Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)

We have investigated the formation of cloud droplets under (pyro-)convective conditions using a cloud parcel model with detailed spectral microphysics and with the Kappa -Koehler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol partic...

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Veröffentlicht in:Atmospheric chemistry and physics discussions 2009-04, Vol.9 (2), p.8635-8665
Hauptverfasser: Reutter, P, Trentmann, J, Su, H, Simmel, M, Rose, D, Wernli, H, Andreae, MO, Poeschl, U
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Sprache:eng
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Zusammenfassung:We have investigated the formation of cloud droplets under (pyro-)convective conditions using a cloud parcel model with detailed spectral microphysics and with the Kappa -Koehler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol particles. Assuming a typical biomass burning aerosol size distribution (accumulation mode centred at 120 nm), we have calculated initial cloud droplet number concentrations (N sub(CD)) for a wide range of updraft velocities (w=0.5-20 m s super(− 1)) and aerosol particle number concentrations (N sub(CN)=10 super(3)-1 0 super(5) cm super(− 3)) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/N sub(CN)), we found three distinctly different regimes of CCN activation and cloud droplet formation: 1. An aerosol-limited regime that is characterized by high w/N sub(CN) ratios (> approximately 10 super(− 3) m s super(− 1) cm super(3)), high maximum values of water vapour supersaturation (S sub(max)> approximately 0.5%), and high activated fractions of aerosol particles (N sub(CD)/N sub(CN)>&#x22 48; 90%). In this regime N sub(CD) is directly proportional to N sub(CN) and practically independent of w. 2. An updraft-limited regime that is characterized by low w/N sub(CN) ratios (< approximately 10 super(− 4) m s super(− 1) cm super(3)), low maximum values of water vapour supersaturation (S sub(max)< approximately 0.2%), and low activated fractions of aerosol particles (N sub(CD)/N sub(CN)
ISSN:1680-7367
1680-7375