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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]

Abstract 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.

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]

Abstract 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.

The atmosphere can self-organize into long-lasting large-scale overturning circulations over an ocean surface with uniform temperature. This phenomenon is referred to as convective self-aggregation and has been argued to be important for tropical weather and climate systems. Here we present a boundary layer centric framework based on the available potential energy budget of convective self-aggregation. We show that boundary layer diabatic processes dominate the available potential energy production and are, therefore, essential to convective self-aggregation. We further show that the enhanced virtual effect of water vapor can lead to convective self-aggregation.

We conduct radiative convection equilibrium experiments using a global nonhydrostatic model to investigate the dependence of convective self‐aggregation (CSA) on domain size and sea surface temperature (SST). We use a spherical domain with radii varying from 1 to 1/16 of Earth's radius, as well as SSTs between 290 and 310 K. A single aggregation occurs at small domain sizes, whereas multiple aggregations emerge at sufficiently large domain sizes. Domain‐averaged atmospheric temperature and humidity gradually increase with domain size until they reach convergence in cases of multiple aggregations. As domain size increases, surface wind speed increases, and the boundary layer becomes more humid. Because the radiative cooling in the free atmosphere also converges in cases of multiple aggregations, the surface evaporation must be limited due to the constraint of the energy balance. Thus, the convective region shifts from single to multiple aggregations to avoid increasing the surface wind speed. Our results show that the dependence of the horizontal scale of CSA on SST is not monotonic. This dependency is closely related to changes in the structure of the detrainment from the convective region at the melting level, resulting in enhanced radiative cooling at the top of the boundary layer near the convective region for cooler and warmer SSTs. This change in the circulation structure leads to increased surface wind speed with increasing domain size. This process affects the non‐monotonic dependence of the horizontal scale of CSA on SST. Plain Language Summary: Convective activity and its aggregation play an essential role in the tropical climate. Convective activity sometimes organizes into massive structures, such as the Madden‐Julian oscillation or tropical cyclones. We used idealized radiative‐convective equilibrium experiments over a sphere to examine the horizontal scale of convective aggregation and its dependence on sea surface temperature (SST). The results show that aggregation shifts from a single convective region to multiple regions at a sufficiently large domain size. The domain‐averaged temperature and humidity increase until convergence in cases with multiple convective regions. Surface wind speed increases with domain size for cases with a single convective region. The energy balance constrains the upper limit of the horizontal scale of a single convective region. The horizontal scale of convective aggregation changes with SST, but not monotonically; it reaches a maximum at an SST of around 300 K. The horizontal scale of convective aggregation becomes smaller when radiative cooling at the topo of boundary layer is stronger near the convective region by shifting the melting level. Key Points: The size limit of a single convective aggregate is constrained by the energy balance and by the dependence of surface winds on domain sizeThe horizontal scale of the convective region does not change monotonically with sea surface temperature (SST)The dependence of the horizontal scale of convective self‐aggregation on SST is related to radiative cooling near the melting layer [ABSTRACT FROM AUTHOR]

Abstract We conduct radiative convection equilibrium experiments using a global nonhydrostatic model to investigate the dependence of convective self‐aggregation (CSA) on domain size and sea surface temperature (SST). We use a spherical domain with radii varying from 1 to 1/16 of Earth's radius, as well as SSTs between 290 and 310 K. A single aggregation occurs at small domain sizes, whereas multiple aggregations emerge at sufficiently large domain sizes. Domain‐averaged atmospheric temperature and humidity gradually increase with domain size until they reach convergence in cases of multiple aggregations. As domain size increases, surface wind speed increases, and the boundary layer becomes more humid. Because the radiative cooling in the free atmosphere also converges in cases of multiple aggregations, the surface evaporation must be limited due to the constraint of the energy balance. Thus, the convective region shifts from single to multiple aggregations to avoid increasing the surface wind speed. Our results show that the dependence of the horizontal scale of CSA on SST is not monotonic. This dependency is closely related to changes in the structure of the detrainment from the convective region at the melting level, resulting in enhanced radiative cooling at the top of the boundary layer near the convective region for cooler and warmer SSTs. This change in the circulation structure leads to increased surface wind speed with increasing domain size. This process affects the non‐monotonic dependence of the horizontal scale of CSA on SST.

In radiative-convective equilibrium (RCE), radiative cooling of the troposphere is roughly balanced by the vaporization enthalpy set free by precipitating moist convection. Many earlier studies restricted the investigation of RCE to the dynamics of the atmosphere with constant boundary conditions including prescribed surface temperature. We investigate a GCM setup where a slab ocean is coupled to the atmosphere, and we explore a wide range of CO2 concentrations. We obtain reliable statistical quantities from thousand-year-long simulations. For moderate CO2 concentrations, we find unskewed temporal variations of 1–2 K in global mean surface temperature, with an almost constant climate sensitivity of 2 K. At CO2 concentrations beyond four times the preindustrial value, the climate sensitivity decreases to nearly zero as a result of episodic global cooling events as large as 10 K. The dynamics of these cooling events are investigated in detail and shown to be associated with an increase in large-scale low-level stratiform cloudiness in the subsiding region, which is a result of penetrative shallow convection being capped by an inversion and thus not ventilating the lower troposphere. These dynamics depend on the CO2 concentration: both through the effect of temperature on stratification and through the changing spatial scale of organization of the flow, which determines the spatial scale and temporal coherence of the stratiform cloud sheets.

Abstract Deep convection over tropical oceans often appears intensified at the edge of convectively active regions, both in idealized studies and in observations. This edge intensification of convection is studied in detail here, using the steady state of a radiative‐convective equilibrium study, marked by a single convective cluster with deep convection intensified at the edge of this cluster. The cause for edge intensification and its dependence on the cluster area is investigated by comparing the spatial distribution of deep convection to different variables known to be important for convection. Analysis of the simulation suggests that the edge is marked by an increased probability for the triggering of convection rather than by stronger updrafts. In particular, while the edge of the moist region is not thermodynamically more favorable, we find strong surface convergence and therefore dynamical lifting at this edge. The surface convergence is shown to result from two opposing flows. On the one hand, there is, as expected from previous radiative‐convective equilibrium simulations, a low‐level inflow directed toward the moist region. On the other hand, there is a positive density anomaly at the surface which is the result of continuously forming cold pools within the convectively active region, creating a super‐cold‐pool. As the velocity of the low‐level inflow approximately matches the potential propagation speed of the super‐cold‐pool boundary, these opposing flows explain the presence of strong convergence at the edge of this region. Whether the resulting lifting induces the formation of deep convection is shown to depend on the large‐scale instability.

An ensemble of 10 radiative‐convective equilibrium (RCE) simulations near the sharp transition zone between scattered and aggregated states are examined in a square‐domain cloud‐resolving model. Surprisingly, the occurrence of self‐aggregation is not deterministic near the boundary line of the two states, with six runs reaching aggregated states and four runs returning to scattered states. Spatial autocorrelation length analysis reveals that the development of moisture contrast in the boundary layer (BL) is the key indicator for self‐aggregation. To reach aggregated RCE states, a part of the BL needs to be dry and extensive sufficiently to suppress convection triggered by collisions of intruding cold pools. Furthermore, the relation between moisture aggregation and convective organization is elucidated. In a shorter time‐scale, convective organization is governed by cold pool dynamics. Meanwhile, large‐scale moisture determines the region of active convection for time‐scale longer than 2 days. Plain Language Summary: Radiative‐convective equilibrium (RCE) describes a balance between the cooling of the atmosphere by radiation and the heating of that through latent heat release and surface heat fluxes. Under certain conditions, initially scattered convection and moisture can spontaneously organize into one or more spatially coherent clusters in RCE simulations. Understanding this spontaneous clumping phenomenon helps better understand the organizing processes of clouds in the tropics. Here, we investigate the behaviors of 10 RCE simulations using a model setting that marginally separates aggregated and scattered RCE states. Clear contrast between wet and dry regions is established in six simulations, but the other four return to a scattered situation. This result suggests that the marginal boundary between the aggregated and the scattered regimes might have a range of probability rather than a deterministic line. Key Points: Occurrence of self‐aggregation is not deterministic near the marginal boundary between scattered and aggregated regimesDevelopment of moisture contrast within the boundary layer is the key indicator for the occurrence of self‐aggregationConvective organization and moisture inhomogeneity do not evolve synchronously [ABSTRACT FROM AUTHOR]

In radiative‐convective equilibrium simulations, convective self‐aggregation (CSA) is the spontaneous organization into segregated cloudy and cloud‐free regions. Evidence exists for how CSA is stabilized, but how it arises favorably on large domains is not settled. Using large‐eddy simulations, we link the spatial organization emerging from the interaction of cold pools (CPs) to CSA. We systematically weaken simulated rain evaporation to reduce maximal CP radii, Rmax, and find reducing Rmax causes CSA to occur earlier. We further identify a typical rain cell generation time and a minimum radius, Rmin, around a given rain cell, within which the formation of subsequent rain cells is suppressed. Incorporating Rmin and Rmax, we propose a toy model that captures how CSA arises earlier on large domains: when two CPs of radii ri,rj∈[Rmin,Rmax] collide, they form a new convective event. These findings imply that interactions between CPs may explain the initial stages of CSA. Plain Language Summary: Convective self‐aggregation (CSA) describes the emergence of persistently dry, cloud‐free areas in numerical simulations. It has been suggested as a possible mechanism for tropical cyclone formation and large‐scale events such as the Madden‐Julian Oscillation. Some understanding of the persistence of CSA exists. However, how CSA initially emerges remains poorly understood. Recently, the dynamics of cold pools (CPs) have been associated with the organization of convective events. CPs are radially expanding pockets of dense air that form under precipitating thunderstorms. In this work, we ask how weakening CPs could facilitate the emergence of CSA. By analyzing high‐resolution numerical simulations, we show that reducing rain evaporation shortens the time before CSA starts. These simulations demonstrate that CPs reach greater radii when rain evaporation is large. Besides, we find that new convective events occur near the point where two CPs collide. Finally, we report a minimum CP radius within which CPs are too negatively buoyant to initialize new convective events. Building on these numerical findings, we propose a simple idealized mathematical model that approximates CPs as expanding and colliding circles. We show that this model can capture the emergence of CSA. We conclude that the lack of CPs facilitates CSA. Key Points: Smaller cold pool radii in large‐eddy simulations diminish the time to reach convective self‐aggregationWe report cold pools' generation time and suppression radius by evaluating the distance between rain events connected in timeA mathematical model captures the effect of domain size, suppression radius, and maximum cold pool radius in convective self‐aggregation [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]

Abstract A 3‐D cloud‐resolving model has been used to investigate the domain size dependence of simulations of convective self‐aggregation (CSA) in radiative‐convective equilibrium. We investigate how large a domain is needed to allow multiple convective clusters and also how the properties equilibrated CSA depend on domain size. We used doubly periodic square domains of widths 768, 1,536, 3,072, and 6,144 km, over 350 simulated days. In the 768‐, 1,536‐, and 3,072‐km domains, the simulations produced circular convective clusters surrounded by broader regions of dry, subsiding air. In the 6,144‐km domain, the convection ultimately forms two semiconnected bands. As the domain size increases, equilibrated CSA moistens in two ways. First, as the circulation widens, this leads to stronger boundary layer winds and a more humid boundary layer. Second, the stronger inflow into the convective region boundary layer is associated with a warmer convective region boundary layer, which leads to intensified deep convection, more melting and freezing near the freezing level, enhanced midlevel stability, increased congestus activity, and detrainment of moist air into the dry region. In the larger domains, the deep convection and congestus slowly oscillate out of phase with each other with a time period of about 25 to 30 days. We hypothesize that other important domain size sensitivities, including a decrease in net moist static energy export from the convective region, are fundamentally linked to the increasing relationship between domain size and boundary layer wind speed. Our results suggest that the statistics of CSA converge only for domains wider than about 3,000 km.

In radiative-convective equilibrium (RCE), radiative cooling of the troposphere is roughly balanced by the vaporization enthalpy set free by precipitating moist convection. Many earlier studies restricted the investigation of RCE to the dynamics of the atmosphere with constant boundary conditions including prescribed surface temperature. We investigate a GCM setup where a slab ocean is coupled to the atmosphere, and we explore a wide range of CO2 concentrations. We obtain reliable statistical quantities from thousand-year-long simulations. For moderate CO2 concentrations, we find unskewed temporal variations of 1–2 K in global mean surface temperature, with an almost constant climate sensitivity of 2 K. At CO2 concentrations beyond four times the preindustrial value, the climate sensitivity decreases to nearly zero as a result of episodic global cooling events as large as 10 K. The dynamics of these cooling events are investigated in detail and shown to be associated with an increase in large-scale low-level stratiform cloudiness in the subsiding region, which is a result of penetrative shallow convection being capped by an inversion and thus not ventilating the lower troposphere. These dynamics depend on the CO2 concentration: both through the effect of temperature on stratification and through the changing spatial scale of organization of the flow, which determines the spatial scale and temporal coherence of the stratiform cloud sheets. [ABSTRACT FROM AUTHOR]

Abstract The Radiative‐Convective Equilibrium Model Intercomparison Project (RCEMIP) consists of simulations at three fixed sea‐surface temperatures (SSTs: 295, 300, and 305 K) and thus allows for a calculation of the climate feedback parameter based on the change of the top‐of‐atmosphere radiation imbalance. Climate feedback parameters range widely across RCEMIP, roughly from −6 to 3 W m−2 K−1, particularly across general‐circulation models (GCMs) as well as global and large‐domain cloud‐resolving models (CRMs). Small‐domain CRMs and large‐eddy simulations have a smaller range of climate feedback parameters due to the absence of convective self‐aggregation. More than 70–80% of the intermodel spread in the climate feedback parameter can be explained by the combined temperature dependencies of convective aggregation and shallow cloud fraction. Low climate sensitivities are associated with an increase of shallow cloud fraction (increasing the planetary albedo) and/or an increase in convective aggregation with warming. An increase in aggregation is associated with an increase in outgoing longwave radiation, caused primarily by mid‐tropospheric drying, and secondarily by an expansion of subsidence regions. Climate sensitivity is neither dependent on the average amount of aggregation nor on changes in deep/anvil cloud fraction. GCMs have a lower overall climate sensitivity than CRMs because in most GCMs convective aggregation increases with warming, whereas in CRMs, convective aggregation shows no consistent temperature trend.

The Radiative‐Convective Equilibrium Model Intercomparison Project (RCEMIP) consists of simulations at three fixed sea‐surface temperatures (SSTs: 295, 300, and 305 K) and thus allows for a calculation of the climate feedback parameter based on the change of the top‐of‐atmosphere radiation imbalance. Climate feedback parameters range widely across RCEMIP, roughly from −6 to 3 W m−2 K−1, particularly across general‐circulation models (GCMs) as well as global and large‐domain cloud‐resolving models (CRMs). Small‐domain CRMs and large‐eddy simulations have a smaller range of climate feedback parameters due to the absence of convective self‐aggregation. More than 70–80% of the intermodel spread in the climate feedback parameter can be explained by the combined temperature dependencies of convective aggregation and shallow cloud fraction. Low climate sensitivities are associated with an increase of shallow cloud fraction (increasing the planetary albedo) and/or an increase in convective aggregation with warming. An increase in aggregation is associated with an increase in outgoing longwave radiation, caused primarily by mid‐tropospheric drying, and secondarily by an expansion of subsidence regions. Climate sensitivity is neither dependent on the average amount of aggregation nor on changes in deep/anvil cloud fraction. GCMs have a lower overall climate sensitivity than CRMs because in most GCMs convective aggregation increases with warming, whereas in CRMs, convective aggregation shows no consistent temperature trend. Plain Language Summary: To determine how much Earth will warm in response to anthropogenic greenhouse gas emissions, we need to understand the atmospheric response to this forcing. The amount of warming in response to a given forcing is called climate sensitivity. Although global climate models are a useful tool to estimate climate sensitivity, estimates remain uncertain, in particular because the response of tropical clouds to warming is uncertain. The weakness of climate models is their coarse grid spacing, with which they cannot resolve important aspects of the weather like clouds and convection. In this study, we use a popular idealization for the tropics, the radiative‐convective equilibrium setup, to compare climate sensitivities across a wide range of models including global climate models and cloud‐resolving models. We find that more than 70–80% of variations in climate sensitivity across these models result from changes in shallow cloud fraction and changes in the spatial organization of convection with warming. Our results indicate that climate sensitivity might be underestimated by global climate models, in which the amount of spatial organization of convection mostly increases with warming, leading to low climate sensitivities, while the cloud‐resolving models show no consistent trend in spatial organization, and thus have higher climate sensitivities. Key Points: Climate feedback parameter (λ) is impacted by change of convective aggregation with temperature, not by its average valueCombined temperature dependencies of convective aggregation and shallow cloud fraction explain 70–80% of intermodel spread in λλ is more negative in GCMs than in CRMs because only GCMs show a mean increase of convective aggregation with warming [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]

Deep convection over tropical oceans often appears intensified at the edge of convectively active regions, both in idealized studies and in observations. This edge intensification of convection is studied in detail here, using the steady state of a radiative‐convective equilibrium study, marked by a single convective cluster with deep convection intensified at the edge of this cluster. The cause for edge intensification and its dependence on the cluster area is investigated by comparing the spatial distribution of deep convection to different variables known to be important for convection. Analysis of the simulation suggests that the edge is marked by an increased probability for the triggering of convection rather than by stronger updrafts. In particular, while the edge of the moist region is not thermodynamically more favorable, we find strong surface convergence and therefore dynamical lifting at this edge. The surface convergence is shown to result from two opposing flows. On the one hand, there is, as expected from previous radiative‐convective equilibrium simulations, a low‐level inflow directed toward the moist region. On the other hand, there is a positive density anomaly at the surface which is the result of continuously forming cold pools within the convectively active region, creating a super‐cold‐pool. As the velocity of the low‐level inflow approximately matches the potential propagation speed of the super‐cold‐pool boundary, these opposing flows explain the presence of strong convergence at the edge of this region. Whether the resulting lifting induces the formation of deep convection is shown to depend on the large‐scale instability. Key Points: Precipitation tends to intensify at the edge of the convectively active region if the region is large enoughEdge intensification in a RCE simulation with a single convective cluster cannot be explained by spatial variations of CAPE or moistureEdge intensification in this case is linked to the formation of a super‐cold‐pool in the convective cluster balanced by a low‐level inflow [ABSTRACT FROM AUTHOR]

A 3‐D cloud‐resolving model has been used to investigate the domain size dependence of simulations of convective self‐aggregation (CSA) in radiative‐convective equilibrium. We investigate how large a domain is needed to allow multiple convective clusters and also how the properties equilibrated CSA depend on domain size. We used doubly periodic square domains of widths 768, 1,536, 3,072, and 6,144 km, over 350 simulated days. In the 768‐, 1,536‐, and 3,072‐km domains, the simulations produced circular convective clusters surrounded by broader regions of dry, subsiding air. In the 6,144‐km domain, the convection ultimately forms two semiconnected bands. As the domain size increases, equilibrated CSA moistens in two ways. First, as the circulation widens, this leads to stronger boundary layer winds and a more humid boundary layer. Second, the stronger inflow into the convective region boundary layer is associated with a warmer convective region boundary layer, which leads to intensified deep convection, more melting and freezing near the freezing level, enhanced midlevel stability, increased congestus activity, and detrainment of moist air into the dry region. In the larger domains, the deep convection and congestus slowly oscillate out of phase with each other with a time period of about 25 to 30 days. We hypothesize that other important domain size sensitivities, including a decrease in net moist static energy export from the convective region, are fundamentally linked to the increasing relationship between domain size and boundary layer wind speed. Our results suggest that the statistics of CSA converge only for domains wider than about 3,000 km. Key Points: Convection aggregates into one cluster for square domain widths of 768, 1,536, and 3,072, and two bands for domain width of 6,144 kmAs the domain size increases, the dry region moistens and the convection oscillates with a period of about 25 to 30 daysThe domain size sensitivities are centrally related to an increase in boundary layer wind speed as the circulation width increases [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]

The behavior of tropical extreme precipitation under changes in sea surface temperatures (SSTs) is investigated with the Weather Research and Forecasting Model (WRF) in three sets of idealized simulations: small-domain tropical radiative-convective equilibrium (RCE), quasi-global 'aquapatch', and RCE with prescribed mean ascent from the tropical band in the aquapatch. We find that, across the variations introduced including SST, large-scale circulation, domain size, horizontal resolution, and convective parameterization, the change in the degree of convective organization emerges as a robust mechanism affecting extreme precipitation. Higher ratios of change in extreme precipitation to change in mean surface water vapor are associated with increases in the degree of organization, while lower ratios correspond to decreases in the degree of organization. The spread of such changes is much larger in RCE than aquapatch tropics, suggesting that small RCE domains may be unreliable for assessing the temperature-dependence of extreme precipitation or convective organization. When the degree of organization does not change, simulated extreme precipitation scales with surface water vapor. This slightly exceeds Clausius-Clapeyron (CC) scaling, because the near-surface air warms 10-25% faster than the SST in all experiments. Also for simulations analyzed here with convective parameterizations, there is an increasing trend of organization with SST. [ABSTRACT FROM AUTHOR]

Radiative-convective equilibrium simulations with the general circulation model ECHAM6 are used to explore to what extent the dependence of large-scale convective self-aggregation on sea-surface temperature (SST) is driven by the convective parameterization. Within the convective parameterization, we concentrate on the entrainment parameter and show that large-scale convective self-aggregation is independent of SST when the entrainment rate for deep convection is set to zero or when the convective parameterization is removed from the model. In the former case, convection always aggregates very weakly, whereas in the latter case, convection always aggregates very strongly. With a nontrivial representation of convective entrainment, large-scale convective self-aggregation depends nonmonotonically on SST. For SSTs below 295 K, convection is more aggregated the smaller the SST because large-scale moisture convergence is relatively small, constraining convective activity to regions with high wind-induced surface moisture fluxes. For SSTs above 295 K, convection is more aggregated the higher the SST because entrainment is most efficient in decreasing updraft buoyancy at high SSTs, amplifying the moisture-convection feedback. When halving the entrainment rate, convection is less efficient in reducing updraft buoyancy, and convection is less aggregated, in particular at high SSTs. Despite most early work on self-aggregation highlighted the role of nonconvective processes, we conclude that convective self-aggregation and the global climate state are sensitive to the convective parameterization. [ABSTRACT FROM AUTHOR]

Positive feedbacks between precipitable water, reduced radiative cooling and enhanced surface fluxes promote convective self-aggregation in limited-area cloud-resolving model (CRM) simulations over uniform sea-surface temperature (SST). Near-global aquaplanet simulations with 4 km horizontal grid spacing and no cumulus or boundary layer parameterization are used to test the importance of these feedbacks to realistically organized tropical convection. A 20,480 × 10,240 km equatorially centered channel with latitudinally varying SST is used. Realistic midlatitude and tropical cloud structures develop. The natural zonal variability of humidity and convection are studied in a 30 day control simulation. The temporal growth of a small white-noise humidity perturbation and intrinsic predictability implications are explored. Atmospheric column budgets of moist-static energy (MSE) quantify its covariation with precipitation, surface heat flux, and radiative energy loss. Zonal Fourier analysis partitions these budgets by length scale. Radiative feedbacks on MSE natural variability and perturbation growth are found to be positive, broadly similar across scales, and comparable to limited-area CRMs, capable of e-folding a column MSE perturbation in 6-14 days. Surface fluxes are highest in synoptic-scale dry intrusions, inhibiting aggregation by damping tropical MSE perturbations. Sub-4-day MSE variations are due mainly to advection. Both tropics and midlatitudes have large-scale intrinsic predictability horizons of 15-30 days. An identical simulation but with 20 km grid spacing has more mesoscale variability and low cloud. [ABSTRACT FROM AUTHOR]