Aerosols in clouds make big storms huge

An abundance of aerosol particles in the atmosphere can increase the lifespan of large storm clouds by delaying rainfall, which in turn makes clouds grow larger, live longer, and eventually let loose extreme rain storms.

A new study is the first to address the impact that aerosol particles have on the lifespans of large thunderstorm systems called mesoscale convective systems, researchers say. The complex, often violent storms can span several hundred kilometers.

The systems are the primary source of precipitation over the tropics and mid‐latitudes, and their lifetime can have a large influence on the variability of rainfall, especially extreme rainfall that causes flooding.

Published in the Proceedings of the National Academy of Sciences, the study looked at satellite data from 2,430 convective cloud systems and found that aerosols can help increase the lifespans of convective cloud systems by as much as three to 24 hours, depending on regional meteorological conditions.

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“A cloud particle is basically water and aerosols. It’s like a cell. The aerosol is the nucleus, and the water is the cytoplasm,” says lead author Sudip Chakraborty, who recently received his PhD from the Jackson School at the University of Texas at Austin. “The more aerosols you have, the more cells you get. And if you have more water, you should get more rain.”

Aerosols are minute particles in the atmosphere that form the nucleus within a cloud around which water condenses to form the cloud. Aerosols can come from natural sources such as volcanic eruptions or desert dust, or human-made sources such as the burning of wood, coal, or oil.

The study looked at aerosols’ relative importance in the lives of storm clouds compared with meteorological conditions like relative humidity, available convective energy, and wind shear. Although meteorological conditions remain the most important element in the lifetime of a convective cloud system, aerosols can have a significant impact, says coauthor Rong Fu, professor of geological sciences.

One of the difficulties in conducting this type of study is that the satellites that give data on cloud aerosol content generally pass over the same spot on Earth twice a day, which doesn’t provide enough data on the lifetime of a convective cloud system. To combat the problem, Chakraborty turned to data from geostationary satellites that fly much higher and stay in the same location relative to the Earth’s surface.

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He then matched the geostationary satellite data, which gives information about the lifecycle of the convective systems, with data from the polar orbital satellite that passes by twice a day.

Aerosols’ effects on deep convective clouds and climate have been major questions for more than a decade, says Daniel Rosenfeld, a professor at the Hebrew University of Jerusalem who was not involved in the study. Of particular interest is the role of clouds in reflecting solar radiation and emitting thermal radiation to space, which can influence the radiative balance in the atmosphere and the Earth’s temperature.

“This is the first study that shows the full lifecycle of convective clouds in a statistically meaningful way on a climate scale,” Rosenfeld says. “This is an important step towards determining the impact of clouds on radiative forcing. The next step is to quantify.”

Researchers from the University of Colorado Boulder and NASA’s Jet Propulsion Laboratory contributed to the work.

Source: University of Texas at Austin