"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 the corresponding author Yuan Wang, an assistant professor of Earth system science at the Stanford Doerr School of Sustainability.
Earlier studies indicated that Saharan dust transport might significantly decline in the coming decades, and hurricane rainfall is expected to increase due to human-induced climate change. However, uncertainties remain regarding how climate change will influence dust outflows from the Sahara and the amount of additional rainfall future hurricanes might bring. The intricate relationships among Saharan dust, ocean temperatures, and hurricane formation, intensity, and precipitation are still not fully understood. Addressing these gaps is crucial for predicting 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."
Competing Effects
Dust can have competing effects on tropical cyclones, which are categorized 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 aimed to develop a machine learning model to predict hurricane rainfall and identify the underlying mathematical and physical relationships.
The researchers analyzed 19 years of meteorological data and hourly satellite precipitation observations to predict rainfall from individual hurricanes.
The findings show that a key predictor of rainfall is dust optical depth, a measure of how much light passes through a dusty plume. They discovered a boomerang-shaped relationship where rainfall increases with dust optical depths between 0.03 and 0.06 and sharply decreases beyond that point. In essence, at high concentrations, dust shifts from enhancing 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 the 'radiative suppression effect' will be dominant," Wang said.
Research Report:Leading Role of Saharan Dust on Tropical Cyclone Rainfall in the Atlantic Basin
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