Aggregation-Disaggregation Cycles in ERA5

Understanding convective aggregation is very important for understanding tropical climate and climate sensitivity. However, we still lack a full understanding of how aggregation evolves in the real world or what real world phenomena can be the counter parts to the phenomena of self-aggregation observed in idealized models. In this study, we apply the moist static energy (MSE) variance budget framework to ERA 5 reanalysis data to study the evolution of large-scale aggregation in the real world.

We develop a novel phase space diagnostic that highlights the cyclic nature of real world aggregation, compared to the quasi-stationary features of the idealized aggregated state. We visualize real world aggregation as evolving in the form of anomalies that are evolving about a mean state forming aggregation - disaggregation cycles.

In this framework, we find horizontal advection to play the primary role in determining when the domain aggregates or disaggregates in the real world. In contrast, all advective, radiative and surface flux feedbacks are found important for determining the magnitude of the aggregation anomalies. Surface fluxes and horizontal advection tend to dampen aggregation anomalies, while radiative fluxes and vertical advection tend to amplify aggregation anomalies. Looking deeper into the advection terms, we find that changes in vertical advection are dominated by an enhanced low level subsidence over the dry columns during the more aggregated states which creates an anomalous drying tendency for the dry columns. In contrast, horizontal advection changes are found to be dominated by increased moisture advection out of the moist columns with stronger aggregation.