Correlations of small cumuli droplet and drizzle drop concentrations with cloud condensation nuclei concentrations

Abstract

Aircraft field measurements of cloud condensation nuclei (CCN) and cloud microphysics in maritime air masses showed ubiquitous influence of CCN. Flight averages of CCN concentrations and cloud droplet and drizzle drop concentrations were examined for as many as 17 flights during the Rain in Cumulus over the Ocean (RICO) project. CCN concentrations at only one supersaturation (S) of 1% measured at 100‐m altitude were compared with cloud droplet and drizzle drop concentrations at six altitude bands between 600 and 3000 m. High positive correlations (R) between these CCN concentrations and the small size threshold of the cumulative cloud droplet concentrations (i.e., total activated cloud droplets) were found at all altitudes. These high R values also persisted for cloud parcels with a wide span of liquid water contents (LWCs), most of which were far below adiabatic (unmixed) values. For all but the lowest LWC parcels, R was essentially constant. There was an even more consistent negative R between CCN and large cloud droplet and drizzle drop concentrations. There was a sharp transition from positive to negative R over a small size range. The size at which this R transition occurred increased with altitude and LWC as overall droplet sizes increased with altitude or LWC. Entrainment seemed to show an opposite effect on R, but this was only apparent at the highest altitudes where entrainment was greatest and only for the smallest droplet sizes. These results indicate that the effect of CCN concentrations on cloud microphysics was pervasive with altitude, LWC, cloud droplet, and drizzle drop concentrations. This indicates greater impact of the indirect aerosol effect (IAE) in both of its manifestations, first IAE cloud radiation and second IAE precipitation.