Relationships between CCN and cloud microphysics variations in clean maritime air

Flight-averaged CCN concentrations during the Rain in Cumulus over the Ocean (RICO) field project varied by a factor of four. This was sufficient to provide correlations with flight-averaged total cloud droplet concentrations (N-c) and mean diameters (MD) that showed greater influence of CCN than updraft velocity (w) on Nc and especially MD. The product of CCN and w produced even stronger correlations with Nc and MD. An earlier RICO study had shown an inverse relationship of Nc and giant nuclei (GN) with large cloud droplet concentrations (N-LD). These two RICO studies now show that the aerosol with the most influence on NLD (MD) is CCN. The results of the present study are similar to earlier studies in more polluted clouds that also showed more influence of CCN than GN on MD/ N-LD/drizzle. The present study indicates that variations in CCN concentrations even within maritime air masses modulate precipitation.