The team found that in all cases, pollution increased the size, thickness and duration of the anvil-shaped clouds. However, only two locations -- the tropics and China -- showed stronger convection. The opposite happened in Oklahoma -- pollution made for weaker convection.
This inconsistency suggested that stronger convection isn't the reason. Taking a closer look at the properties of water droplets and ice crystals within clouds, the team found that pollution resulted in smaller droplets and ice crystals, regardless of location.
In addition, the team found that in clean skies, the heavier ice particles fall faster out of the anvil-shaped clouds, causing the clouds to dissipate. However, the ice crystals in polluted skies were smaller and too light to fall out of the clouds, leading to the larger, longer-lasting clouds.
Lastly, the team estimated how much warming or cooling the storm clouds contributed. Overall, the polluted clouds cooled the day and warmed the night, decreasing the daily temperature range.
Most models don't simulate convection well, take into account the microphysical processes of storm clouds, nor address how pollution interacts with those processes. Accounting for pollution effects on storm clouds in this way could affect the ultimate amount of warming predicted for the earth in the next few decades. Accurately representing clouds in climate models is key to improving the accuracy of predicted changes to the climate.
|Contact: Franny White|
DOE/Pacific Northwest National Laboratory