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Catastrophic Chinese Floods Triggered By Air Pollution
sciencehabit writes: The worst flooding to hit China in 50 years may have been caused by air pollution, according to a new study. Soot in particular contributed to the catastrophic flooding. It prevented rainclouds from forming over the Sichuan basin, which is surrounded by mountains that trap smoke billowing from its industrial centers, and is 'notorious' for its dirty air. That in turn lead to more intense rainfall in the mountains that evening, which eventually led to the massive flooding.
Fan and her co-authors ran two forecasts for the weather system that passed over the Sichuan basin during the peak of the floods: one with the thick blanket of smoke that covered the region and one with the kind of clean air that existed 40 years ago, before the Chinese economic boom. In the clean air model, moist air at Earth’s surface was heated by the daytime sun, became buoyant, and rose to great heights, triggering a convective cycle that led to storm clouds and mild daytime rainfall. But in the dirty air model, the dark veil over the plain soaked up much of the sun’s warmth high in the atmosphere, while simultaneously cooling the streets and fields below. This altered thermal structure stabilized the daytime atmosphere and suppressed rainfall. But as night fell, the moist air mass moved northward toward the Longmen Mountains, which tower some 2000 meters above the basin. The weather system that had been building energy over the plains for 12 hours was driven upward as it collided with the range’s steep contours, triggering the postponed convection. A day’s worth of rainfall from the plains was focused into a few hours over a handful of mountain valleys.
We play the game with the bravery of being out of range
I'm not a meteorologist, but this is my understand: It depends on the size and temperature of the particles, and potentially on their shape and concentration. If the particulate matter is too hot and in the air too thick, not enough water vapor condenses on each to form droplets heavy enough to fall.
I think what they are saying in this case is once the particles cool and thin a bit, they end up carrying that moisture with them until the condensation is complete. Hitting a mountainside and mixing with the cool air above is a perfect trigger to release the rain.
Large rain events often happen when a warm, wet mass of air mixes with a cooler, dryer mass of air that can't hold that level of moisture. The particulate matter is just one of a number of variables.
The number one cause of flooding in China is, and has been for a long time, that the ability of the watershed to retain water and let it move through slowly has been degraded by some of the longest-term use of the plow on the face of the planet. The Loess Plateau, the original homeland of the Han people, shows some of the most horrific erosion ever. It's been reduced to near-desert conditions, and rainwater washes off from it unabsorbed, carrying silt, right into the river that's called Yellow because of just that.
Air pollution just adds to this.
https://www.youtube.com/watch?...