Opposite Effects of Bimodal CCN Spectra on Stratus and Cumuli

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, usually to lower values. But 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.

Opposite effects on cloud droplet concentrations (N_c) were observed in the Marine Stratus/Stratocumulus Experiment (MASE) project compared to the Ice in Clouds Experiment Tropical (ICE-T) project (Hudson et al. 2015). Bimodal CCN seemed to enhance N_c in stratus but decreased N_c in cumuli when compared to clouds associated with unimodal CCN spectra. Unimodal spectra had apparently not been cloud processed. Hudson et al. (2018) showed that enhanced N_c associated with bimodal CCN also decreased droplet sizes, droplet spectral width and thus suppressed drizzle in MASE stratus clouds (Fig. A). Now we show that the lower N_c of ICE-T cumulus clouds associated with bimodal CCN led to larger droplet sizes, broader droplet spectra and enhanced drizzle (Fig. B). In this figure CCN modality (bimodal versus unimodal) is quantified by the difference between the concentrations within the two modes (Aitken minus accumulation mode normalized by dividing by the sum of Aitken and accumulation concentrations; ndf). Cloud data were then divided according to ndf of the nearest below cloud CCN spectra into halves (65/130 in MASE and 22/45 in ICE-T) and extreme octiles (17/130 in MASE and 6/45 in ICE-T). Thus, CCN modality (spectral shape; i.e., whether bimodal or unimodal) seemed to have opposite effects on all aspects of stratus and cumulus cloud microphysics and drizzle.

Chemical cloud processing, which reduces ndf, is more prevalent in stratus clouds because the droplets are smaller, there is less cloud liquid water content (LWC_c) and lower vertical wind, W (Hegg et al. 1992; Feingold and Kreidenweis 2000; Hudson et al. 2015, 2018). 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). On the other hand, larger droplets and greater LWC_c that occur in thicker cumulus clouds such as ICE-T, especially when there is greater droplet spectral width that provided greater differences in droplet fall speeds that fostered coalescence by autoconversion makes more drizzle. This was more the case for clouds grown on bimodal CCN in cumuli, which reduce N_c and ndf. Greater W of cumuli also produce higher cloud S that can activate more of the smaller Aitken particles in addition to the larger accumulation particles. This greater S_c diversity provided more droplet size diversity that should foster autoconversion to drizzle.

These opposite effects of bimodal CCN spectra on cloud and drizzle microphysics were probably largely due to chemical processing dominance in stratus compared to coalescence processing dominance in deeper cumuli (Hudson et al. 2015). Lower W of stratus that limit cloud S compared to cumuli tend to restrict activation to only the accumulation mode, which means less S_c diversity and thus less droplet size diversity, which in turn restricts autoconversion to drizzle. Thus the predominant cloud processing types in the two projects and the differences in W, which produce different cloud S, were apparently the factors that produced the opposite effects of bimodal aerosol on cloud microphysics and drizzle.

In both MASE and ICE-T all of these results were independent of the LWC_c thresholds used to the define clouds. In the ICE-T cumulus clouds the greatest drizzle amounts were found in intermediate LWC_c bands where there was the greatest contrast in drizzle between clouds associated with bimodal or unimodal CCN. These drizzle differences between clouds associated with bimodal versus unimodal CCN exceeded an order of magnitude in both projects for all LWC_c thresholds and LWC_c bands. But in ICE-T these drizzle differences exceeded two orders of magnitude in the intermediate LWC_c bands (i.e., 0.1-0.01 g/m³). These LWC_c bands also exhibited the broadest cloud droplet spectra that should thus enhance autoconversion to drizzle. Drizzle thus seemed to reduce LWC_c to produce these intermediate LWC_c band values.

Thus, cloud processing tended to enhance both the cloud brightness aerosol indirect effect (AIE) of higher N_c (first AIE) and the cloud lifetime AIE (drizzle suppression, second AIE) in MASE stratus. But cloud processing reduced both of these AIE effects in ICE-T cumuli. However, since these results were obtained in only these two field projects they will now be tested in other projects in other environments.

Feingold, G., and S. Kreidenweis, 2000. Does cloud processing of aerosol enhance droplet concentrations? J. Geophys. Res., 105, 2435124361.

Hegg, D.A., P. Yuen, and T.V. Larson, 1992: Modeling the effects of heterogeneous cloud chemistry on the marine particle size distribution. J. Geophys. Res., 97, 1292712933.

Hoose, C., U. Lohmann, R. Bennartz, B. Croft, and G. Lesins, 2008a: Global simulations of aerosol processing in clouds. Atmos. Chem. Phys., 8, 69396963.

Hoppel, W.A., J.W. Fitzgerald, and R.E. Larson, 1985: Aerosol size distributions in air masses advecting off the East Coast of the United States. J. Geophys. Res., 90, 23652379.

Hudson, J.G., S. Noble, and S. Tabor, 2015: Cloud supersaturations from CCN spectra Hoppel minima. J. Geophys. Res., Atmos., 120, 3436–3452, doi:10.1002/2014JD022669.

Hudson, J.G., S. Noble, and S. Tabor, 2018: CCN spectral shape and stratus cloud and drizzle microphysics. Accepted 26 July, J. Geophys. Res.