Association for Public Policy Analysis & Management

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 the offshore wind farm due to the extraction of kinetic energy from the wind flow by the turbines, a pattern of convergence upstream and divergence downstream of the farm is formed, which potentially enhances precipitation offshore and reduces precipitation near the shore.

To verify this hypothesis, we used observed rainfall data collected near two large 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. Using the approach called differences-in-differences (DiD) between the rainfall data at the treatment and control sites before and after the construction of the wind farms, we found the following statistically significant signals at Walney post-farm: reduced wind speed (-1.0 knots on average), reduced hourly rainfall rate (-0.003 mm/hr), reduced hourly positive rainfall rate (i.e., only including hours with non-zero precipitation, -0.08 mm/hr), and reduced monthly total precipitation (-0.42 mm). However, at Burbo Bank the signal was not as clear; wind speed was reduced after the farm (-1.1 knots), but the precipitation variables, although generally lower after the farm, were not so with statistical significance. We speculate that, since Burbo Bank is very close to the shore (distance <10 km), the divergence pattern cannot form with sufficient strength to cause the precipitation suppression at the shore. By contrast, at Walney the distance is to shore is approximately 15 km. Despite the difficulties in finding adequate pairs of treatment and control sites near offshore wind farms with long enough records of hourly precipitation data before and after the farm, we plan to extend this analysis to more sites and more countries in the future.

These results tend to demonstrate that the large-scale deployment of technologies such as offshore wind turbines will significantly impact local weather conditions and therefore should be accounted for as part of the environmental impact assessment of related energy projects. For instance, the design and location of offshore wind farms can be such that the impact on onshore precipitation is minimized, as argued in this study. Ultimately, this paper addresses the potential implications of decreased precipitation on human and agricultural systems located downwind from offshore wind farms.