Abstract
Abstract. We have investigated the formation of cloud droplets under pyro-convective conditions using a cloud parcel model with detailed spectral microphysics and with the κ-Köhler 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 (NCD) for a wide range of updraft velocities (w=0.25–20 m s−1) and aerosol particle number concentrations (NCN=200–105 cm−3) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/NCN), we found three distinctly different regimes of CCN activation and cloud droplet formation: (1) An aerosol-limited regime that is characterized by high w/NCN ratios (>≈10−3 m s−1 cm3), high maximum values of water vapour supersaturation (Smax>≈0.5%), and high activated fractions of aerosol particles (NCN/NCN>≈90%). In this regime NCD is directly proportional to NCN and practically independent of w. (2) An updraft-limited regime that is characterized by low w/NCN ratios (
Highlights
Clouds cover about 60% of the Earth’s surface and have a strong influence on the global radiative balance, water cycle and climate (IAPSAG, 2007; IPCC, 2007)
Reutter et al.: Aerosol- and updraft-limited regimes of cloud droplet formation. The results of this and related studies suggest that the variability of initial cloud droplet number concentration in convective clouds is mostly dominated by the variability of updraft velocity and aerosol particle number concentration in the accumulation and Aitken mode
A crucial factor for the dynamical and microphysical evolution of clouds is the activation of aerosol particles as cloud condensation nuclei (CCN), i.e., their hygroscopic growth into aqueous droplets that can freely grow by condensation of water vapor
Summary
Clouds cover about 60% of the Earth’s surface and have a strong influence on the global radiative balance, water cycle and climate (IAPSAG, 2007; IPCC, 2007). Modifications of convective cloud properties can affect weather and climate on local and global scales (Rosenfeld, 2006). A crucial factor for the dynamical and microphysical evolution of clouds is the activation of aerosol particles as cloud condensation nuclei (CCN), i.e., their hygroscopic growth into aqueous droplets that can freely grow by condensation of water vapor. Enhancing the number of aerosol particles that can serve as CCN generally leads to more and smaller cloud droplets at cloud base. The consequences of enhanced aerosol concentration are nonlinear and depend strongly on meteorological parameters The consequences of enhanced aerosol concentration are nonlinear and depend strongly on meteorological parameters (e.g. Khain et al, 2008; Rosenfeld et al, 2008)
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