Convective self‐aggregation (CSA) in an idealized modeling framework is key to understanding the role of clouds. To investigate the existence of characteristic length of CSA onset, we conducted systematic cloud‐resolving simulations, with a scope covering the horizontal domain size and resolution. In the high‐resolution simulation, CSA can occur with a square domain larger than ~500 km. Based on the competition between two near‐surface horizontal divergent flows, we discuss the characteristic length existence. 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. The study suggests characteristic length existence for the organization of moist convection, even in real‐world conditions.

The organization of clouds has been widely studied by numerical modeling as an essential problem in climate science. Convective self-aggregation (CSA) occurs in radiative-convective equilibrium when the model domain size is sufficiently large. However, we have not yet reached a comprehensive understanding of the mechanism of CSA onset. This study argues that low-level circulation is responsible for horizontal moisture transport and that its coupling with variabilities of diabatic heating and moisture in the free troposphere is essential. We simulated scattered and aggregated convection by varying the domain size as a control parameter constraining the horizontal scale associated with the CSA onset. Based on a new analysis method quantifying the circulation spanning dry and moist regions, we found that 1) the upgradient moisture transport in the aggregated cases is associated with low-level circulation development, amplifying the horizontal moisture contrast, 2) the horizontal buoyancy gradient due to strong radiative cooling in the dry region intensifies the low-level circulation, 3) the free-tropospheric subsidence intrudes into the boundary layer in the dry region preceding the intensification of low-level circulation, and 4) the subsidence intrusion is due to a weakening of convective heating in the free troposphere associated with the moisture variability at a larger horizontal scale. This study provides new insights into the organization mechanism of clouds unifying the different mechanisms impacting CSA: the free-tropospheric moisture, radiation, convection, and low-level circulation.

This study investigates the representation of the diurnal variation of cumulus convection in radiative-convective equilibrium states in an area of 200 km by 200 km without large-scale forcing by using a non-hydrostatic model with sub-kilometer horizontal resolutions. The experiment with the horizontal resolution of 200 m successfully reproduced the diurnal variability of the trimodal characteristics of cumulus convection. We demonstrated that the horizontal resolution dependence largely affects the trimodal structure of clouds and the characteristics of precipitation and its diurnal variation. With the coarse resolution of 1600 m, a signature of convective aggregation appeared and the diurnal variation of convection was not clearly seen. We further examined the mechanisms for the diurnal variation of cumulus convection by focusing on the temporal and vertical variations of radiative and latent heating anomalies. The diurnal variability of the static stability caused by both radiative and latent heating plays a role in characterizing the diurnal variation of the cumulus convection.(Citation: Yanase, T., and T. Takemi, 2018: Diurnal variation of simulated cumulus convection in radiative-convective equilibrium. SOLA, 14, 116−120,

The nature of convective organization remains elusive, despite its importance in understanding the role of clouds in climate systems. This study reports a new type of large‐scale structure formed by the self‐organization of deep moist atmospheric convection in radiative‐convective equilibrium. To understand the natural behavior of convection unaffected by the computational domain, we conducted cloud‐resolving simulations by systematically increasing the horizontal domain size to approximately 25,000 km. We found that if the domain side length exceeded 5,000 km, the domain‐averaged thermodynamic fields and the horizontal characteristic length converged in quasi‐equilibrium, and the cloud aggregation area exhibited a mesh‐like pattern, analogous to the shallow convective organizations despite their different scales. Its characteristic length was estimated to be approximately 3,000–4,000 km. Further work is needed to understand how convective self‐aggregation simulated without the artificial domain constraint is related to the large‐scale organization of tropical convection in the real world.

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This study investigated a rainfall event under a typhoon influence using a 2D video disdrometer and weather radar observations to characterize raindrop size distribution (DSD) in a mixed convective and stratiform precipitating system. During the time period when both convective and stratiform rainfalls existed, the DSDs generally indicated a monotonically decreasing shape with increasing particle size, with a relatively gradual decrease at intermediate particle size observed at certain times; this feature is attributed to the combined effect of convective and stratiform rainfalls. During the transitional period between convective and stratiform rainfalls, the DSDs exhibited a bimodal shape. The DSDs were well approximated by a newly proposed gamma raindrop distribution combined with exponential (GRACE) distribution function, which was defined as the sum of the exponential distribution and the gamma distribution. A comparison of the volume ratio of the exponential and gamma components of the GRACE distribution revealed that the exponential component of the DSD was larger than the gamma component in the bimodal DSD. These results suggest that the DSD became bimodal during the period when stratiform rainfall predominated because of the weakening of convective rainfall. The GRACE distribution is useful for understanding cloud‐microphysical processes in mixed stratiform and convective precipitation conditions.