Cloud Microphysics and Drizzle Relationships With CCN Modality in Southern Ocean Stratocumuli

Abstract Extended‐range high resolution cloud condensation nuclei (CCN) modality relationships to cloud microphysics and drizzle of the First Aerosol Characterization Experiment (ACE 1) were consistent with Physics of Stratocumulus Tops (POST) and Marine Stratus/Stratocumulus Experiment (MASE) observations. Stratus clouds in these three projects demonstrated that bimodal CCN spectra often caused by cloud processing were associated with clouds that contained higher concentrations of smaller droplets and less drizzle than clouds associated with unimodal CCN spectra. Subsequent brighter clouds and greater cloudiness could enhance both indirect aerosol effects (IAE). However, ACE 1 clouds within lower CCN concentration air displayed bimodal/unimodal reversals within the drizzle size range so that clouds associated with bimodal CCN exhibited less small drizzle but more large drizzle than clouds associated with unimodal CCN. On the other hand, ACE 1 clouds associated with higher CCN concentrations exhibited order of magnitude more drizzle in clouds associated with unimodal CCN. Thus, these higher concentration ACE 1 observations agreed better with MASE and POST stratus observations than the complete ACE 1 data set. Despite robust ACE 1 associations of CCN modality with droplet and drizzle microphysics it is not possible to demonstrate independent causal attribution in an observational aircraft study.