Analysis of near-surface relative humidity in a wind turbine array boundary layer using an instrumented unmanned aerial system and large-eddy simulation

Simulation and modeling have shown that wind farms have an impact on the near-surface atmospheric boundary layer as turbulent wakes generated by the turbines enhance vertical mixing. While a few observational data sets that focus on near-surface temperature changes exist, these studies lack high spatial resolution and neglect the combined effect of these temperature changes with an altered humidity profile. With a large portion of wind farms hosted within an agricultural context, changes to relative humidity can potentially have secondary impacts, such as to the productivity of crops. The goal of this study is to gather high-resolution in situ field measurements in the wake of a single wind turbine in order to differentially map downstream changes to relative humidity. These measurements, obtained by an instrumented unmanned aerial system, are complemented by numerical experiments conducted using large-eddy simulation. Observations and numerical results are in good general agreement around a single wind turbine and show that downstream relative humidity is differentially altered in all directions, specifically decreased below the turbine hub height. Large-eddy simulation is then used to determine the effect of a large 7x4 turbine array on the relative humidity distribution in compounding wakes. It is found that the region of relative humidity decrease below the turbine hub height and the region of increase above the hub height both intensify, differentially extend in the lateral directions, and move slightly upward with downstream distance.