The Diurnal Susceptibility of Subtropical Clouds to Aerosols

Abstract. The diurnal susceptibility of clouds to aerosols is examined during the transition from subtropical stratocumulus to shallow cumulus regimes. Using large-eddy simulations, a six-day air mass evolution along a 3800-km observed trajectory from the coast of Peru toward the equator is analyzed. Pristine and polluted scenarios are simulated with forcing imposed from weather reanalysis. The polluted scenario exhibits stronger diurnal variations in cloud water, cloud fraction, and albedo, with enhanced entrainment and suppressed precipitation. The overall response of cloud properties and outgoing shortwave radiation to droplet number concentration follows a distinct diurnal pattern: strong positive adjustments dominate at night and in the morning, while weak negative adjustments prevail in the afternoon. This cycle is driven by the competition between precipitation suppression, which enhances cloud water and coverage, and entrainment drying, which depletes them. In polluted conditions, enhanced entrainment leads to a deeper and more decoupled boundary layer that cannot be sustained by surface fluxes in the afternoon, resulting in negative cloud adjustments. This entrainment enhancement is mediated by the sedimentation of cloud and precipitation water from the entrainment zone. While the Twomey effect dominates the diurnal average albedo response, the diurnal variation in the competing cloud adjustments lead to a near-neutral net adjustment effect, highlighting the critical role of diurnal processes in aerosol-cloud interactions.