Cloud-Top Relative Humidity Modulates Aerosol Effects Across Marine Cloud Regimes

Aerosol-cloud interactions are estimated to offset ∼1/4 of greenhouse gas-induced warming. However, significant uncertainty remains, largely due to the counteracting influence of fine and coarse aerosols as well as the contrasting responses of various cloud regimes. Using a decade-long data set of geostationary satellite-retrieved cloud properties, reanalysis aerosol and meteorological data, along with strict meteorological binning and bivariate regression, we quantify the regime-dependent susceptibilities of cloud properties and radiative effects (CRE) to fine aerosols (FA) and coarse sea-salt aerosols (CSA). FA consistently decreases droplet size, while enhancing albedo, cloud fraction, and net cooling across stratocumulus (MSC), trade-wind cumulus (Cu), and shallow tropical convection (STC). A 50% increase in FA mass concentrations induces average Net CRE changes of −14.7 W m⁻² (MSC), −5.1 W m⁻² (Cu), and −4.1 W m⁻² (STC), far exceeding CSA effects, which results in a cooling enhancement of less than −3 W m⁻² and even slight warming in STC. The magnitude and pathway of these regime-dependent responses are strongly modulated by cloud-top relative humidity (RH). Under dry cloud-top conditions (RH < 35%), liquid water path (LWP) decreases with increasing FA, as enhanced evaporation outweighs precipitation suppression. Under humid cloud-top conditions (RH > 70%), LWP consistently increases with FA across regimes, as suppressed evaporation allows precipitation suppression to accumulate cloud water. At intermediate humidity (35%–70%), MSC shows a non-monotonic LWP response, marking a transition between the two mechanisms. CSA effects show the opposite RH-dependent behavior. Thus, cloud-top RH plays a crucial role in modulating aerosol forcing and the efficacy of marine cloud brightening.