Polluted subtropical clouds brighten more in the morning and barely at all in the afternoon

Why it matters

Aerosol-cloud interactions are a major uncertainty in how human activity changes Earth’s radiation balance. This study suggests the effect depends on time of day. Satellites that see clouds at one fixed hour could therefore miss part of the picture. Models may also need to treat both cloud and rain water sedimentation to capture entrainment drying.

What they did

The authors ran large-eddy simulations of an air mass moving 3800 km along an observed trajectory for six days. The air mass moved from stratocumulus to shallow cumulus. Weather reanalysis supplied the forcing. They compared pristine and polluted droplet number scenarios, with intermediate cases. They used offline radiation calculations to split the change in reflected sunlight into a Twomey effect, a liquid water adjustment and a cloud fraction adjustment. Extra experiments applied the polluted droplet number to only one process at a time.

Key findings

• Polluted clouds had stronger daily swings in cloud water, cloud fraction and albedo, with more nighttime entrainment and less precipitation.
• The daylight averages of the Twomey, liquid water and cloud fraction terms were 17.0, -6.0 and 5.6 W m−2.
• The two cloud adjustments were positive in the morning and negative in the afternoon, so net susceptibility was close to zero in the afternoon.
• Polluted clouds deepened overnight, then became decoupled from the surface in the afternoon, which cut cloud water and cover.
• The adjustments were positive in pristine conditions and negative in polluted ones. Reduced sedimentation of cloud and rain water from the cloud top drove the stronger entrainment.

Limitations

• The results come from a single suite of simulations along one trajectory, so the authors caution against extending them to other conditions.
• The simulated clouds did not fully match observations. The model produced too little nighttime cloud on days 2–4, and the polluted runs grew deeper than observed.
• Precipitation suppression drives the diurnal response here. The authors do not expect the same result in non-precipitating clouds.

Glossary

• Twomey effect: More cloud droplets at the same water amount make a cloud optically thicker and brighter.
• Cloud adjustment: A change in cloud water or cloud cover that follows a change in droplet number.
• Entrainment: Mixing of dry air from above into the top of the cloud layer.
• Large-eddy simulation: A high-resolution model that resolves the larger turbulent eddies in the atmosphere.