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Convection self-aggregation in CNRM-CM6-1: equilibrium and transition sensitivity to surface temperature
  • David Coppin,
  • Romain Roehrig
David Coppin
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Romain Roehrig
CNRM, Université de Toulouse, Météo-France, CNRS

Corresponding Author:[email protected]

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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 towards 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 300 K, a new, slower, transition towards 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.