Tomoro Yanase

Abstract The intertropical convergence zone (ITCZ) is a central component of tropical climate, yet the conditions under which a tropical rain belt remains zonally extended or becomes unstable to zonal organization remain poorly understood. Here we investigate this problem using idealized non‐rotating kilometer‐scale simulations forced by a prescribed sea surface temperature (SST) distribution that varies only in the meridional direction. This setup produces an ITCZ‐like rain belt while allowing spontaneous zonal convective self‐aggregation (ZCSA) to emerge. A parameter sweep shows that ZCSA occurs preferentially when both the peak SST and the meridional SST amplitude are large. All ZCSA cases exhibit a temporary weakening of the meridional near‐surface inflow that maintains the zonally elongated rain belt. Boundary‐layer momentum and thermodynamic analyses show that this weakening is associated with enhanced lower‐tropospheric stability, which leads to a shallower boundary layer and stronger effective frictional damping of the meridional inflow. However, weak meridional inflow alone does not determine whether ZCSA develops. Cases that develop ZCSA are additionally characterized by a large meridional contrast in moist‐static‐energy forcing, implying a strong demand for meridional energy transport. During the transition, relatively dry zonal sectors develop larger moisture deficits together with stronger radiative cooling, suppressed convective latent heating, and stronger zonal surface divergence. In the mature ZCSA state, meridional transport of moist static energy (MSE) is also reorganized, including enhanced stationary‐eddy export from the warm region. These results suggest that the imposed SST distribution controls the susceptibility of an ITCZ‐like rain belt to zonal symmetry breaking through its effects on the meridional low‐level circulation and the large‐scale demand for meridional energy transport, while the transition itself involves coupled local changes in moisture, radiation, convection, and circulation.

Abstract Clouds play a central role in climate physics by interacting with precipitation, radiation, and circulation. Despite being a fundamental issue in convective organization, the self-aggregation of clouds lacks a theoretical explanation due to its complexity. In this study, we introduce an idealized mathematical model where the system’s state is represented solely by the vertically integrated water vapor content of atmospheric columns under the weak temperature gradient approximation. By analyzing the nonlinear dynamics of this simplified system, we mathematically elucidate the mechanisms that determine the onset of self-aggregation and the spatial scale of the self-aggregated state. Nonlocal coupling between atmospheric columns induces bistability, leading to dry and moist equilibria. This reflects the circulation effects driven by horizontal differential heating due to convection and radiation. The bistable self-aggregated state realizes when destabilization by nonlocal coupling, triggered by finite-amplitude disturbances in the uniform state, overcomes stabilization by diffusion. For globally coupled systems where all columns are equally coupled, perturbations with the maximum wavelength exhibit the highest growth rate. This results in a solution with an infinitely long wavelength, understood as the dynamical system’s heteroclinic trajectories describing the steady state’s spatial evolution. Conversely, for nonlocally coupled systems with finite filter lengths, perturbations with wavelengths close to the characteristic length of the coupling are preferred. The results reveal that the balance between nonlocal coupling and diffusion is essential for understanding convective self-aggregation. Moreover, this study suggests a physical similarity between convective self-aggregation and the moisture mode. Significance Statement Cloud self-organization is a longstanding fundamental problem in climate physics, and its representation in climate models may contribute to uncertainties in future climate projections. Clouds are complex phenomena, intricately connected to latent heat release during water phase changes, buoyancy-driven fluid motion, and interactions with radiation. As a result, their detailed modeling is ongoing. Efforts have also been undertaken to understand the macroscopic behavior of clouds through simple mathematical descriptions. In this study, we semianalytically reveal the mechanisms underlying the spontaneous clustering of clouds and the characteristic distances between clusters using an idealized mathematical model that describes the spatiotemporal variation of water vapor content in tropical atmospheric columns.

Accepted on Sep 8, 2022.

Accepted on Sep 7, 2022.