To picture what happens when a hurricane meets a wind turbine — and how turbines can diminish rainfall — it’s important to understand the two significant factors that influence precipitation, convergence and divergence. Strong hurricane winds slow down when they hit wind turbines, and move upward because they have no other place to go. This brings more moisture into the atmosphere, boosting rainfall. That is convergence.
Divergence is the opposite; it causes downward motion, attracting drier air that is moving downward, which suppresses precipitation.
“Think about convergence like when there’s traffic on a freeway, and everybody is going fast and then — all of a sudden — there’s an accident and everyone slows down,” Archer explains. “You get a convergence of cars that backs up… that’s the convergence upstream of the offshore wind farms. Divergence is similar to what happens when cars finally get past the accident and everybody speeds up.”
Convergence and divergence occur naturally in many situations. “For example, wherever we have a low-pressure center — associated with storms in the mid latitudes — typically the surface winds converge towards it,” Archer says. “Vice versa, wherever we have a high-pressure center, like during clear and warm weather, winds at the surface diverge out of it. When you have a change in land use, like from ocean to land, or from forest to pasture, this causes a change in the winds — deceleration from ocean to land, or acceleration from forest to grass -and thus you have convergence and divergence, respectively.”
Over a wind farm, the wind speed slows “because that’s what turbines do,” she says. “They extract kinetic energy from the wind, therefore the wind behind them, downstream of each turbine, is weaker than it was upstream. Put more and more turbines together in a large farm and you can imagine that you would start causing convergence ahead of the farm, before the winds hit the first turbines, and divergence past it, after the farm, where the winds accelerate again.”
Therefore, in order to have precipitation, there must be upward motion to bring moisture-rich air near the surface into the atmosphere, where it condenses and causes rain. “To get upward motion near the surface, you need to have convergence,” she says. Then, “to kill precipitation, you need downward motion, which is what happens when you have divergence at the surface. Obviously you also need a moisture-rich environment to start with. If you have convergence in the desert, no matter how strong, there will not be precipitation because it is too dry.”
That’s why the scientists studied the impact of offshore wind farms, rather than land-based ones, as hurricanes form over water and dissipate quickly once they reach land. “There is plenty of moisture [offshore], thus it was more likely that we would see an effect,” she says. “We looked at hurricanes because they last long enough that the patterns of convergence/divergence can form and persist. If you think about it, a hurricane lasts for days and causes strong, steady winds from pretty much the same direction — from the ocean to the land.”
One current drawback is that turbines often are now turned off during high winds. Archer said their computer simulations took the “cut-off” wind speed into consideration, which currently is 76 miles per hour for the Enercon 126, a German offshore wind turbine model. However, manufacturers now are developing new turbines that will withstand even stronger winds, such as those in hurricanes and typhoons, and these are expected to be on the market in 2020, she added.
The scientists used numerical computer simulations that covered the coast of Texas and Louisiana, examining what might have occurred in the presence of offshore wind farms during Harvey. They looked at a wide range of numbers of turbines, over different sized areas, and in a variety of layouts.
“By the time the air reaches the land, it’s been squeezed out of a lot of moisture,” Archer says. “We got a 30 percent reduction of the precipitation with the Harvey simulations. That means, potentially, if you have arrays of offshore turbines in an area where there are hurricanes, you will likely see a reduction in precipitation inland.”
The study used hypothetical turbines ranging from a control case of zero turbines to a maximum of 74,619, a number currently unrealistic for the United States, she said. Unlike Europe — where there are more than 100 offshore wind turbines — the United States only has five, all part of the nation’s first and only offshore wind farm located on Block Island, off the coast of Rhode Island.
The largest configuration had 74,000 and the smallest 22,000, she said. “So these are high numbers by all means,” she says. “By comparison, there are about 300,000 turbines in the world today.”
Although she believes the more turbines the better, the scientists found there were positive effects of using fewer turbines or placing turbines over a smaller area. “For example, you do not need to have [turbines covering a large area],” Archer says. You can get pretty much the same benefit with [a smaller layout] placed ‘smartly’ for Houston.”
With 33,363 turbines over a medium sized area, they saw a reduction of 15 percent in the Houston metropolitan area. With 28,197 turbines over the small [area], she says they saw a reduction of 10 percent. “We get greater reductions in other locations, but we focused on the totals for Houston metropolitan.”
At this time, she said the researchers can’t project the optimal number of turbines needed, although “the more [offshore] wind farms you have, the more impact they will have on a hurricane,” she says. “By the time a hurricane actually makes landfall, these arrays of turbines have been operating for days and days, extracting energy and moisture out of the storm. As a result, the storm will be weaker. Literally.”
She points out that communities can build sea walls or artificial barrier islands to protect against storm surge. But these won’t protect against rain. “The costs are high and they only serve one purpose,” she says. “An alternative is to build offshore wind farms, which [ultimately] pay for themselves, generate clean and renewable energy, and — cherry on top — protect against hurricanes, including rain and storm surge.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.