Does the aggregation of tropical deep convection cause a negative cloud feedback in global storm resolving models? 

Global circulation models (GCMs) are too coarse to resolve tropical deep convection and convective aggregation, i.e., the clustering of deep convective cells that leads to the formation of mesoscale convective systems. The need for convective parameterizations leads to high inter-model variability in how convective aggregation responds to surface warming, participating in the uncertainty surrounding cloud feedbacks. Realistic kilometer-scale simulations with Global Storm Resolving Models (GSRMs) have recently been run without the need for convective parameterizations under climate change scenarios. We analyze such simulations from the ICON and IFS models and compare them to observations to evaluate whether tropical convective aggregation changes in a warming world. Using the Iorg aggregation metric, we show that simulated tropical convective aggregation becomes increasingly realistic with enhanced horizontal resolution, and that tropical deep convection becomes more aggregated with surface temperatures under uniform warming and under interannual temperature increases. Because convective aggregation helps to cool down the atmosphere through enhanced clear-sky longwave cooling, this could imply that convective aggregation causes a negative climate feedback. However, long-term climate trends of Iorg are less unequivocal, in both observations and models, and results are very sensitive to the method for Iorg computation. This suggests that process-level studies are needed to better understand what drives convective aggregation in the Tropics.