South SFV Today

Giant plumes of Sahara Desert dust that travel across the Atlantic can suppress hurricane formation over the ocean and affect weather in North America. However, thick dust plumes can also lead to heavier rainfall—and potentially more destruction—from landfalling storms, according to a July 24 study in Science Advances. The research reveals a previously unknown relationship between hurricane rainfall and Saharan dust plumes.

“Surprisingly, the leading factor controlling hurricane precipitation is not, as traditionally thought, sea surface temperature or humidity in the atmosphere. Instead, it’s Sahara dust,” said Yuan Wang, an assistant professor of Earth system science at the Stanford Doerr School of Sustainability.

This “true-color” composite animation of visible satellite imagery shows the movement of the Saharan Dust plume from June 15-25, 2020. It was captured by the VIIRS instrument aboard NASA/NOAA’s Suomi NPP satellite. The bright streaks seen at regular intervals are due to sun glint off the ocean surface.

Previous studies have found that Saharan dust transport may decline dramatically in the coming decades and hurricane rainfall will likely increase due to human-caused climate change. However, uncertainty remains around how climate change will affect outflows of dust from the Sahara and how much more rainfall we should expect from future hurricanes. Additional questions surround the complex relationships among Saharan dust, ocean temperatures, and hurricane formation, intensity, and precipitation. Filling in these gaps will be critical to anticipating and mitigating the impacts of climate change.

“Hurricanes are among the most destructive weather phenomena on Earth,” said Wang. Even relatively weak hurricanes can produce heavy rains and flooding hundreds of miles inland. “For conventional weather predictions, especially hurricane predictions, I don’t think dust has received sufficient attention to this point.”

After storm surges, flooding is the second leading cause of death in the U.S. from landfalling tropical cyclones.

Dust can have competing effects on tropical cyclones, which are classified as hurricanes in the North Atlantic, central North Pacific, and eastern North Pacific when maximum sustained wind speeds reach 74 miles per hour or higher.

“A dust particle can make ice clouds form more efficiently in the core of the hurricane, which can produce more precipitation,” Wang explained, referring to this effect as microphysical enhancement. Dust can also block solar radiation and cool sea surface temperatures around a storm’s core, which weakens the tropical cyclone.

Wang and colleagues set out to first develop a machine learning model capable of predicting hurricane rainfall and then identify underlying mathematical and physical relationships.

The researchers used 19 years of meteorological data and hourly satellite precipitation observations to predict rainfall from individual hurricanes.

The results show a key predictor of rainfall is dust optical depth—a measure of how much light filters through a dusty plume. They revealed a boomerang-shaped relationship in which rainfall increases with dust optical depths between 0.03 and 0.06 but sharply decreases thereafter. In other words, at high concentrations, dust shifts from boosting to suppressing rainfall.

“Normally when dust loading is low, the microphysical enhancement effect is more pronounced. If dust loading is high it can more efficiently shield [the ocean] surface from sunlight and what we call ‘radiative suppression effect’ will be dominant,” Wang said.

Additional authors are affiliated with Western Michigan University, Purdue University, University of Utah, and California Institute of Technology.

This research was supported by the National Science Foundation.