Cloud-Processed Bimodal CCN and Effects on Cloud Microphysics
Abstract
Hudson et al. (2015) showed that bimodal cloud condensation nuclei (CCN) spectra caused by cloud processing have opposite effects on stratus and cumulus clouds. Noble and Hudson (2016; this conference) showed that theoretical predictions of droplet concentrations (Nc) and mean diameters (MD) agree with those observations; bimodal CCN make more Nc with smaller MD in stratus, but lower Nc with larger MD in cumuli. Now we show that the cloud processing influence extends to drizzle drop concentrations (Nd) (Fig. 1); bimodal CCN reduce stratus Nd and increase cumuli Nd. Thus, cloud processing enhances both indirect aerosol effects (IAE) in stratus and reduces both IAE in cumuli. However, since cloud processing makes the best CCN (largest particles; lowest critical supersaturations [Sc]) they can suppress cloud supersaturations (S), which thus renders many higher Sc CCN irrelevant to cloud microphysics. Anthropogenic CCN that cause IAE generally have high Sc. Moreover, many high Scparticles are removed by Brownian capture by cloud droplets, especially in stratus, which have lower S due to lower vertical wind. Thus, cloud processing may actually reduce IAE, especially for maritime stratus, which produces most of IAE. Fig. 2 substantiates cloud processing as the cause of aerosol bimodality. Nu is the concentration of the unprocessed mode, those smaller than the Hoppel minima, which define aerosol bimodality. Np is particle concentration of the larger size (or lower Sc) mode, those larger than Hoppel minima that result from cloud processing, which is mainly chemistry in stratus and drop coalescence in cumuli. Thus, smaller (Nu-Np)/(Nu+Np) indicates bimodality and higher (Nu-Np)/(Nu+Np) indicates unimodality. Panel A shows bimodal aerosol response to the lowest cloud base altitudes (CBA) and unimodal aerosol response to the highest CBA. Panel B shows bimodal aerosol response to the highest cloud fractions (CF) and unimodal response to the lowest CF. Two different instruments (ceilometer and LIDAR) show these same aerosol responses to CBA and four different instruments show these same aerosol responses to CF. The fact that clouds make the best CCN must be accounted for in order to understand and quantify IAE. Hudson et al. (2015), JGRA, 120, 3436-3452.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.A52C..05H
- Keywords:
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- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSESDE: 3354 Precipitation;
- ATMOSPHERIC PROCESSES