NEWARK, Del. — While offshore wind projects provide clean and renewable energy, a new study from the University of Delaware shows that they can also have unintended effects on local weather in the form of minimal, though statistically significant, impacts when it comes to wind speed and reduced precipitation at nearby onshore locations.
The study, which was recently published in the Bulletin of Atmospheric Science and Technology, was led by Nicolas Al Fahel, a doctoral level student in Energy and Environmental Policy (ENEP), and his adviser, Cristina Archer, a professor in UD’s College of Earth, Ocean and Environment (CEOE) with a joint appointment between the School of Marine Science and Policy and the Department of Geography and Spatial Sciences. They found that not all wind farms affect onshore precipitation in the same way.
Specifically, the impacts of wind speed and precipitation depend on the farm size and its distance from the shore. There is also a sweet spot — approximately 10 kilometers offshore for a mid-size wind farm — such that a wind farm built there will have a greater impact on onshore precipitation than if it were closer to or farther from the shore.
To conduct their study, Al Fahel and Archer analyzed precipitation data collected near two wind farm sites in the western United Kingdom before and after the construction of the farms: Walney and Burbo Bank which were built in 2011-2014 and 2005-2007, respectively. For each site, the researchers identified a treatment and a control weather station from which to collect data. The treatment station had to be likely affected by the wake of the wind farm for a certain wind direction range while the control station had to remain unaffected by the farm for that same wind direction range.
By comparing the precipitation data collected at the treatment and control stations before and after the construction of the wind farms, the authors were able to determine the net impact of these farms on the precipitation patterns at the shore.
At both sites, they discovered a reduction in both wind speed and precipitation.
Archer said that, while both of the impacts they found were statistically significant, they were also minimal in magnitude.
“We don’t want our study to be interpreted as wind farms causing droughts,” said Archer. “There’s a very small reduction in precipitation. It’s statistically significant, but it’s not large.”
Yet, the authors believe that the monitoring of this impact on precipitation remains important, as it could help determine where wind farms ought to be placed throughout the world. In areas that get a lot of rain, like England, the residents probably wouldn’t complain about getting a touch less rain. For areas that don’t get much rain or grow crops that rely on rain, however, it would be more important to know if it was going to rain less.
Al Fahel added that some areas might want to take advantage of the reduced precipitation.
“Dr. Archer has published an earlier paper that asked the question: what if you had a very big offshore wind farm in the Gulf of Mexico, and if a hurricane hit that farm? There might be a decrease in precipitation, which may lower flooding risks for instance,” said Al Fahel. “So alternatively, you could take advantage of the farm to fight adverse consequences of climate change.”
Slowing down and speeding up
As for how the wind farms are causing this reduction of precipitation onshore, Archer and Al Fahel cite a phenomenon known as wind convergence and divergence.
When wind approaches a wind farm, it essentially hits the brakes and slows down. This causes a convergence of mass and vertical motion which enhances precipitation at the farm. After the farm, the wind accelerates and if given enough space, a minimum of 10 kilometers, the divergence causes the wind to reach or exceed its original speed, which reduces vertical motion leading to a small decrease in precipitation.
Archer likened it to when traffic comes to a halt before and while approaching an accident and then accelerates afterwards.
“When we drive in our cars ahead of an accident, we’re all slowing down and getting closer to one another before the accident is even reached. Then we drive by the accident kind of slowly, we look, and we accelerate,” said Archer. “Half a mile after the accident, we’re actually going as fast as we were before the accident. The accident is the wind farm and the point where we start accelerating is the beginning of the divergence zone.”
This is where the location of the wind farm comes into play, as there is a connection between the location of the farm offshore and its impact onshore and it takes a bit of time and distance for the divergence pattern that suppresses precipitation to develop.
“If the farm is too close to the shore, the divergence may not form so you’re not going to have this effect,” said Archer. “We found that it takes some distance for these divergence patterns to form. Just because there’s a wind farm offshore, it doesn’t mean that you will get this reduction in precipitation. It depends on the scale of the farm and the distance from the land. Vice versa, if the wind farm is too far, the wind speed will have recovered and there might be no divergence at the shore.”
For this study, the wind farm at Walney was located about 15 kilometers from the shore which was sufficient a distance for the divergence zone to reach the shore, while at Burbo Bank, which is less than eight kilometers from shore, the divergence pattern did not always form with sufficient strength to cause precipitation suppression at the shore. In addition, the authors observed that the size of a farm also affects the wind divergence that it induces. The reduction in precipitation was greater at Walney, which has a far larger wind farm, than at Burbo Bank.
While this phenomenon was observed at the two wind farms in western England, the researchers are hoping to expand the study in the future.
“We went through an extensive survey of all of the offshore sites that exist in the world and the only two that we concluded could be used for this particular study were the two that we picked,” said Al Fahel. “We should look more into this phenomenon and expand our analysis to more sites and potentially, build weather stations at those sites closer to the farm to be able to capture the potential for the change in onshore precipitation.”
–Adam Thomas, University of Delaware
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