Ice initiation and ice multiplication processes in a warm-based precipitating cumulus cloud model

This study focuses on ice initiation and ice multiplication processes in warm-based precipitating shallow cumulus clouds. The five principal components of the investigation are: (1) development and application of a two-cylinder cloud and aerosol interaction model which allows sensitivity tests on the microphysical processes; (2) analysis of the role of perturbation pressure in the evolution of cloud drop spectra; (3) analysis of the impacts of cloud drop spectra on ice formation; (4) evaluation of the impacts of concentration of cloud condensation nuclei on the ice formation and the dynamics of warm-based precipitating shallow cumulus clouds; and (5) analysis of the influences of ice-active bioaerosols on ice bursts.;The existence of the cloud drop activation process at the interface of cloud and clear air at the cloud summit allows the occurrence of the condensation freezing process. On the other hand, the processes of immersion freezing and contact freezing become significant when precipitation-sized water drops appear in the interior cloud. Both of these ice nucleation modes contribute to ice crystal bursts when the small rain drops reach subfreezing heights. This finding indicates that the Hallett-Mossop mechanism can explain ice crystal multiplication in warm-based precipitating shallow cumulus clouds. An increase in concentration of CCN can strengthen the ice formation and the cloud development. The different ice-nucleating efficiencies of bioaerosols can lead to the different times of ice initiation and ice production speeds.;A time-dependent cloud and aerosol interaction two-cylinder model is formulated which incorporates the explicit microphysical processes of cloud condensation nuclei (CCN) and ice nuclei (IN). Perturbation pressure is determined explicitly by a Fourier Bessel series expansion. The aerosol masses of CCN and IN in hydrometeors are calculated explicitly in the warm rain formation and the ice crystal riming processes. Simulation results show that the updraughts induced by the gradient force of the dynamic pressure result in the new activation of cloud droplets at the cloud-clear air interface. The broadening of the droplet spectra at the cloud top results in a continuous feeding process of small cloud droplets. This feeding process can accelerate the speed of warm rain formation due to large differences in gravitational settling velocities between the small-sized cloud drops originally activated at the cloud top and large-sized cloud drops activated at the cloud base.