A hurricane can dump nearly 2 trillion liters of water a day. But this massive rainfall has an upside, according to a new study. Researchers have discovered that the deluge can weaken the tempest by up to 30%—a finding that may improve future storm predictions.
The hurricane engine has many parts. The calmest part—the eye—is surrounded by the most active: a narrow, 15-kilometer-wide band of wind and rain known as the eyewall. Through the eyewall flows the updraft, a vertical wind that draws water vapor from the ocean surface. At the top of the eyewall, water condenses; as the resulting drops fall back down to the ocean surface they lose power through friction with the surrounding air. The sum of this lost energy per unit time has been dubbed “rainpower.”
“The rainpower is colossal, comparable in magnitude to the ocean-derived power that fuels the hurricane,” says Pinaki Chakraborty, a fluid mechanician at the Okinawa Institute of Science and Technology Graduate University in Japan. Activity within the eyewall is closely connected to the hurricane’s overall intensity, with the vertical updraft fed by an inward-spiraling, ocean-hugging wind whose average speed is the highest across the whole storm.
Previously, researchers thought rain in the eyewall increased a hurricane’s intensity, as heat released from the condensing water added to the overall power of the storm. In contrast, Chakraborty and his colleagues found the opposite to be true: Rainpower acts to significantly lessen, not increase, hurricane intensity.
To calculate rainpower, the researchers culled 15 years of data from the Tropical Rainfall Measuring Mission, a joint NASA–Japan Aerospace Exploration Agency satellite. For every hurricane in the North Atlantic Basin between 1997 and 2013, they pulled information such as mean sea-level pressure and temperature as well as vertical temperature and humidity profiles, and entered it into a thermodynamic hurricane model that treats each storm as a gigantic heat engine. Based on these factors alone, they then predicted the intensity of each hurricane, with and without rainpower.
The results were surprising. The team’s model predicted a 10% to 30% reduction in storm intensity that varied with the overall strength of the storm. What’s more, whereas many models tend to overestimate the intensity of hurricanes in their predictions, theirs was a much closer match to historical observations, the researchers report online in Geophysical Research Letters.
“The authors have honed in on a potentially ignored effect of the friction of falling condensate on the intensity of tropical cyclones,” says Vasubandhu Misra, a meteorologist from Florida State University in Tallahassee. He says the finding should be placed in the context of other under-researched factors that could affect hurricane intensity, including vertical changes in wind speed and the ocean surface cooling created by the storms themselves.
Dave Nolan, an atmospheric scientist at the University of Miami in Florida, calls the paper “thought-provoking,” and says it makes a good case for considering the effect of rainpower. Still, he also cautions against the assumption that rainpower is the only factor impacting hurricane intensity, as not all the eyewall’s rain falls directly down through the updraft. “The eyewall and its updraft typically slope outward at angles from 30 to 60 degrees,” he explains. “Thus, much of the rain is falling down through a less active part of the storm.”
Regardless, with their initial work complete, the scientists are now moving to incorporate rainpower into the models of hurricane evolution used for weather and climate change forecasting. Their hope is to improve storm intensity predictions for the future.