Gravity wave observations by (offshore) short-range wind lidars

Short-range vertical profiling wind lidars are extensively used in the context of wind energy. These instruments measure wind profiles up to 200-300 m and their 10-minute averaged wind products are commonly used. Here we discuss their ability to observe atmospheric gravity waves (GW), using the unaveraged data. Trapped GWs in the stable boundary layer that propagate horizontally can result in a strong modulation of wind (and other meteorological variables) with periods similar or smaller than 10 minutes. These periods are such that GWs might be missed in the standard wind lidar data products. In general, the measurements of vertical velocity profiles are the most suitable for GW detection and characterization, while the horizontal wind profiles directly link to the possible impact on wind energy.

Here we consider the ZX300M (ZX Lidars) wind lidar, which is operationally deployed on offshore platforms within the Dutch offshore wind farms [1]. The ZX300M is a continuous-wave focusing vertical profiling wind lidar. It measures horizontal wind speed, wind direction and vertical velocity in a range of 10 m to 300 m above the instrument, using velocity azimuth display (VAD) scans of 1 second and a scanning cone angle of 30° (with respect to zenith). Up to 11 heights are measured sequentially.

To demonstrate the ability to observe GWs with these instruments we have identified two GW events, in the stable nocturnal boundary layer, during the two-year measurement campaign of the ZX300M wind lidar at our atmospheric supersite Cabauw in 2018-2020 [2]. We compare the unaveraged ZX300M data with in-situ wind measurements in the 213-m tall mast, in particular the vertical velocity as measured by 3D sonic anemometers. We observe a high correlation between the ZX300M wind lidar and in-situ measurements for those GW events. We conclude that the ZX300M nicely captures those GWs, limited by its maximum range of 300 m.

Finally, we present our first GW observations from the operational ZX300M instruments within the offshore wind farms. We highlight the GW event on 2022-05-19 observed at platforms Borssele Alpha and Borssele Beta (7 km apart) within offshore wind farm Borssele. Our growing offshore network of wind lidars provided a unique opportunity to detect offshore gravity waves and study their impact on the offshore wind energy.

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

[2] Knoop, S., Bosveld, F. C., de Haij, M. J., and Apituley, A.: A 2-year intercomparison of continuous-wave focusing wind lidar and tall mast wind measurements at Cabauw, Atmos. Meas. Tech., 14, 2219, 2021, https://doi.org/10.5194/amt-14-2219-2021