An evapotranspiration, convection, polluted air connection?

Can air pollution affect cloud formation and can this, in turn, impact planet weather and climate?

In a Nov. 25, 2013 Pacific Northwest National Laboratory (PNNL) news release, discussed is how air pollution may effect cloud formation and what, if any, affect this might have on warming/cooling among other considerations.

Dexter Lawn, Cal Poly, SLO

“Researchers had thought that pollution causes larger and longer-lasting storm clouds by making thunderheads draftier through a process known as convection,” information in the PNNL release brought to light. “But atmospheric scientist Jiwen Fan and her colleagues show that pollution instead makes clouds linger by decreasing the size and increasing the lifespan of cloud and ice particles. The difference affects how scientists represent clouds in climate models.”

Storm clouds absent pollution can contain bigger, heavier water droplets which, in turn, fall to Earth as rain. Contrast this with clouds impacted by air pollution and not only is there the potential for smaller water droplet formation, but that may be coupled with higher altitude formation of such. Higher-altitude clouds, according to information in the news release in question, are apparently better at facilitating ice-crystal creation and, as I understand things, it is that which can potentially affect both weather and climate.

Pollution influencing cloud action may also affect temperature.

As pointed out in the release, “High clouds left after a thunderstorm spread out across the sky and look like anvils. These clouds cool the earth during the day with their shadows but trap heat like a blanket at night. Pollution can cause clouds from late afternoon thunderstorms to last long into the night rather than dissipate, causing warmer nights.”

Why the pollution-caused longer, lingering anvil-shaped clouds compared to those formed under clean-sky conditions?

“Possible reasons revolve around tiny natural and manmade particles called aerosols that serve as seeds for cloud droplets to form around. A polluted sky has many more aerosols than a clean sky – think haze and smog – and that means less water for each seed. Pollution makes more cloud droplets, but each droplet is smaller,” release information revealed.

Adding to this said Fan of the Department of Energy’s PNNL in the release: “‘Observations consistently show taller and bigger anvil-shaped clouds in storm systems with pollution, but the models don’t always show stronger convection. Now we know why.’”

Moreover, upon closer examination of cloud-based ice crystals and water droplets, the study team discovered that irrespective of location, pollution was behind the ice crystal and smaller-sized water droplet formation, information in the release noted.

All part of a month-long study, thunderstorm activity was tracked in three regions: the Great Plains in Oklahoma, the western Pacific tropics and in southeastern China. Interestingly, the clouds that were actually observed nearly replicated storm-model simulations.

For more, see: “The lingering clouds” and “Tracking pollution: Research helps explain air-contaminant survival.”

Image above: Gregg Erickson

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