The production of warm rain in shallow maritime cumulus clouds

Abstract

The problem of the production of warm rain by collision and coalescence has been studied for over half a century and several processes have been suggested to explain the observed production, which is more rapid than generally possible with models. A straightforward scenario in relatively shallow maritime cumulus clouds is one where cloud drops simply grow by condensation and coalescence, with no appeal to enhancement due to entrainment or turbulence or indeed the presence of giant and ultragiant aerosols that may exist in the boundary layer. However, it is difficult in a field experiment to measure the concentrations of aerosols and the time evolution of the droplet size distribution or reflectivity in individual clouds. The Rain in Cumulus Over the Ocean (RICO) field experiment overcame some of these difficulties due to the abundance of clouds and the statistical sampling strategy at all significant altitudes. In this article, we present the results of the rate of increase in the radar reflectivity in a couple of cases. Comparisons with a cloud model strongly suggest that the development of warm rain can be explained using the observed aerosol distribution alone. Sensitivity studies suggested that giant and ultragiant aerosols were unimportant for the production of rain in these clouds. © 2012 Royal Meteorological Society.