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Abstract

The organization of deep moist convection is considered to play an important role in the water and energy cycles in the tropical atmosphere. The nature of convective organization has been studied by idealized numerical simulations of clouds in radiative–convective equilibrium. However, because the behavior of simulated clouds is significantly affected by the computational domain, natural characteristics inherent in the moist convective system are not fully understood. To investigate the natural horizontal scale and horizontal form of the large-scale organization of deep moist convection, a series of numerical simulations were performed to systematically extend the horizontal domain size from about 1000 km to about 25000 km. When the domain size was larger than approximately 5000 km, domain mean variables in statistical equilibrium converged to identical values. In the wide domain simulations, the humid–cloudy area exhibited a mesh-like horizontal pattern analogous to shallow convective organizations. The horizontal length of the organization was approximately 3000–4000 km based on spectral analyses. Further, the factors responsible for determining the length scale were diagnosed based on a previous theory focused on boundary layer dynamics. The diagnosis suggests that the boundary layer buoyancy field is closely related to the length scale. Further study using a hierarchy of models is needed to bridge the self-aggregation of clouds in the idealized atmosphere and the convective organization in the real-world atmosphere.