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<p>Aerosols influence clouds, and therefore Earth’s radiation budget, through processes that operate across multiple and interacting time scales, making aerosol-cloud interactions (ACI) a persistent source of uncertainty in estimates of effective radiative forcing (ERF). Here we examine the time-dependent response of the local, convection-focused ERF<sub>ACI</sub> using an ensemble of high-resolution simulations initialized from different atmospheric states and subjected to an instantaneous aerosol perturbation, together with simulations in which aerosol concentration changes with prescribed periods. We find that the transient ERF<sub>ACI</sub> during the first &#xa0;~&#xa0;2 days is positive, driven by rapid microphysical invigoration, enhanced high-cloud fraction, and increased longwave trapping. In contrast, the equilibrium ERF<sub>ACI</sub> becomes negative as upper-tropospheric warming increases static stability and reduces anvil cloud fraction. As a result, the time-mean forcing depends on the ratio between the environmental adjustment time scale (<i>τ</i><sub>adj</sub>) and the aerosol-perturbation time scale (<i>τ</i><sub>aer</sub>). For intermediate regimes, where <i>τ</i><sub>aer</sub> is only moderately longer than <i>τ</i><sub>adj</sub>, the system exhibits pronounced hysteresis: ERF<sub>ACI</sub> depends not only on the instantaneous aerosol loading but also on its recent history. These results imply that snapshot-based observational constraints and near-instantaneous-equilibrium convective parameterizations may systematically misestimate ERF<sub>ACI</sub>.</p>

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Opposing transient and equilibrium effective radiative forcing from aerosol-cloud interactions

  • 期刊论文

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  • 发表日期：

    2026年5月12日

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  • Guy Dagan

摘要

Aerosols influence clouds, and therefore Earth’s radiation budget, through processes that operate across multiple and interacting time scales, making aerosol-cloud interactions (ACI) a persistent source of uncertainty in estimates of effective radiative forcing (ERF). Here we examine the time-dependent response of the local, convection-focused ERFACI using an ensemble of high-resolution simulations initialized from different atmospheric states and subjected to an instantaneous aerosol perturbation, together with simulations in which aerosol concentration changes with prescribed periods. We find that the transient ERFACI during the first  ~ 2 days is positive, driven by rapid microphysical invigoration, enhanced high-cloud fraction, and increased longwave trapping. In contrast, the equilibrium ERFACI becomes negative as upper-tropospheric warming increases static stability and reduces anvil cloud fraction. As a result, the time-mean forcing depends on the ratio between the environmental adjustment time scale (τadj) and the aerosol-perturbation time scale (τaer). For intermediate regimes, where τaer is only moderately longer than τadj, the system exhibits pronounced hysteresis: ERFACI depends not only on the instantaneous aerosol loading but also on its recent history. These results imply that snapshot-based observational constraints and near-instantaneous-equilibrium convective parameterizations may systematically misestimate ERFACI.