Influence of entrainment and mixing in the production of warm rain in Trade-Wind cumulus clouds

The problem of the production of rain by collision and coalescence has remained unsolved for so long partly because good enough observations have not been made of the sub-cloud aerosol particles and of the drop size distributions at the critical altitudes of the cloud where collision and coalescence begins to dominate over condensational growth. Likewise, the models have not been able to simultaneously reproduce the 3-D dynamics of the cloud and the drop size distribution at each grid point at appropriate spatial and temporal resolutions. 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, provided unique observations mainly because (a) the sub-cloud aerosol particles were well measured in long legs and (b) many clouds were present on most days of the project so that many penetrations could be made at several altitudes by each of the three research aircraft. The NCAR S-PolKA radar also provided the critical information on the time of cloud development.

Our research is focused on quantifying the evolution of the droplet size distribution (DSD) in-cloud, by exploring whether various mechanisms, including entrainment and mixing may be responsible for the initiation of precipitation. It has been hypothesized that the growth of the largest cloud droplets can be enhanced in ascending regions of cloud that have experienced entrainment and mixing due to enhanced supersaturation during the ascent. Recent detailed modelling results have supported the idea. However, the so-called "superadiabatic" growth of the largest drops has not been observed in cumulus clouds. More droplet size distributions were measured at more altitudes than ever before during RICO. Droplet size distributions observed in key regions of clouds and critical tests of the entrainment and mixing hypothesis will be presented.

As part of this study we have examined the dynamics of the clouds and, in particular, the relative importance of cloud-top eddies, shear and mixing at the cloud edges and ascending regions with low liquid water content. Observations of the structure of the updraft and of cloud top and edges made in the cumulus clouds measured during RICO will be compared with model results.