How do convective cold pools influence the boundary-layer atmosphere near two wind turbines in northern Germany?

Abstract. With increasing wind energy in the German energy grid, it is crucial to better understand how particular atmospheric phenomena can impact wind turbines and the surrounding boundary-layer atmosphere. Deep convection is one source of uncertainty for wind energy prediction, with the near-surface convective outflow (i.e., cold pool) causing rapid kinematic and thermodynamic changes that are not adequately captured by operational weather models. Using 1-minute meteorological mast and remote-sensing vertical profile observations from the WiValdi research wind park in northern Germany, we detect and characterize 120 convective cold pool passages over a period of 4 years in terms of their temporal evolution and vertical structure. We particularly focus on variations in wind-energy-relevant variables (wind speed and direction, turbulence strength, shear, veer and static stability) within the turbine rotor layer (34–150 m height) to isolate cold pool impacts that are critical for wind turbine operations. Near hub-height (92 m) during the gust front passage, there are relatively increased wind speeds up to +4 m s⁻¹ in addition to the background flow, a relative wind direction shift up to +15°, and increased turbulence strength for a median cold pool. Given hub-height wind speeds lying within the partial load region of the power curve for the detected cases, there is an increase in estimated power of up to 50 % which lasts for 30 minutes. We find a ’nose shape’ in relative wind speeds and θ_v at hub-height during gust front passages, with larger wind direction changes closer to the surface. This manifests as asymmetric fluctuations in positive shear, negative veer, and stability across the rotor layer, with relative variations below hub-height at least twice as large compared with above hub-height and temporarily opposite signs for stability that have complex implications for turbine wakes. Doppler wind lidar profiles indicate that kinematic changes associated with the gust front extend to a height of 650–700 m, providing an estimate for cold pool depth and highlighting that cold pool impacts would typically extend beyond the height of current wind turbines. After the cold pool gust front passage, there is gradually increasing stability, with a decrease in θ_v to -2 K, a gradual decrease in turbulence strength, and faster recovery of wind speed than wind direction.