, without the mechanisms of entrainment and mixing, and turbulence, account for the production of the largest drops observed at various altitudes in-cloud? Secondly, what effect does entrainment and mixing (varying from homogeneous to inhomogeneous) have on both the breadth of the DSD, and the concentration of the largest drops within different regions of a cloud? In order to examine the influence of the sub-cloud aerosol size distribution on droplet spectral broadening, comparisons will be made between observed DSDs and those produced by an adiabatic Lagrangian parcel model, at various altitudes above cloud-base. Furthermore, the impact of entrainment and mixing will be investigated by comparing observations with output from a modified version of this model run along trajectories produced by a 3D cloud model. The talk will highlight significant findings from each of these studies.