Forced Convective Aggregation

The phenomenon of convective aggregation in idealised radiative convective equilibrium simulations has the ability to change the mean state of its domain. When compared to non-aggregation conditions, these simulations usually have warmer drier mean atmospheres, with stronger precipitation in the convective areas. Many of these idealised experiments use a fixed sea surface temperature (SST), where higher temperatures generally increase the scale of aggregation. SST gradients have been shown to organise convection, yet there has been no work done to investigate the impact of heating perturbations in the air on the aggregation of convection. Here we investigate how strong diabatic heating of the atmospheric column affect the existence and properties of convective aggregation. These perturbations provide a link to studying the effect of large pollution plumes on convection, for example during the Indian monsoon season.

An aerosol model is used to insert plumes of strongly absorbing aerosols into aquaplanet, non-rotating, global RCE simulations. We study the sensitivity of the response to aerosol optical depth (AOD) and aerosol radiative properties under different SSTs.

Without any forcing, the simulations at low SST do not aggregate while at high SST they do. We also see that adding the forcing causes aggregation at both temperatures for a wide range of AODs. Detailed investigation shows that the diabatic heating source causes two circulations to develop, one with low-level convergence towards the plume and high-level divergence away from the plume. A secondary circulation works tangentially to the plume, again with low-level convergence and high-level divergence, driving the formation of several radial branches of aggregated convection. We argue that, as we see this aggregation for plumes with realistic AODs, this could be an analogue for real-world organisation during high pollution events. Future work will investigate the difference in mechanisms between forced and unforced convective aggregation as well as conducting similar experiments in smaller, cloud resolving domains.