Doppler lidar gravity wave observations within North Sea wind farms

Atmospheric gravity waves (GW) are small-scale propagating disturbances that arise due to the vertical forcing of air parcels by topography, convection, wind shear, jet streams, frontal systems and other tropospheric sources. GWs can be trapped in the stable boundary layer, propagate horizontally and cause a strong modulation of wind, temperature and humidity [1]. When passing a wind farm those AGWs cause sudden sharp increases and decreases in wind speed at rotor height and might affect the power generation.

Here we present GW observations from short-range wind lidars that are deployed on platforms within offshore wind farms in the Dutch North Sea for operational wind profile measurements [2]. The GWs are most easily recognized in the vertical velocity profiles, while the simultaneously measured horizontal wind reveals the potential impact on the wind farm performance. Typical GW periods are about 5 to 10 minutes, meaning that those GWs are easily missed when considering only the standard 10-minute averaged wind lidar data. Collocated automatic ceilometer lidars can provide additional information on the GWs.

Our North Sea wind lidar network currently consists of six platforms within four Dutch offshore wind farms (Borssele, Hollandse Kust Zuid, Hollandse Kust Noord and Hollandse Kust West) and grows together with the Dutch offshore wind energy development. We have collected about 20 GWs cases in the last five years, of which a few examples will be shown. Special emphasis will be made on GWs events that are observed throughout the whole network, including our onshore Cabauw atmospheric research site, highlighting the mesoscale nature of those GWs. Our observations offer the possibility to study offshore GWs, while the particular siting of our observations allows to directly relate the GWs to wind farm performance.

[1] Knoop, S., Assink, J., Tijm, S., and Leijnse, H.: High-resolution observations of a gravity wave event over the Netherlands, EMS 2024, https://doi.org/10.5194/ems2024-445

[2] Knoop, S. and de Jong, M.: Wind lidars within Dutch offshore wind farms, EMS 2023, https://doi.org/10.5194/ems2023-271