How does dust from African and Asian deserts affect rainfall over California?

Featured image: Sand Dunes by Free-Photos on Pixabay

Paper: Dusty Atmospheric Rivers: Characteristics and Origins

Authors: Kara K. Voss, Amato T. Evan, Kimbery A. Prather, and F. Martin Ralph

Atmospheric rivers, narrow plumes of highly concentrated water vapor in the atmosphere, can cause heavy rain over the coastal western United States and southwest Canada. In fact, up to half of California’s annual rainfall comes from atmospheric rivers, and while this rain helps replenish California’s water sources, it can also cause flooding and mudslides. A new study sheds light on how dust kicked up from deserts halfway around the world in Africa and Asia may influence these atmospheric rivers and control California’s rain patterns.

Previous work has shown that dust in the atmosphere facilitates rainfall. For water vapor in the atmosphere to condense into liquid water or transition straight to ice, it usually needs a particle surface to attach to. Dust is particularly effective for ice formation because its properties allow ice to form at relatively warm temperatures compared to other common atmospheric particles. After millions of water vapor molecules deposit into ice on the dust particle, it can either fall as snow or melt into rain. High concentrations of atmospheric dust can therefore produce a lot of rainfall if enough water vapor is present in the air.

While it is known that dust from Africa and Asia transported across the Pacific Ocean accounts for a significant portion of dust in the western United States, this study is the first to specifically explore atmospheric dust concentrations around atmospheric rivers when they are moving over land. In this study, Voss and coauthors used a newly developed dataset containing over 18 years of atmospheric dust concentrations derived from satellite measurements to analyze how this dust influences atmospheric rivers.

By utilizing this new dataset, the researchers explored the seasonality of atmospheric rivers with high amounts of dust. The transport of dust across the Pacific is strongest in spring (March, April, and May), while atmospheric rivers typically make landfall in the United States in the fall and winter (October to March). The atmospheric rivers that contain the most dust occur in the early spring, at the intersection of the atmospheric river season and trans-Pacific dust season.

Next the researchers focused on studying why the amount of dust in atmospheric rivers changes year-to-year. One important control on the amount of dust in atmospheric rivers is the amount of dust emitted from the deserts, which changes year-to-year. Another factor the researchers discovered is the correlated changes in the amount of dust being emitted and the positions of atmospheric rivers.

These correlated changes in dust amount and atmospheric river position could indicate that there are certain weather patterns that result in atmospheric rivers with a lot of dust in them. Voss and coauthors plan to identify what these patterns are in future research. Understanding the weather patterns could help meteorologists better forecast atmospheric rivers in the future. Better forecasting leads to improved preparation, especially for strong atmospheric rivers that result in dangerous flooding in California and other western states.

How does dust from African and Asian deserts affect rainfall over California? by Alyssa Stansfield is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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