How do convective cold pools influence the atmospheric boundary layer 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 atmospheric boundary layer. 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 min 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 of up to +4 m s-1 in addition to the background flow, a relative wind direction shift of 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 min. We find a "nose shape"in relative wind speeds and virtual potential temperature (θv) 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 static 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 700-800 m, providing an estimate for cold-pool depth and highlighting that cold-pool impacts would typically extend beyond the height of current onshore wind turbines. After the cold-pool gust front passage, there is gradually increasing static stability, with a median decrease in near-surface θv of-2.7 K, a gradual decrease in hub-height turbulence strength, and faster recovery of wind speed than wind direction.