Opposite Effects of Bimodal CCN Spectra on Stratus and Cumuli
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 (ICE-T) 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). Now we show that the lower Nc of ICE-T cumulus clouds associated with bimodal CCN leads to larger droplet sizes, broader droplet spectra and enhanced drizzle (Fig. B). In the figures the quantified CCN modalities are divided into two halves (65/130 in MASE and 22/45 in ICE-T) and extreme octiles (17/130 in MASE and 6/45 in ICE-T) as they are compared with measurements within the closest clouds.
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 vertical winds of stratus that limit cloud supersaturations compared to cumuli were also a factor in these opposite effects. All of these results were independent of the cloud droplet liquid water content (LWCc) thresholds used to the define clouds. In the cumulus clouds the greatest drizzle amounts were found in intermediate LWCc bands where there was the greatest contrast in drizzle between clouds associated with bimodal or unimodal CCN. Drizzle thus seemed to reduce LWCc. These intermediate LWCc bands also exhibited the broadest droplet spectra that should thus enhance drizzle. Thus, cloud processing tended to enhance both aerosol indirect effects in MASE stratus but reduced both of these 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. 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. Accepted July 26, J. Geophys. Res.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A43K3219H
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0320 Cloud physics and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0368 Troposphere: constituent transport and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES