Convection self‐aggregation in CNRM‐CM6‐1: Equilibrium and transition sensitivity to surface temperature

This study investigates the spontaneous self-aggregation of convection in non-rotating Radiative-Convective Equilibrium (RCE) simulations performed by the CNRM-CM6-1 General Circulation Model within the framework of the RCE Model Intercomparison Project (RCEMIP). In this model, the level of convection self-aggregation at equilibrium, as quantified by metrics based on moisture or moist static energy, strongly increases with sea surface temperature (SST). As it gets warmer, the troposphere gets drier, high cloud cover diminishes in dry regions, the top of high cloud rises and their thickness increases in moist regions, and low-cloud cover increases. At high SSTs, the large-scale circulation exhibits a shallow component, stronger than its deep counterpart. The transition toward self-aggregation has a similar first 20-day phase for all SSTs within the 295-305 K range. It primarily involves radiative positive feedback processes. Then, for SSTs above approximately 298 K, a new, slower, transition toward higher levels of self-aggregation occurs. It is concomitant with a shift from a top-heavy to a more bottom-heavy large-scale circulation, a strengthening of the shallow circulation and a reduced mobility of convective aggregates. This second transition is mostly driven by the dry regions, still involves longwave radiative positive feedbacks, but also advective positive feedbacks in the driest regions. It is argued that boundary-layer radiative cooling difference between moist and dry regions, which is stronger at high SSTs, is instrumental in this second phase of self-aggregation. The sensitivity of deep convection to environmental dry air also likely acts as a positive feedback on the system. Plain Language Summary In idealized configurations of the Earth, convective clouds can spontaneously organize into large clusters: this is convective self-aggregation. We investigate the sensitivity of this process to surface temperature in the atmospheric component of the state-of-the-art global climate model CNRM-CM6-1. For surface temperatures spanning a typical range of tropical conditions (295-305 K), the model exhibits an aggregated state when equilibrium is reached. As the surface gets warmer, convection is more aggregated, the troposphere gets drier, high clouds get less frequent in dry regions and low-cloud cover increases. When starting from homogeneous conditions, an initial rapid phase of self-aggregation occurs in all experimented sea surface temperatures. Radiative processes are instrumental in leading to self-aggregation. For warm surface temperature above approximately 298 K, a second, slower, transition occurs and leads to higher levels of self-aggregation. It is associated with an adjustment of the large-scale circulation, in which shallow circulations in the lower troposphere (surface-700 hPa) and between dry and moist regions strengthen. The radiative loss of energy within the boundary layer, and its unbalanced state between dry and moist regions after the initial transition is argued to be the main process at play.