New Critical Length for the Onset of Self-Aggregation of Moist Convection

Abstract:

Convective self-aggregation (CSA) in an idealized radiative-convective equilibrium (RCE) framework is key to understanding the role of moist convection in the tropical weather and climate. To investigate the existence of characteristic length of CSA onset, we conducted systematic convection-resolving simulations, with a scope covering the horizontal domain size and resolution. Firstly, we reproduced the results of Muller and Held (2012) that CSA is unlikely to occur from a homogeneous initial field with smaller domains and/or finer resolutions. Next, by extending the domain size (from ~100 km to ~1000 km) while maintaining a high resolution (not coarser than 1 km), we found that CSA robustly occurs with a domain larger than ~500 km. Based on a number of experiments, we discovered the critical domain size which allowed CSA sharply changes at the resolution of ~2 km and converges toward ~500 km as the resolution increases. Our results update the RCE regime diagram consistently. Furthermore, we discuss the mechanism of CSA onset and the characteristic length existence, based on the competition between two near-surface horizontal divergent flows. While the flow induced by radiative cooling in the subsidence region acts as positive feedback for moisture perturbation and scales with the domain size, the other flow induced by evaporative cooling of falling rain in the convective region acts as negative feedback and does not scale. CSA occurs when the former positive feedback overwhelms the latter negative feedback. Our results support the theory of the homogenizing effect of cold pools proposed by Jeevanjee and Romps (2013). Additionally, the sharp change of the critical domain size at the resolution ~2 km can be interpreted by the resolution dependence of low-cloud amount and radiative cooling: the low-resolution experiments produce a larger amount of low-cloud, the enhanced radiative cooling strengthens the positive feedback, and the critical length becomes smaller than that of high-resolution experiments. This is consistent with the result of Muller and Bony (2015) and Yanase and Takemi (2018). Finally, this idealized study suggests the existence of characteristic length of moist convection organization at a meso-α spatial scale, whose role in real-world conditions should be clarified in future works.