Cloud fraction response to aerosol driven by nighttime processes

[Geoengineering News]

https://www.pnas.org/doi/full/10.1073/pnas.2509949122

Authors: Geoffrey Pugsley, Edward Gryspeerdt, and Vishnu Nair

November 21, 2025

https://doi.org/10.1073/pnas.2509949122

Significance

The effect of airborne particulates-called aerosols-on climate is highly uncertain due to their complex interactions with clouds. A significant source of this uncertainty comes from the aerosol influence on large, low-lying clouds over the oceans, known as stratocumulus. These clouds are known to behave differently between day and night, yet most previous observational studies have focused on the daytime. This study shows that the aerosol impact on stratocumulus is strongly time-dependent, with cloud fraction changes primarily driven by nighttime processes. These results highlight the need for more observations of nighttime cloud behavior and a better representation of the diurnal cycle in models, particularly when considering the impact of marine cloud brightening.

Abstract

Aerosol–cloud interactions remain one of the largest uncertainties in the anthropogenic forcing of the climate; a significant contribution to this is due to the aerosol effect on the development of cloud fraction and liquid water path in stratocumulus clouds. Stratocumulus are strongly modulated by the diurnal cycle, but many previous observational studies have primarily focused on the daytime behavior of these clouds. In this work, a Lagrangian framework is used to characterize the day-night variation in the cloud sensitivity to aerosol. It is shown that the cloud fraction response to aerosol is driven by nighttime processes, whereas aerosols play a lesser role in daytime cloud fraction breakup. The liquid water path response reveals that aerosols act to thin the cloud during the daytime; however, this effect is partially offset by other processes during the nighttime. These nighttime cloud processes play an important role in setting the cloud state at the start of the day and hence the daytime cloud evolution, during which stratocumulus clouds have the greatest radiative impact. Our findings are consistent with an aerosol induced suppression of precipitation that acts most effectively at night, when stratocumulus precipitation is strongest. These results highlight a requirement for nighttime observations of marine clouds and an improved representation of the diurnal cycle in model-observation comparisons, especially when assessing climate forcing and the viability of marine cloud brightening.

Source: PNAS