Cloud Processed CCN Affect Cloud Microphysics

Abstract:

Variations in the bimodality/monomodality of CCN spectra (Hudson et al. 2015) exert opposite effects on cloud microphysics in two aircraft field projects. The figure shows two examples, droplet concentration, N_c, and drizzle liquid water content, L_d, against classification of CCN spectral modality. Low ratings go to balanced separated bimodal spectra, high ratings go to single mode spectra, strictly monomodal 8. Intermediate ratings go merged modes, e.g., one mode a shoulder of another. Bimodality is caused by mass or hygroscopicity increases that go only to CCN that made activated cloud droplets. In the Ice in Clouds Experiment-Tropical (ICE-T) small cumuli with lower N_c, greater droplet mean diameters, MD, effective radii, r_e, spectral widths, σ, cloud liquid water contents, L_c, and L_d were closer to more bimodal (lower modal ratings) below cloud CCN spectra whereas clouds with higher N_c, smaller MD, r_e, σ, and L_d were closer to more monomodal CCN (higher modal ratings). In polluted stratus clouds of the MArine Stratus/Stratocumulus Experiment (MASE) clouds that had greater N_c, and smaller MD, r_e, σ, L_c, and L_d were closer to more bimodal CCN spectra whereas clouds with lower N_c, and greater MD, r_e, σ, L_c, and L_d were closer to more monomodal CCN. These relationships are opposite because the dominant ICE-T cloud processing was coalescence whereas chemical transformations (e.g., SO₂ to SO₄) were dominant in MASE. Coalescence reduces N_c and thus also CCN concentrations (N_CCN) when droplets evaporate. In subsequent clouds the reduced competition increases MD and σ, which further enhance coalescence and drizzle. Chemical transformations do not change N_c but added sulfate enhances droplet and CCN solubility. Thus, lower critical supersaturation (S) CCN can produce more cloud droplets in subsequent cloud cycles, especially for the low W and effective S of stratus. The increased competition reduces MD, r_e, and σ, which inhibit coalescence and thus reduce drizzle. These observations are consistent with cloud droplet growth models for the higher vertical wind (W) of cumuli and lower W of stratus. Coalescence thus reduces the indirect aerosol effect (IAE) whereas chemical cloud processing enhances both aspects of IAE; 1^st IAE by greater N_c, 2^ndIAE by suppression of drizzle.

Hudson, Noble and Tabor: JGRA, 120.