On the optimal aerosol concentration (Nop) that marks the transition from cloud invigoration to suppression (Invited Presentation)

The invigoration effect implies that increase in aerosol loading enhances the development of convective clouds. Out of the variety of aerosol effects on clouds, invigoration is the most elusive. In particular in the case of warm convective clouds where some numerical modeling studies suggest suppression under high aerosol loading, while observational studies that do show clouds' invigoration require aerosol measurements in the close vicinity of clouds, that questions the quality of the retrievals as well as the physical explanation.

Here we suggest a physical scheme that incorporates many of the conflicting results into one coherent theory. We propose to view invigoration as an extension of the case where cloud development is limited by the availability of cloud condensation nuclei (CCN). For any given thermodynamic conditions that support convective cloud formation, there will be an optimal CCN concentration (N_op) that allows a maximal cloud development (by meaning of mass, depth or rain amount). For any concentration below N_op the forming cloud would be aerosol limited, while for concentrations above N_op, cloud periphery processes such as enhanced entrainment, takeover and suppress the cloud development. Moreover, we show that N_op is a function of the thermodynamic conditions. Thus, profiles that favor deeper clouds would dictate larger values of N_op, whereas for profiles of shallow convective clouds, N_op is in the pristine range of the aerosol concentration levels.

Such physical scheme bridges the gap between the contradictory results of numerical models and observations. Satellite studies are biased in favor of larger clouds that are characterized by larger N_op values and therefore invigoration is more likely to be observed. While mondeling studies are often biased to small, trade-like convective clouds characterized by low N_op values and therefore cloud suppression is likely to be found as a response to elevated aerosol concentrations.