CCN Spectral Shape and Cumulus Cloud and Drizzle Microphysics

The shapes of high- resolution cloud condensation nucleus (CCN) spectra are compared with cloud and precipitation characteristics observed in the Ice in Clouds Experiment-Tropical. These high-resolution spectra often revealed bimodality that is usually caused by in-cloud processing. Bimodal CCN spectra were associated with clouds that had a third of the droplet concentrations, twice as broad droplet spectra and 2 to 3 orders of magnitude more drizzle than clouds associated with unimodal CCN spectra. These findings are opposite of a similar earlier study of stratus clouds that suggested enhancement of the indirect aerosol effect. The observations were consistent over the full range of cloud droplet liquid water content (LWCc) thresholds and bands. But the largest differences between clouds associated with bimodal and unimodal CCN were at intermediate LWCc bins where drizzle and droplet spectral width was greatest in the clouds associated with bimodal CCN.