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The elimination of rain evaporation in the planetary boundary layer (PBL) has been found to lead to convective self‐aggregation (CSA) even without radiative feedback, but the precise mechanisms underlying this phenomenon remain unclear. We conducted cloud‐resolving simulations with two domain sizes and progressively reduced rain evaporation in the PBL. Surprisingly, CSA only occurred when rain evaporation was almost completely removed. The additional convective heating resulting from the reduction of evaporative cooling in the moist patch was found to be the trigger, thereafter a dry subsidence intrusion into the PBL in the dry patch takes over and sets CSA in motion. Temperature and moisture anomalies oppose each other in their buoyancy effects, hence explaining the need for almost total rain evaporation removal. We also found radiative cooling and not cold pools to be the leading cause for the comparative ease of CSA to take place in the larger domain. Plain Language Summary: Convective clouds are not randomly scattered across the sky but tend to clump together, a phenomenon known as convective self‐aggregation (CSA). The interaction between clouds and radiation is a key mechanism for CSA to occur. Curiously, CSA can still take place without this radiative feedback, provided that rain is prohibited from evaporating in the lowest layers of the atmosphere (∼1 km), called the planetary boundary layer (PBL). To investigate the physical processes behind this type of CSA (no‐evaporation CSA, or "NE‐CSA"), we ran high resolution atmospheric model simulations and reduced rain evaporation in steps in the PBL. We found that the additional heat resulting from the reduction of evaporative cooling is crucial in triggering NE‐CSA, thereafter the invasion of dry air into the PBL in the dry region takes over and intensifies aggregation. Surprisingly, allowing even a minuscule amount of rain to evaporate prevents NE‐CSA from taking place. This is because removing rain evaporation has two opposing effects on convection: heating and drying. The former aids convection while the latter hinders it. Only when rain evaporation is almost completely eliminated can the heating effect be powerful enough to overcome the drying effect and kick‐start NE‐CSA. Key Points: When rain evaporation is removed in the PBL, convective self‐aggregation (CSA) is triggered by convective heating of the moist regionsSurprisingly, CSA only occurs when rain evaporation is almost totally removed in the PBL, due to opposing temperature and moisture effectsCSA occurs more easily in a larger domain due to stronger radiatively induced subsidence, while cold pools play a less significant role [ABSTRACT FROM AUTHOR]

Previous research showed that radiative feedbacks are essential to the spontaneous development of convective aggregation (CSA) in idealized atmosphere models. We find that the contribution of radiative feedbacks decreases with warming and that, in warm climates, CSA occurs without radiative feedbacks. We perform 2D simulations in different climates using a cloud‐resolving model and use a layerwise moist static energy (LMSE) framework to quantify the contribution of radiative feedbacks to the increase of LMSE variance, which characterizes the development of CSA. The result shows that positive radiative contribution dominates the LMSE variance production when SST is below 300 K; when SST exceeds 300 K, adiabatic contribution shifts from negative to positive and dominates the variance production. Then we turn off radiative feedbacks by horizontally homogenizing radiative heating rates at all model levels. CSA still occurs in warmer climates (310–320 K). This result agrees with the LMSE diagnosis and additional 3D simulations. Plain Language Summary: Convective clouds often come together and combine to create larger storms. Previous research indicated that this aggregation is mainly driven by a feedback loop influenced by atmospheric radiation. However, our study reveals that this feedback mechanism becomes less effective as the climate warms. In particular, in a warmer climate where surface temperatures are around 10 or 20° higher than present, convective clouds can still aggregate even in the absence of this radiative feedback loop. These findings are derived from atmosphere simulations at high resolution and analyses of the variability of moist static energy at different altitudes. Key Points: Radiative feedbacks become less important to the development of convective self‐aggregation (CSA) with surface warmingIn warm climates, adiabatic production switches from negative to positive, becoming essential to the CSA developmentAt sea surface temperatures of 310 and 320 K, CSA can still occur without radiative feedbacks [ABSTRACT FROM AUTHOR]

Convective self‐aggregation in radiative‐convective equilibrium (RCE) is a phenomenon of significant interest because of its potential relevance to the organization of tropical storms. However, some previous theories on RCE instability neglected or simplified cloud‐radiative interaction in theoretical models and suggested that water vapor‐radiation feedback is responsible for destabilizing a homogeneous RCE state. Here, through numerical experiments of nonrotating RCE, we first confirm that the predictions from these theories are valid insomuch that dry patches can form in an initially homogeneous domain with uniform sea‐surface temperature when cloud‐radiation interaction is suppressed or eliminated. However, sustained growth of those dry patches is not guaranteed. Developing deep convective circulation can engulf dry patches, and water vapor feedback is not sufficient for overcoming this barrier. Only when the early‐stage cloud radiative forcing is sufficiently strong can self‐aggregation fully develop. Therefore, cloud‐radiative feedback is essential for the eventual realization of self‐aggregation in nonrotating RCE. Plain Language Summary: Radiative‐convective equilibrium (RCE) refers to a balanced atmospheric state with radiative cooling and moist convective heating, a representative idealization of the tropical atmosphere. In RCE simulations, a homogeneous initial state may spontaneously evolve into an inhomogeneous state in which convective clouds cluster in a moist region and are surrounded by dry air. This spontaneous development of organized convection is termed convective self‐aggregation and is thought to be relevant to the organization of tropical weather systems. Convective self‐aggregation starts with the formation of small dry patches in the simulation domain. Previous theories explained their emergence as the result of water vapor‐radiative interaction constrained by atmospheric dynamics. In this study, we find that those theories neglecting cloud‐radiative feedback are only partially correct. After the initial formation of dry patches, the developing deep convection in moist regions may engulf those dry patches due to the downgradient transport of moisture and energy. Only when the cloud‐radiative feedback is sufficiently strong can the barrier due to deep convective circulation be overcome. Thus, cloud‐radiative interaction is essential for the complete development of self‐aggregation and is of great practical importance because clouds are infamous for their uncertainties. Key Points: The initial emergence of dry patches in nonrotating radiative‐convective equilibrium is insensitive to cloud‐radiation interactionStrong cloud forcing is necessary for sustained dry patch growth and the realization of convective self‐aggregationCirculation feedback is initially negative and restrains self‐aggregation but becomes positive as dry patches grow to substantial sizes [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: In the tropics, deep convective clouds play significant roles in the redistribution of heat and water. They are often organized into hierarchically clustered forms, such as tropical cyclones. We investigated the condition for the onset of spontaneous clustering by a series of simulations in an idealized situation. We discovered that clusters appear if the horizontal domain size is larger than 500 km. Thus, behind a complex tropical atmospheric phenomenon, a characteristic length exists. The relationship between two near‐surface horizontal divergent flows explains the characteristic length existence; cloud clusters can occur when the flow induced by subsidence in the cloud‐free area overwhelms the flow induced by the cooling of rainfall evaporation in the cloudy area. Key Points: Convective self‐aggregation occurs in subkilometer cloud‐resolving simulations with domain sizes larger than 500 kmThe critical domain size allowing the onset of aggregation sharply varies across a grid spacing of approximately 2 kmThe competition of near‐surface horizontal moisture transport sets the critical length in the order of hundreds of kilometers [ABSTRACT FROM AUTHOR]

A new observational measure, the Morphological Index of Convective Aggregation (MICA), is developed to objectively detect the signs of convective self-aggregation on the basis of a simple morphological diagnosis of convective clouds in satellite imagery. The proposed index is applied to infrared imagery from the Meteosat-7 satellite and is assessed with sounding-array measurements in the tropics from Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY2011)/Dynamics of the Madden Julian Oscillation (MJO) (DYNAMO)/Atmospheric Radiation Measurement (ARM) MJO Investigation Experiment (AMIE). The precipitation events during the observational period are first classified by MICA into "aggregation events" and "nonaggregation events". The large-scale thermodynamics implied from the sounding-array data are then examined, with a focus on the difference between the two classes. The composite time series show that drying proceeds over 6 - 12 h as precipitation intensifies in the aggregation events. Such drying is unclear in the nonaggregation events. The moisture budget balance is maintained in very different manners between the two adjacent sounding arrays for the aggregation events, in contrast to the nonaggregation events that lack such apparent asymmetry. These results imply the potential utility of the proposed metrics for future studies in search of convective self-aggregation in the real atmosphere. [ABSTRACT FROM AUTHOR]