Opposite Effects of Bimodal CCN on Stratus and Cumulus Clouds
Abstract
Bimodal CCN spectra had opposite effects on cloud droplet concentrations (Nc) in the Marine Stratus/Stratocumulus Experiment (MASE) project compared to the Ice in Clouds Experiment-Tropical (ICET) project (Hudson et al. 2015). Compared with nearby unimodal CCN spectra, bimodal CCN enhanced Nc in stratus but decreased Nc in cumuli. Hudson et al. (2018) showed that enhanced Nc associated with bimodal CCN decreased droplet sizes, droplet spectral width and thus suppressed drizzle in MASE stratus clouds (Fig. A). Hudson and Noble (2019) showed that the lower Nc of ICET cumulus clouds associated with bimodal CCN led to larger droplet sizes, broader droplet spectra and enhanced drizzle (Fig. B). We now confirm these results in another cumulus project Rain in Cumulus Over the Ocean (RICO) (Fig. C). Quantified CCN modalities are divided into halves (65/130 in MASE and 27/54 in ICET and 14/18 in RICO) and extreme quartiles and octiles as they are compared with measurements within the closest clouds. The figures show that these opposite effects of bimodal CCN spectra on cloud and drizzle microphysics are consistent for a wide range of cloud liquid water content (LWCc) bins. These contrasting results were probably largely due to chemical processing dominance in stratus compared to coalescence processing dominance in deeper cumuli (Hudson et al. 2015). Lower vertical winds of stratus that limit cloud supersaturations compared to cumuli were also a factor. In all 3 projects drizzle peaked at intermediate LWCc where there were the greatest drizzle contrasts between clouds associated with bimodal and unimodal CCN. Drizzle thus seemed to reduce LWCc. These intermediate LWCc bands also exhibited the broadest droplet spectra that enhanced drizzle. Thus, cloud processing tended to enhance both aerosol indirect effects (AIE) in MASE stratus but reduced both AIE in ICET and RICO cumuli. By December we will have similar tests in at least one other stratus cloud experiment (e.g., POST). Hudson, J.G., S. Noble, & S. Tabor, 2015: Cloud supersaturations from CCN spectra Hoppel minima. J. Geophys. Res., 120, 3436-3452, doi:10.1002/2014JD022669. Hudson, J.G., S. Noble, & S. Tabor, 2018: CCN spectral shape and stratus cloud and drizzle microphysics. 123, 9635-9651. http://doi.org/10.1029/2017JD027865.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A11E..05H
- Keywords:
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- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSES;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3314 Convective processes;
- ATMOSPHERIC PROCESSES