Bimodal CCN Effects on Cloud and Drizzle Microphysics

Bimodal aerosol size distributions first observed by Hoppel et al. (1985) under marine stratus were attributed to sulfur dioxide conversion to sulfate that made the larger sizes of the accumulation mode of cloud residual particles after water evaporated. Particles that nucleate activated cloud droplets thus have altered hygroscopic mass that changes their critical supersaturations, S_c, while S_c of particles that did not nucleate cloud droplets are unchanged. This results in an S_c and size gap after cloud passage. Aerosol bimodality has since been numerously reported. Hudson et al. (2015) showed bimodal CCN spectra and higher cloud droplet concentrations (N_c) in stratus and lower N_c in cumuli of clouds associated with and probably grown upon bimodal CCN compared to clouds grown on unimodal CCN spectra that had not been cloud processed. Here we show that CCN modality (spectral shape; i.e., whether bimodal or unimodal) affects all aspects of stratus cloud microphysics and drizzle. Panel A shows mean differential cloud droplet spectra that have been divided into six groups according to the measured CCN spectra that was closest to each of these 131 cloud measurements during the Marine Stratus/Stratocumulus Experiment (MASE) off the Central California coast. These divisions were done according to traditional slopes, k, of cumulative CCN spectra plotted log-log within the critical supersaturation, S_c, range pertinent to these clouds, 0.5-0.1%. Since cloud processing usually decreases S_c of the accumulation mode, it reduces k. Thus, more bimodal CCN have lower k. Chemical cloud processing that reduces k is more prevalent in stratus clouds because the droplets are smaller, there is less LWC and lower W (Hegg et al. 1992; Feingold and Kreidenweis 2000; Hudson et al. 2015). Because of supersaturation, S, variations among clouds there is an intrinsic advantage of lower S_c particles for making higher N_c, in subsequent clouds. Cloud S are seldom the same as the cloud S that produced the accumulation particles. Thus, lower S_c accumulation particles can increase N_c in subsequent cloud cycles (Hoose et al. 2008). This higher N_c for clouds associated with CCN spectra with lower k is demonstrated in panel A. This shows higher concentrations of small cloud droplets associated with and probably grown upon lower k CCN than clouds grown on higher k CCN. The small droplet concentrations follow the k order of the legend, black, blue, cyan, pink, orange, and then red (lowest k to highest k). Greater competition among droplets in clouds with greater small droplet concentrations limits concentrations of larger droplets compared to clouds with lower small droplet concentrations grown on more unimodal CCN, which then allow more larger droplets. Thus, above 13 µm diameter the lines in panel A cross and the hierarchy reverses so that red (highest k) has the highest concentrations followed by orange, pink, cyan, blue and finally black (lowest k). This is more apparent at larger droplet sizes in panel B.

Steeper slopes, higher k of unimodal CCN spectra describe greater concentration variations with S. This means greater diversity of CCN concentrations with S. Panels A and B display greater droplet size diversity (greater sigma, σ) for clouds associated with higher k CCN; i.e., red and orange display the lowest small droplet concentrations in panel A and the greatest large droplet concentrations in panel B. This describes the largest σ. Thus, the greater CCN diversity of higher k gives rise to more diverse cloud droplet size spectra (i.e., greater σ).

The reversed droplet concentration hierarchy shown on right side of panel A and in panel B continues up into the drizzle size range where it is more explicit in panel C. Here the most drizzle drops, N_d, occur in clouds grown on the least cloud-processed CCN (highest k, red), while clouds grown on the most processed CCN (black) have the lowest N_d. The N_d differences are greater than the N_c differences. Clouds associated with the most unimodal CCN (highest k, red) have two orders of magnitude greater N_d than clouds associated with the second most bimodal CCN (blue). In clouds associated with the most bimodal CCN (black for lowest k) there are no drops larger than 250 µm diameter. Lower N_d for more bimodal CCN is largely due to more uniform droplet sizes, i.e., smaller σ that inhibits autoconversion to drizzle. Drizzle results show that cloud processing suppresses precipitation in MASE stratus. This suppression of stratus cloud drizzle by cloud processing is an additional 2^nd indirect aerosol effect (IAE) that along with the enhancement of 1^st IAE by higher N_c due to cloud processing (panel A) are above and beyond original IAE due to anthropogenic CCN.

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