Climate change is one of the most pressing challenges of our time, requiring accurate predictions to guide effective mitigation and adaptation strategies. Central to this effort is the need to estimate and better understand the physical mechanisms behind effective radiative forcing (ERF) from anthropogenic activities. ERF encompasses both the instantaneous radiative forcing from external forcing agents, such as greenhouse gases and aerosols, and the subsequent radiative adjustments, particularly those involving cloud changes. A major and persistent source of uncertainty in ERF estimates is anthropogenic aerosols, especially absorbing aerosols.

In this paper, we present a hierarchy of convective-permitting simulations to investigate ERF and cloud adjustments to absorbing aerosols. This first-of-its-kind model hierarchy spans small-domain simulations (capturing local responses), large-domain simulations (representing convective aggregation and large-scale tropical circulation), and mock Walker simulations (accounting for geographically oriented, sea surface temperature gradient-driven circulation). Our results demonstrate that ERF is primarily driven by cloud adjustments and that the baseline cloud regime distribution plays a crucial role in determining ERF. Specifically, as the simulation scale shifts from small-domain to large-domain and to mock Walker setup, the baseline cloud regime transitions from ice-dominated to shallow-cloud-dominated. Since shallow clouds are more susceptible to absorbing aerosol perturbations, cloud adjustments—and consequently ERF—increase across this hierarchy, from small to large to mock Walker simulations.

Aerosol-cloud interactions are a persistent source of uncertainty in climate research. This study presents findings from a model intercomparison project examining the impact of aerosols on clouds and climate in convection permitting Radiative-Convective Equilibrium (RCE) simulations. Specifically, 11 different modeling teams conducted RCE simulations under varying aerosol concentrations, domain configurations, and sea surface temperatures (SSTs). We analyze the response of domain-mean cloud and radiative properties to imposed aerosol concentrations across different SSTs. Additionally, we explore the potential impact of aerosols on convective aggregation and large-scale circulation in large-domain simulations.

The results reveal that the cloud and radiative responses to aerosols vary substantially across models. However, a common trend across models, SSTs, and domain configurations is that increased aerosol loading tends to suppress warm rain formation, enhance cloud water content in the mid-troposphere, and consequently increase mid-tropospheric humidity and upper-tropospheric temperature, thereby impacting static stability. The warming of the upper troposphere can be attributed to reduced lateral entrainment effects due to the higher environmental humidity in the mid-troposphere. However, models do not agree on aerosol impacts on convective updraft velocity based on the preliminary examination of high-percentiles of vertical velocity at a single mid-troposheric layer (500hPa). In large-domain simulations, where convection tends to self-organize, aerosol loading does not consistently influence self-organization but tends to reduce the intensity of large-scale circulation forming between convective clusters and dry regions. This reduction in circulation intensity can be explained by the increase in static stability due to the upper tropospheric warming. 

Clouds are a key player in the global climate system, affecting the atmospheric water and energy budgets, and they are strongly coupled to the large-scale atmospheric circulation. Here we examine the co-variability of the atmospheric energy and water budget imbalances in three different global model configurations - radiative-convective equilibrium, aqua-planet and global simulations with land. The gradual increase in the level of complexity of the model configuration enables an investigation of the effects of rotation, meridional temperature gradient, land-sea contrast and seasonal cycle on the co-variability of the water and energy imbalances. We demonstrate how this co-variability is linked to both the large-scale tropical atmospheric circulation and to cloud properties. Hence, we propose a co-variability-based framework that connects cloud properties to the large-scale tropical circulation and climate system and is directly linked to the top-down constrains on the system – the water and energy budgets. In addition, we examine how the water and energy budget imbalances co-variability depends on the temporal averaging scale, and explain its dependency on how stationary the circulation is in the different model configurations. Finally, we demonstrate the effect of an idealized global warming and convective aggregation on this co-variability.