Assessment of nocturnal low-level jets during the FESSTVaL campaign 2020 for wind energy applications

Due to the increasing contribution of wind power to electricity in Europe, an exact wind characterization at the height of wind turbines is important. Nocturnal Low Level Jets (NLLJ) can influence the winds at typical blade heights and therefore influence the wind power production. However, due to the often missing measurements with a sufficient precision and resolution, the occurrence frequency and spatio-temporal characteristics of NLLJs are still poorly understood. The present work characterizes the properties of NLLJs, measured with a Doppler lidar at the Lindenberg Meteorological Observatory – Richard Aßmann Observatory (Germany), during the period of June–August 2020, and evaluates the representation of NLLJs in state-of-the-science re-analysis products. The vertical profiles of 10-minute mean winds from the lidar are statistically analysed using automated detection tools. These allow to determine the frequency of occurrence, height and wind speed in the core of NLLJs as well as the vertical wind shear and momentum transport with a high temporal resolution. We intercompare NLLJ results from different previously-used identification tools to estimate the uncertainty. Our automatic detections identified NLLJs in more than about 60% of the summer nights in 2020, with NLLJ cores between 70m and 500 m above ground level and a core speed of ~3–25 m/s. The prevailing wind direction in NLLJ cores is southwest. A considerable amount of NLLJ cores occurred at heights that are in the range of modern wind turbines and rotor sizes on land, with wind speeds of ~3-12 m/s. We use the measurements of NLLJs to evaluate their representation in the ERA5 re-analysis of the European Centre for Medium-Range Weather Forecasts and plan to compare the NLLJs to regional high-resolution re-analyses developed in the research area Climate Monitoring and Diagnostics in the Hans-Ertel Centre for Weather Research.  The first comparisons suggest a frequent co-occurrence of NLLJs in the measurements and ERA5 re-analysis, but the strength and height of NLLJ cores often differ. Possible reasons are the model’s vertical resolution and the parameterization of vertical mixing in the stable boundary layer. Future work includes extending the NLLJ analysis to more lidar measurements and other regional re-analysis data.