Influence of cloud condensation and giant cloud condensation nuclei on the development of precipitating trade wind cumuli in a large eddy simulation

Abstract

To investigate the effects of both cloud condensational nuclei (CCN) and giant CCN (GCCN), the Regional Atmospheric Modeling System was used to investigate the effects of various CCN and GCCN concentrations on the development of precipitating trade wind cumuli in a large eddy simulation (LES) framework. The sounding to initialize the LES was taken from the Rain in Cumulus over the Ocean Experiment archive for 11 January 2005. Several sensitivity experiments were performed in which two levels of CCN (GCCN) concentrations were used: 100 (0.01) and 1000 (0.1) cm -3 corresponding to low and high values, respectively. Both CCN and GCCN can affect the precipitation processes. With low GCCN concentration, raising the CCN concentration from low to high reduced the precipitation rate as well as the accumulated precipitation due to the presence of a large number of small cloud droplets that are inefficient in forming drizzle. However, GCCN can have a greater response in increasing the precipitation rate and accumulation when the cloud system has a high CCN concentration. The total cloud coverage (TCC) was reduced for the higher CCN concentration experiments because of the susceptibility of evaporation of cloud droplets in the upper parts of the cloud as a result of entrainment. On the other hand, the TCC was increased for the higher GCCN concentration experiments. For this trade wind cumuli case, the time- and domainaveraged albedo changed very slightly with increased [CCN] and/or [GCCN] because of a compensating increase/decrease among the optical depth, liquid water path, cloud coverage, and cloud droplet concentration. Copyright 2009 by the American Geophysical Union.