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

Abstract. Aerosol-cloud interaction (ACI) is the critical yet most uncertain process in future climate projections. A major challenge is the sign and magnitude of cloud liquid water path (LWP) response to aerosol perturbations (represented by cloud droplet number concentration, N_d) at different temporal and spatial scales are highly variable, but potential microphysical-dynamical mechanisms are still unclear, especially at a diurnal scale. Here, robust observational evidence from geostationary satellite reveals that the diurnal variation of LWP adjustments is driven primarily by diurnal-related boundary layer decoupling and cloud-top entrainment. Strikingly, these diurnal adjustments exhibit a distinct regional pattern associated with cloud regimes. We find that the cooling effect of LWP adjustments would be underestimated by up to 86 % in study regions if neglecting their diurnal variations, leading to a further 45 % offset of Twomey effect, thus biasing aerosol indirect effect toward a warming direction. Our findings highlight the key role of diurnal variation of ACI in reducing the uncertainty in climate projections.