Strong aerosol indirect radiative effect from dynamic-driven diurnal variations of cloud water adjustments

Abstract

Aerosol-cloud interaction (ACI) remains a key uncertainty in climate projections. A major challenge is that the sign and magnitude of cloud liquid water path (LWP) response to aerosol perturbations (represented by cloud droplet number concentration, Nd) at different temporal and spatial scales are highly variable, but potential microphysical-dynamical mechanisms are still unclear, especially at a diurnal scale. Here, Himawari-8 retrievals were used to investigate LWP adjustments in two distinct cloud regions: the stratocumulus region off the western Australia (AUW) and clouds over the East China Sea (ECS) characterized by a transition from stratocumulus to cumulus under strong anthropogenic influences. In the ECS region, LWP exhibits a unique pronounced rising (positive LWP adjustments) at high Nd. Results indicate that this pattern is driven by northerly cold-air advection during the cold seasons, which enhances surface fluxes and subsequently leads to increases in both LWP and Nd. Furthermore, the diurnal variation of LWP adjustments is likely driven by cloud-top entrainment in the ECS region, but is primarily associated with diurnal-related boundary layer decoupling in the AUW region. The results indicate that neglecting diurnal variations of LWP adjustments leads to an underestimation (up to 89 %) of the cooling effect induced by changes in cloud albedo due to aerosol perturbations in the AUW. The bias spans from a 32 % overestimation to a 37 % underestimation in the ECS. Our findings highlight the key role of diurnal variations of ACI in reducing the uncertainty in climate projections.