Effects of Cloud-Processed CCN on Warm Clouds

Cloud condensation nuclei (CCN) distributions are transformed by in-cloud processing. This can be chemical: aqueous oxidation; or physical: Brownian scavenging, collision and coalescence. Droplet evaporation then leaves behind the cloud-processed CCN. Chemical processing increases CCN size (lower critical supersaturation; S_c) but does not change CCN concentration (N_CCN) (Feingold and Kreidenweis, 2000). Physical processing leads to an increase in size (lower S_c) and decrease of N_CCN. These processes are especially important in stratus clouds that cover large areas and persist for long periods. Modified CCN in turn modify cloud droplet spectra. Both chemical and physical processing were observed during the 2005 MArine Stratus/stratocumulus Experiment (MASE) field campaign. Higher concentrations of SO₄ and NO₃ anions with lower SO₂ and O₃ were associated with bimodal CCN spectra whereas monomodal spectra had lower SO₄ and NO₃ and higher SO₂ and O₃. These are consistent with chemical processing.

Two nearby MASE CCN spectra, one bimodal and one monomodal were input to an adiabatic cloud droplet growth model. Model runs at various updrafts (W) show that the low S_c cloud processed mode of the bimodal CCN spectrum augmented droplet activation creating higher cloud droplet concentrations (N_c) for low W characteristic of stratus clouds (Fig. 1a, black). Higher N_CCN at low S_c (black data) also increased condensation competition and thus reduced cloud effective S (S_eff) (Fig.1b). This increases W importance for determining N_c (Hudson and Noble, 2014). These high N_CCN at low S_c and lower S_eff of the bimodal CCN spectrum reduce droplet mean diameter (MD; Fig. 1c) and broaden droplet distributions (sigma; Fig. 1d). Increased N_c and decreased MD of chemical processing seems to augment the indirect aerosol effect (IAE) whereas inherently decreased N_c and increased MD of coalescence processing reduces IAE. CCN cloud-processing alters cloud microphysics (N_c, S_eff, MD, and sigma) thus affecting IAE and is therefore as influential as CCN sources.

Feingold, G., and S. Kreidenweis, 2000. J. Geophys. Res., 105(D19), 24351-24361.

Hudson, J. G., S. Noble, 2014. J. Atmos. Sci., 71, 312–331.