Clouds over Bombai
A new study reveals how pollution produces storms that leave us with longer lasting, larger and denser clouds. During the month of November the Proceedings of the National Academy of Sciences (PNAS), published some results that close a long debate. They reveal how pollution affects global warming. This work will help improve the accuracy of weather and climate models.
Most researchers thought that atmospheric pollution it causes larger, longer-lasting storm clouds by making storm fronts more susceptible to drafts, and causing internal convection. In this study, he observed that pollution, as a phenomenon, makes clouds more durable but, in a different way than previously thought, by a decrease in the size of their ice particles and a decrease in the total size of the cloud. This difference directly affects the way scientists represent clouds in climate models.
This study reconciles what we see on a daily basis with what is indicated in computer models. Observations show anvil-shaped clouds (columonimbus) higher and larger in storm systems that contain pollution, but models do not always show stronger convection, thanks to this study we see why.
The Secret Life of Clouds
Anvil or Comulonimbus clouds over polluted area
Models that predict weather and climate do not reconstruct the life of storm clouds well, since they represent them with simple equations that fail to give a complete picture. This poor reconstruction created a dilemma for the researchers: "Pollution causes anvil clouds to last longer than in the case of clear skies", but why?
One possible reason revolves around aerosols (small particles of natural or human origin) that serve as the basis for cloud droplets to form around them. A polluted sky has many more aerosols (smog and haze) than a clean one and this translates to less water for each particle. Pollution produces more droplets, but smaller.
A larger number of smaller droplets changes the characteristics of the clouds. It has long been thought that larger and smaller droplets start a chain reaction that leads to larger, longer-lasting clouds instead of precipitating. The lighter droplets cause your water to rise by freezing and this freezing extracts the heat contained in the droplets and produces a change in temperature that generates internal convection. A more intense convection makes more drops of water rise, thus building the cloud.
But researchers do not always observe a more intense convection related to larger and more durable clouds in polluted environments, indicating that we were missing something to take into account.
To solve this dilemma, the team responsible for this study decided to compare actual summer storms with computer-generated models. The model included the physical properties of cloud particles as well as the ability to observe whether the convection becomes stronger or softer. The simulations in this study spanned 6 months.
Convection is not the culprit.
Data were collected from three locations with varying degrees of pollution, humidity, and wind: the western Pacific tropics, southeastern China, and the great plains of Oklahoma. Data were obtained from DOE's (US Department of Energy) ARM Climate Research System.
Simulations were carried out on the Olympus supercomputer from PNNL (Pacific Northwest National Laboratory). These simulations of a month of storms are very similar to the clouds currently observed, determining that the models recreated storm clouds well.
Observing these models it was found that in all cases, pollution increases the size, thickness and duration of the anvil clouds. But only in two places (the tropics and China) is more intense convection observed. In Oklahoma, pollution led to gentler convection. This inconsistency with what has been thought so far suggests that the reason is not intense convection.
By reviewing in more detail the properties of the water droplets and ice crystals within the clouds, the research team concluded that the pollution produced smaller drops and ice crystals, regardless of their location.
Also, in clear skies, ice particles are heavier and precipitate faster from anvil clouds, causing them to dissipate rapidly. In polluted skies, the ice crystals were smaller and too light to precipitate, thus creating larger and more durable clouds.
Contribution to global warming.
On the other hand, the team estimated how storm clouds contribute to the heating or cooling. These clouds cool the Earth during the day with their shadows but trap heat like a blanket at night, making nights warmer.
Taking into account the effects of pollution on storm clouds we understand that they could affect the amount of definitive warming predicted for the earth in the coming decades. Making more accurate representations of clouds in climate models is the key to improving the accuracy of climate change predictions.
More information: The Cumulonimbus, Important Findings on Atmospheric Particles in Cities, Lightning bolts get stronger with global warming
Source: PNAS