Impact of solar radiation on aerosol-cloud interactions in thin stratocumulus clouds

Abstract. This study examines the role of solar radiation in the effect of aerosols on liquid-water path (LWP) in thin, marine stratocumulus clouds with LWP of ~50 g m⁻² or less by performing four sets of simulations with different solar radiation. Each set is composed of a simulation with present-day (PD) aerosols and a simulation with preindustrial (PI) aerosols. As solar radiation increases, decoupling within the marine boundary layer (MBL) becomes stronger, leading to less condensation and less LWP and thus the absence of the surface precipitation. This enables the evaporation of rain to affect the cloud-base instability. As rain evaporation increases due to more conversion of cloud liquid to rain in the PI case, the cloud-base instability increases and thus updrafts increase which leads to larger LWP in the PI case than in the PD case. In the cases with no surface precipitation, when solar radiation decreases and thus decoupling becomes weaker, rain evaporation and cloud-base instability become larger, which increases the LWP more with PI aerosols than with PD aerosols. As solar radiation decreases further, condensation and, thus, the LWP increase, which leads to the presence of the surface precipitation. This stabilizes the entire MBL and thus prevents the interactions that cause the evaporation of rain to enhance the cloud-base instability. In cases with the surface precipitation, the in-cloud interactions among cloud droplet number concentration (CDNC), supersaturation, and updrafts play an important role in the effect of aerosols on the LWP; these in-cloud interactions produce larger LWP with the PD aerosols than with the PI aerosols. In a case with lower solar radiation and with surface precipitation, weaker decoupling induces stronger in-cloud interactions, which results in larger increases in LWP with PD aerosols compared to PI aerosols than that in a case with higher solar radiation.

The results of this study demonstrate that solar radiation can act as an important environmental factor by inducing a large variation in the LWP and by changing the sign of aerosol effects on the LWP of thin stratocumulus clouds. Hence, the effect of solar radiation on decoupling and thus on the feedbacks between microphysics and dynamics needs to be included in climate models for a better prediction of the effect of aerosols on clouds and thus climate.