Comparisons of cloud microphysics with cloud condensation nuclei spectra over the summertime Southern Ocean

Abstract

In spite of the many factors that determine cloud droplet concentrations, relationships were found between cloud condensation nuclei (CCN) and cloud droplet concentrations for the First Aerosol Characterization Experiment (ACE 1). Measurements were made in summertime stratocumulus clouds over the Southern Ocean far from anthropogenic sources. The closest relationship of CCN and droplet concentrations was found for CCN measured just below near‐adiabatic cloud parcels. Moreover, an adiabatic droplet growth model successfully predicted these droplet concentrations from the CCN spectra and updraft velocities. Correlations of the averages of CCN and cloud droplet concentrations over the entirety of each of the flights were also good. The model also predicted, with reasonable accuracy, these flight‐wide average droplet concentrations from flight‐wide average CCN spectra and flight‐wide average cloud updraft velocities. Drizzle was a major reason for the larger variations in droplet concentrations than in CCN concentrations. Vertical differences in CCN concentrations may have also affected cloud droplet concentrations. Despite variabilities in droplet concentrations, there was a clear relationship between CCN and droplet concentrations. Many of the inferred supersaturations (0.5 to 1.4%) in the “adiabatic” clouds ranged higher than some previous estimates (e.g., 0.2%) for stratus clouds. This may have been due to the cleaner air of the Southern Ocean but is more likely attributable to the more intensive analysis used in this study. Nonetheless, even where the inferred cloud supersaturations were lower (i.e., 0.2%), due to reductions of the droplet concentrations, there was still a relationship between CCN and droplet concentrations.