Aerosol effects on microphysics and precipitation: classification of numerical results

It is widely accepted that aerosols affect cloud microphysics and precipitation. At the same time many results of both observations and numerical studies are contradictory. In some studies a decrease in cloud intensity and precipitation (including the total precipitation depletion) with the increase in the aerosol concentration was reported. Other studies report convection invigoration, which however, leads to the precipitation inhibition. There are also studies reporting both convective invigoration and an increase in the precipitation. According to some studies an increase in the aerosol concentration leads to the decrease in the amount of ice precipitation, other studies report the increase in the ice precipitation. Some studies do not show any aerosol effects on precipitation. Being taken without proper classification, the results can be interpreted in the sense that aerosols effects are variable, contradictory and not clear from physical point of view. Wide utilization of numerical models that unable to simulate aerosol effects adequately increases the "scientific noise", which creates additional difficulties in retrieval of useful information. In the review an attempt is performed to classify aerosol effects both observed and simulated numerically. It is shown that aerosol effects should be considered only in combination with thermodynamic conditions under which the effects were evaluated. Different physical mechanisms, by means of which aerosols affect cloud microphysics are analyzed. One or another mechanism starts dominating depending on temperature, atmospheric stability, air humidity, wind shear and even, on cloud size. Difference in the aerosol effects are illustrated using (as examples) results obtained in simulations of maritime and continental shallow and deep convective clouds, squall lines, multicell storms, mixed phase and winter orographic clouds. Variability of aerosol effects is closely related to the variability of cloud types. The problem of the aerosol effects is actually a long-lasting problem in the Cloud Physics that concerns the precipitation formation in different clouds.