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Abstract

The activation of aerosol particles and its contribution to cloud droplet size distributions in shallow cumulus clouds are investigated using a Lagrangian cloud model (LCM), allowing explicit simulations of the activation and tracking of each simulated hydrometeor. This analysis is focused on the differences in activations for aerosol particles following central updraft from a cloud base (CB) and particles laterally entrained (LE) aloft. Strong supersaturation fluctuations induced by entrainment and turbulent mixing at cloud edges cause most LE particles to be deactivated soon after activation, substantially limiting their lifetime and growth relative to CB particles. Most net activation—the difference between activation and deactivation—occurs at the cloud base and vanishes aloft, thus making the cloud droplet concentration vertically uniform above the cloud base. Time series of various variables following individual particles confirm the distinctive nature of CB and LE activation. Activation spectra follow the theoretical prediction at high supersaturation conditions, but their magnitudes are smaller because of particle deactivation. At low supersaturation conditions, the spectra deviate from the theoretical prediction since they are affected by the transport of the CB particles activated elsewhere.