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

Abstract 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.