Satellite observations spanning 35 years reveal that desert dust plays a surprising role in cloud freezing across the Northern Hemisphere.
Credit: SciTechDaily.com
New research based on decades of satellite observations reveals an unexpected atmospheric connection: mineral dust from distant deserts can trigger the freezing of clouds in the Northern Hemisphere.
A new study has found that tiny dust particles traveling from distant deserts can help trigger the freezing of clouds in the Northern Hemisphere. This subtle atmospheric process influences how much sunlight clouds reflect back into space and how they produce rain and snow. Because of these effects, the mechanism could play an important role in improving climate projections.
Using 35 years of satellite data, an international team led by ETH Zurich investigated how mineral dust affects cloud formation. These particles are lifted from desert surfaces by strong winds and transported high into the atmosphere. Once there, they can act as seeds that cause droplets inside clouds to freeze. The effect is particularly important in northern regions, where clouds frequently form at temperatures slightly below freezing.
“We found that where there’s more dust, clouds are much more likely to freeze at the top,” explains Diego Villanueva, a Post-doctoral researcher for Atmospheric Physics at ETH Zurich and lead author of the study. “This has a direct impact on how much sunlight is reflected back into space and how much precipitation is generated.”
Dust turns clouds to ice
The scientists concentrated on mixed phase clouds, which contain both supercooled liquid droplets and ice crystals. These clouds form in temperatures between −39 °C and 0 °C (−38 °F to 32 °F). They are widespread across mid and high latitude regions, particularly over the North Atlantic, Siberia, and Canada.
Mixed-phase clouds are extremely sensitive to environmental conditions. One key factor is the presence of ice nucleating particles, which often originate from desert dust aerosols that have traveled long distances through the atmosphere.
Credit: Diego Villenueva Ortiz / ETH Zurich
To investigate the relationship, the researchers compared satellite observations of dust concentrations with how often clouds developed ice at their tops. A clear pattern emerged. When dust levels were higher and temperatures were lower, ice-topped clouds appeared more frequently.
According to the team, this large-scale pattern closely matched predictions from laboratory studies that examine how mineral dust causes water droplets to freeze.
“This is one of the first studies to show that satellite measurements of cloud composition match what we’ve known from lab work,” says Ulrike Lohmann, senior co-author and Professor of Atmospheric Physics at ETH Zurich.
A new benchmark for climate models
The freezing of clouds plays a key role in the climate system. It affects the amount of sunlight clouds reflect into space and determines how efficiently they release precipitation. Climate models rely on accurate descriptions of these processes, yet researchers have long lacked global observations that clearly connect airborne dust with cloud freezing.
The new results establish a measurable relationship between atmospheric dust and the occurrence of ice at cloud tops. This provides an important reference point that could help scientists refine climate simulations.
“It helps identify one of the most uncertain pieces of the climate puzzle,” says Villanueva.
To investigate the relationship, the researchers compared satellite observations of dust concentrations with how often clouds developed ice at their tops. A clear pattern emerged. When dust levels were higher and temperatures were lower, ice-topped clouds appeared more frequently.
According to the team, this large-scale pattern closely matched predictions from laboratory studies that examine how mineral dust causes water droplets to freeze.
“This is one of the first studies to show that satellite measurements of cloud composition match what we’ve known from lab work,” says Ulrike Lohmann, senior co-author and Professor of Atmospheric Physics at ETH Zurich.
A new benchmark for climate models
The freezing of clouds plays a key role in the climate system. It affects the amount of sunlight clouds reflect into space and determines how efficiently they release precipitation. Climate models rely on accurate descriptions of these processes, yet researchers have long lacked global observations that clearly connect airborne dust with cloud freezing.
The new results establish a measurable relationship between atmospheric dust and the occurrence of ice at cloud tops. This provides an important reference point that could help scientists refine climate simulations.
“It helps identify one of the most uncertain pieces of the climate puzzle,” says Villanueva.
A complex picture – with a clear signal
Scientists have studied the freezing of individual water droplets for decades, often focusing on microscopic processes. The new research shows that cloud glaciation follows the same basic behavior seen in those small-scale experiments, but across far larger atmospheric systems.
This connection links extremely small structures on the surfaces of dust particles, measured in nanometers (1 nanometer equals about 0.00000004 inches), with cloud systems that extend for kilometers (1 kilometer equals about 0.62 miles) and can be observed from satellites in orbit.
However, the influence of dust on cloud freezing is not uniform worldwide. In desert regions such as the Sahara, clouds are relatively rare, and rising warm air may limit freezing. In the Southern Hemisphere, sea salt and other marine aerosols may play a larger role than desert dust.
The researchers say additional studies are needed to determine how other factors, including updraft strength and atmospheric humidity, shape the freezing process. Even so, the study highlights an important conclusion. Tiny grains of dust from distant deserts help influence the clouds overhead, and those clouds play a role in shaping Earth’s future climate.
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