CCN Spectral Modality Compared to Droplet Spectra and Drizzle in RICO Cumuli

Abstract

Rain in Cumulus over the Ocean (RICO) warm cloud microphysics and drizzle comparisons with cloud condensation nuclei (CCN) spectra displayed similarities with earlier analyses of low‐altitude Ice in Clouds Experiment‐Tropical (ICE‐T) warm maritime cumuli. These comparisons of high‐resolution CCN spectra measured at 100 m altitude with cloud and drizzle measurements were consistent within three altitude bands between 600 and 3,700 m. For both projects, clouds associated with bimodal CCN (accumulation mode dominant) displayed more drizzle than clouds associated with unimodal CCN (Aitken mode dominant). Higher concentrations in clouds associated with bimodal CCN than clouds associated with unimodal CCN extended throughout the ranges of 3 drizzle drop probes (260X, 2DC, and 2DP) between 60 and 5,200 μm diameter. Ratios of drizzle drop concentrations in clouds associated with bimodal CCN to those in clouds associated with unimodal CCN were as much as an order of magnitude. Confirmation of a relationship between CCN modality and cloud droplet spectra found in ICE‐T was often obscured in RICO by drizzle effects on droplet spectra. There was also an association between cloudiness and CCN bimodality that is consistent with clouds as a source of accumulation mode particles. These consistent results now found in two warm maritime small low‐altitude cumulus cloud experiments show that CCN bimodality could inhibit the indirect aerosol effect (IAE), especially second IAE (cloud lifetime) in warm maritime cumuli. These RICO and ICE‐T observations in maritime cumuli are opposite of the results in the Marine Stratus/Stratocumulus Experiment, which indicated that CCN bimodality could enhance both IAE.

A Note on Aerosol Processing by Droplet Collision‐Coalescence

“…Furthermore, cloud droplets can collect unactivated aerosol particles by Brownian capture (e.g., Svenningsson et al., 1997). The subject of this study will, however, be the collision and subsequent coalescence of cloud droplets, merging the dissolved aerosol masses inside them to form larger particles upon evaporation (e.g., Flossmann et al., 1985; Hudson & Noble, 2020, 2022; Noble & Hudson, 2019). As these processes shape the aerosol size distribution, they alter the ability of aerosol particles to activate to cloud droplets and to act as drizzle embryos, and thus feed back on the aforementioned effects of aerosol particles on clouds and the climate.…”