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

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2025JD045791

Authors: Fan Liu, Feiyue Mao, Zengxin Pan, Daniel Rosenfeld, Wei Gong, Lin Zang

First published: 03 April 2026

https://doi.org/10.1029/2025JD045791

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.

Plain Language Summary

Aerosols, tiny particles in the atmosphere, interact with clouds and influence Earth's climate. These interactions can either cool or warm the planet depending on the type of aerosol and cloud. This study focuses on two types of aerosols: fine aerosols (FA) with radii between 0.05 and 1 μm, and coarse sea-salt aerosols (CSA) with radii larger than 1 μm. By analyzing satellite observations from various warm cloud regimes over the oceans, including marine stratocumulus, trade-wind cumulus, and shallow tropical convection, we found that FA generally cools the Earth by making clouds brighter, particularly in humid conditions. In contrast, CSA has a weaker cooling effect and may even cause slight warming in humid shallow tropical convection regions. The way clouds respond to these aerosols is largely modulated by the cloud-top relative humidity. The cloud physics behind these aerosol-cloud interactions involves the balance between cloud droplet formation, cloud cover, albedo (reflectivity), precipitation, and evaporation processes, all of which are shaped by the aerosol size and the moisture content above the cloud. These findings enhance our understanding of how aerosols affect clouds and help reduce uncertainties in predicting their role in climate change.

Source: AGU