Idealised studies of aerosol effects on precipitation – from aqua-planets to global km-scale models

Aerosol effects on precipitation can be broadly categorized into radiatively and microphysically mediated effects – all of which remain highly uncertain. Their assessment in atmospheric models generally relies on the simulation of a complex chain of microphysical process growing aerosols into radiatively active size ranges and into size ranges suitable to act as cloud condensation nuclei, subsequently perturbing radiative fluxes, diabatic heating, cloud microphysical processes and ultimately precipitation formation.  The associated uncertainties along each step in these complex process chains remain significant and make it difficult to disentangle uncertainties in aerosol and cloud processes. 

Here we present results from a hierarchy of highly idealised model simulations in which aerosols are prescribed as fixed plumes of radiative properties, with an optional associated semi-empirical scaling of droplet number perturbations. These idealised simulations provide fascinating insights into the physical processes underlying aerosol effects on precipitation and into the interaction of local perturbations with the larger scale dynamics. 

Idealised aqua-planet general circulation model simulations reveal that the response of regional precipitation to idealised and realistic aerosol radiative perturbations can be well explained in an energetic framework (because associated changes in the net diabatic heating needs to be balanced by latent heat release, surface or top-of-atmosphere fluxes or compensated for by energy divergence/convergence). Extending this framework by adding land and realistic sea surface temperatures in an AMIP setup, we probe the regional sensitivity of precipitation changes to absorbing aerosol perturbations across the globe. Our results confirm the findings from the aqua-planet studies that that the local precipitation response to aerosol absorption is opposite in sign between the tropics and the extratropics and we show that this contrasting response can be understood in terms of different mechanisms by which the large-scale circulation responds to heating in the extratropics and in the tropics. Finally, we apply our framework in cloud resolving km-scale model simulations regionally and globally, which highlights the importance of radiative perturbations as well as a complex interplay of aerosol effects with the diurnal cycle of precipitation.