Airborne dust has been shown to speed up snowmelt in the Colorado Rockies, but there’s more to the global dust and snow story, according to a NOAA-led study showing that dust and microorganisms from as far away as the Sahara help spur the precipitation that California counts on for its water supply.
The CalWater field campaign, funded by the California Energy Commission and led by NOAA and the University of California San Diego, could help western states better understand the future of their water supply and hydropower generation as climate change influences how much and how often dust travels around the world and alters precipitation far from its point of origin.
“We were able to show dust and biological aerosols that made it from as far as the Sahara were incorporated into the clouds to form ice, then influenced the formation of the precipitation in California,” said Jessie Creamean, a postdoctoral associate at NOAA’s Earth System Research Laboratory in Boulder, Colorado. “To our knowledge, no one has been able to directly determine the origin of the critical aerosols seeding mid-level clouds which ultimately produce periods with extensive precipitation typically in the form of snow at the ground.”
Researchers have long known that winds can carry aerosols such as dust at altitudes above 5,000 meters (16,400 feet) from continent to continent. An unrelated 2009 study found that in one instance, Asian dust made a complete circuit around the planet in 13 days.
These dust particles can act as ice nuclei within clouds at warmer temperatures than would occur in their absence. They initiate the freezing of water vapor and water droplets, then precipitate as rain, snow, or hail depending on whether meteorological conditions enable them to attain sufficient mass to fall from the sky before evaporating. Without ice nuclei, ice would likely not form in clouds with temperatures above minus 38 degrees Celsius.
Besides dust, aerosols can be composed of sea salt, bits of soot and other pollution, or biological material. Bacteria, viruses, pollen, and plants, of both terrestrial and marine origin, also add to the mix of aerosols making the transcontinental voyage.
The researchers’ analysis of winter storms in 2011 found that dust and biological aerosols tend to enhance precipitation-forming processes in the Sierra Nevada. In previous studies, researchers have found that pollution particles have the opposite effect, suppressing precipitation in the Sierra Nevada.
Using specialized instruments, the researchers were able to determine that at least some of the dust and bioparticles detected during February 2011 flights through Sierra Nevada storm clouds were in the skies over Oman 10 days earlier, having likely originated in the Sahara a few days before thatn.
Along the journey, the Saharan dust and microbes mixed with other aerosols from deserts in China and Mongolia before wafting over the Pacific Ocean. Upon arrival in California, the aerosols effectively seeded the storm clouds and contributed to the efficiency of clouds in producing precipitation.
The researchers said it is a major challenge to sort out the relative impacts of meteorology, atmospheric dynamics, and the original sources of the cloud seeds on precipitation processes. They added that further studies like CalWater are necessary to further identify which aerosols are conducive to precipitation formation and which aerosols stifle its production.
“Due to the ubiquity of dust and co-lofted biological particles such as bacteria in the atmosphere, these findings have global significance,” the study concludes. “Furthermore, the implications for future water resources become even more substantial when considering the possible increase in [wind-blown] dust as a result of a warming climate and land use changes.”