The Diurnal Susceptibility of Subtropical Clouds to Aerosols

The influence of boundary-layer aerosols on low-level clouds is studied in the context of the transition from subtropical stratocumulus to cumulus cloud regimes. Specifically, a six-day evolution of an air mass originating from the region off the coast of Peru and propagating towards the equator along an observed 3800-km-long trajectory is investigated using large-eddy simulation. Pristine and polluted scenarios are simulated along the trajectory with forcing imposed from weather reanalysis. The focus is on estimating the diurnal susceptibility of cloud properties and cloud albedo to cloud droplet number concentration. Overall, the polluted scenario undergoes significantly stronger diurnal changes of the cloud water path, cloud fraction, and albedo, with much larger entrainment rates and a delayed cloud breakup as compared to the pristine scenario. The cloud water path and cloud fraction adjustments (or sensitivity to number concentration) tend to be negative during the afternoon and positive overnight into the early morning. The diurnal cycle of the cloud adjustments is driven by the competition between precipitation suppression and entertainment drying where the positive adjustments are dominant at night into the morning and the negative adjustments dominate in the afternoon hours. This diurnal pattern results from a significant enhancement of the cloud top entrainment and deepening of the boundary layer in the polluted scenarios, which leads to a dryer more decoupled boundary layer that cannot be sustained by surface fluxes through the afternoon. The enhanced entrainment in the polluted scenario is mediated by the sedimentation of both cloud and precipitation water out of the cloud top entrainment zone. The diurnal average albedo response is dominated by the Twomey effect, while a diurnal variation in the sign of the cloud adjustments leads to a near neutral effect on the average albedo.