The effects of aerosol particles produced by forest fires and urban pollution on the evolution of warm-rain processes in clouds over tropical regions were studied with a one-dimensional numerical model.

The parcel model with detailed microphysics simultaneously calculates condensation and coalescence growth, including activation of CCN. Both microphysical and meteorological data were obtained from various field projects in several areas of Mexico and Indonesia. Calculations included different parameterization schemes for the CCN spectra and simulated both "background" and "polluted" scenarios. Comparisons between the results obtained from different scenarios and field data for a given region allow one to infer the effects of pollution, mainly on the processes of initiation of warm rain.

The results show that droplet concentrations in clouds forming in contaminated air may be greatly increased and that coalescence would be slower. Thus, it might be anticipated that the production of rain by coalescence processes would be much retarded and that less rain would result. However, slight differences on both the concentration and the quality of pollutants may result even in an increase of the precipitation rate at cloud-base.