Observed onshore precipitation changes after the installation of offshore wind farms

The goal of this study is to assess the extent to which offshore wind farms can inadvertently affect precipitation at nearby onshore locations. As the winds slow down over an offshore wind farm due to the extraction of kinetic energy from the air flow by the turbines, a pattern of convergence upstream and divergence downstream of the farm is formed, which respectively may enhance precipitation offshore and reduce precipitation near the shore. To verify this hypothesis, we used observed precipitation data collected near two offshore wind farms in the western UK: Walney and Burbo Bank, built in 2011–2014 and 2005–2007, respectively. For each farm, we identified a “treatment” site, a meteorological station likely to be affected by the wake of the wind farm for a certain wind direction range, and a “control” site, unaffected by the farm for the same wind direction range. We used a difference-in-differences (DiD) approach between the precipitation data at the treatment and control sites before and after the construction of the wind farms to determine the net impact of the offshore farms on precipitation patterns at the shore. We found the following statistically significant signals at Walney after the construction of the wind farms: decreased wind speed (− 1.1 knots on average), reduced precipitation rate (− 0.003 mm/h), and reduced positive precipitation rate (i.e., only including hours with non-zero precipitation, − 0.080 mm/h). At Burbo Bank, wind speed and positive precipitation were reduced after the farm was built (− 1.45 knots and − 0.080 mm/h, respectively) with statistical significance, but no statistically significant drop in the hourly precipitation rate was found. All observed statistically significant changes were small in magnitude (order of 11% of the average or less). We speculate that, since Burbo Bank is very close to the shore (distance < 8 km), the divergence pattern cannot always form with sufficient strength to cause a precipitation suppression at the shore, which may explain the a less clear signal at that site. By contrast, the Walney farm is located ∼ 15 km away from the shore, which, we believe, is sufficient for the divergence zone to reach the shore. © 2020, Springer Nature Switzerland AG.