Notes on state-of-the-art investigations of aerosol effects on precipitation: a critical review

There is no agreement between the results of different studies as regards quantitative and even qualitative evaluation of aerosol effects on precipitation. While some observational and numerical studies report a decrease in precipitation in polluted areas, in some other observations and numerical studies aerosol-induced precipitation enhancement was reported.

This study analyses possible reasons for the discrepancy between the results. The analysis of aerosol effects on precipitation is performed using the mass and heat budgets. The analysis is concentrated on clouds and cloud systems arising in the environment with relatively high freezing level. It is shown that for such clouds aerosols increase both the generation and the loss of the condensate mass. The net effect of aerosols on the precipitation depends on the environment conditions (air humidity, buoyancy, and wind shear) as well as on the cloud type determining whether the increase in the condensate generation or in the condensate loss will dominate with increase in the aerosol concentration. In the case when the loss increases more than the generation, a decrease in precipitation will take place. If the increase in the condensate generation dominates, an increase in precipitation will take place. A classification scheme of aerosol effects on precipitation is proposed and its relation to the observational and numerical results available is analysed. Possible reasons for the uncertainties and discrepancies of the numerical results, as well as between measurements, are analysed. A discussion of unsolved problems is presented in the conclusion.