Alterations of Cloud Microphysics Due to Cloud Processed CCN

Abstract:

High-resolution CCN spectra have revealed bimodality (Hudson et al. 2015) similar to aerosol size spectra (e.g., Hoppel et al. 1985). Bimodality is caused by chemical and physical cloud processes that increase mass or hygroscopicity of only CCN that produced activated cloud droplets. Bimodality is categorized by relative CCN concentrations (N_CCN) within the two modes, N_u-N_p; i.e., N_CCN within the higher critical supersaturation, S_c, mode that did not undergo cloud processing minus N_CCN within the lower S_c mode that was cloud processed. Lower, especially negative, N_u-N_p designates greater processing. The table shows regressions between N_u-N_p and characteristics of clouds nearest the CCN measurements.

	ICE-T			MASE
parameter	R	SL		R	SL
Nc	0.17	93.24		-0.26	98.65
MD	-0.31	99.69		0.33	99.78
σ	-0.27	99.04		0.48	100.00
Ld	-0.31	99.61		0.38	99.96

Table. Correlation coefficients, R, and one-tailed significance levels in percent, SL, for N_u-N_p with microphysics of the clouds closest to each CCN measurement, 75 ICE-T and 74 MASE cases. N_c is cloud droplet concentration, MD is cloud droplet mean diameter, σ is standard deviation of cloud droplet spectra, L_dis drizzle drop LWC.

Two aircraft field campaigns, Ice in Clouds Experiment-Tropical (ICE-T) and Marine Stratus/Stratocumulus Experiment (MASE) show opposite R signs because coalescence dominated cloud processing in low altitude ICE-T cumuli whereas chemical transformations predominated in MASE low altitude polluted stratus. Coalescence reduces N_c and N_CCN, which thus increases MD, and σ, which promote L_d. Chemical transformations, e.g., SO₂ to SO₄, increase CCN hygroscopicity, thus reducing S_c, but not affecting N_c or N_CCN. Lower S_c CCN are capable of producing greater N_c in subsequent cloud cycles, which leads to lower MD and σ which reduce L_d (figure). 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) with a positive feedback that further enhances coalescence and drizzle. Chemical cloud processing enhances both components of IAE; 1^st IAE by greater droplet surface area, 2^ndIAE by suppressing drizzle and thus increasing cloudiness.

Hoppel, Fitzgerald and Larson: JGR 90, 2365-79

Hudson, Noble and Tabor: JGRA 120, 3436–52