1179 Drizzle Suppression in Stratus Clouds due to Cloud Processing of CCN

Wednesday, 10 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
James G. Hudson, DRI, Reno, NV; and S. R. Noble

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, Sc, while Sc of particles that did not nucleate cloud droplets are unchanged. This results in an Sc 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 (Nc) in stratus and lower Nc 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 according to traditional slopes, k, of the 131 measured CCN spectra in the Marine Stratus/Stratocumulus Experiment (MASE) off the Central California coast. K is generally high within the supersaturation, S, range of stratus clouds (< 0.5%). High k means greater diversity of CCN concentrations with S. Because cloud processing generally decreases Sc of some particles to create the accumulation mode, it reduces k. Thus lower k indicates more cloud-processed CCN that are more bimodal. Panel A shows higher concentrations of small cloud droplets associated with and probably grown upon lower k CCN than clouds grown on higher k CCN. At small droplet sizes the concentrations follow the k order of the legend, black, red, green, blue (lowest k to highest k). Above 13 µm diameter the lines cross and the hierarchy reverses so that blue (highest k) has the highest concentrations followed by green, then red and finally black (lowest k). This reversal of the droplet hierarchy continues up into the drizzle size range (panel B) where the most drizzle drops, Nd, occur in clouds grown on the least cloud-processed CCN (blue), while clouds grown on the most processed CCN (black) have the lowest Nd. The other major determinant of Nc and other cloud microphysics, is vertical wind, W, which in stratus is actually the variation of W, σw (Hudson and Noble 2014). It seemed unfortunate that σw was anti-correlated with k during MASE, because both lower k and higher σw work toward higher Nc, it was therefore difficult to distinguish these two influences on clouds, i.e., CCN shape effects from dynamic effects. Although higher σw ought to make more Nc, panel B shows by far the highest Nc in clouds with the 2nd highest σw values, while clouds of the highest σw group have only the 3rd highest Nc. Panel D shows the same reversal of the concentration hierarchy of panel C that was shown for panels B and A. Further analysis confirmed what is shown in this figure, that the influence of CCN spectral shape overwhelms the effect of σw on Nc and Nd; i.e., CCN spectral shape interferes with the hierarchy of cloud and drizzle response to σw but σw does not interfere with cloud response to CCN spectral shape. There were also greater k differences among the cloud groups shown in the legends than there were σw differences among these cloud groups. Adjusted coefficients of determination (R2adj) of double regressions of σw and k were greater than these two single regressions with cloud microphysics (e.g., Nc) and especially with Nd. This means that the two factors, σw and k, each independently influenced Nc and Nd. The suppression of stratus cloud drizzle by cloud processing in panel B is an additional 2nd indirect aerosol effect (IAE) that along with the enhancement of 1st IAE by higher Nc due to cloud processing (panel A) are above and beyond original IAE due to anthropogenic CCN. However, further similar analysis is needed in other cloud regimes to determine if what was found in MASE is typical. Such further research in other cloud regimes could also determine whether the correlation between σw and k was coincidental or inherent; i.e., that CCN spectral shape affects σw, e.g., through variations of latent heat exchanges due to condensation or evaporation differences among clouds grown on CCN spectra with different shapes due to various levels of cloud processing.

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, 2365-2379.

Hudson, J.G., and S. Noble, 2014: CCN and vertical velocity influences on droplet concentrations and supersaturations in clean and polluted stratus clouds. J. Atmos. Sci., 71, 312-331. DOI: 10.1175/JAS-D-13-086.1

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.

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