Steven Knoop

Over last few years we have observed tens of atmospheric gravity wave (GW) events on our operational network and research instruments in The Netherlands, including the North Sea. These events are ducted GWs that are trapped in the stable boundary layer and propagate horizontally, characterized by a near-monochromatic wave with vertical velocity amplitudes up to a few m/s, a period of a few minutes, a duration of one hour or more, and a spatial extent on the order of 100 km or more. The origin of those GWs are non-orographic, and likely linked to weather fronts. These GWs can lead to a strong modulation of wind, temperature and humidity in the lower 100m’s of the atmosphere.We observe these GWs by various instruments, including Doppler lidars, automatic lidar ceilometers, weather radars, (tower) in-situ sensors and microbarometers that are part of the KNMI synoptic observation network, Cabauw atmospheric research station or North Sea wind lidar network. Together they provide 3D information on mesoscale GWs, including vertical profiles, horizontal spatial extent, and evolution during propagation. We have recently highlighted two GW events: in the Netherlands and Belgium in the night and early morning of June 30, 2022 [1], over the North Sea in the night and early morning of May 2, 2025 [2].Here we give an overview of our research on boundary-layer GW observations. We explore the occurrence, properties and impact of those GW events in the Netherlands, both onshore and offshore. These comprehensive sets of observations may serve as a testbed for high resolution weather models that aim to capture mesoscale GW events and the effect they have on the (lower) atmosphere. The siting of our North Sea wind lidars, i.e., in the middle of large offshore wind farms, provides the possibility to study the effect of GWs on wind farm performance.[1] Knoop S, Assink J D, Leijnse H, Tijm S, de Haij M J, Bosveld F C, Theeuwes N E, Evers L G, Unal C and Laffineur Q, “High-resolution observations of a mesoscale gravity wave event in the nocturnal boundary-layer over The Netherlands and Belgium”, submitted to Journal of Geophysical Research: Atmospheres (preprint on https://doi.org/10.22541/essoar.176478826.60490095/v1)[2] Knoop S, Assink J D, de Haij M J, de Jong M, “Doppler lidar gravity wave observations within North Sea wind farms”, to be published in Journal of Physics: Conference Series (Torque 2026)

Over the past decade, the Royal Netherlands Meteorological Institute (KNMI) has undertaken significant effort to integrate emerging surface-based remote sensing technology in its services for operations and research. KNMI deploys several types of ground-based lidar instruments to detect clouds and volcanic ash, and measure vertical profiles of aerosols, wind and water vapor. Real-time information on the vertical structure of the atmosphere is becoming increasingly important input for nowcasting purposes and models supporting KNMI’s forecasting, climate, early warning, maritime, and energy tasks.Since the late 1990’s KNMI deploys automatic lidar ceilometers (ALC) for continuous (24/7), fully automated observations of cloud base height and cloud cover. These ALCs are integrated in our nationwide automatic weather station network and currently consists of 45 stations, including civil and defense airports and North Sea platforms. ALC backscatter profile data provides aerosol layer information, including the boundary layer and elevated aerosol layers, allowing to monitor Sahara dust, wildfire smoke and volcanic ash. Those different types of aerosols can be differentiated with the Raman lidars at the Cabauw atmospheric research station, part of the Ruisdael Observatory and ACTRIS National Facility. One Raman lidar also measures vertical profiles of the water vapor mixing ratio. The KNMI ALCs are part of the European ALC network within E-PROFILE. Current research focuses on the real-time retrieval of boundary layer height from ALC backscatter data and its transition from research to operations (R2O). The ground-based lidars at Cabauw play a significant role in validation of space-born lidars, such as EarthCARE.Since end 2010’s KNMI uses Doppler lidars that measure wind. The development of wind farms on the Dutch North Sea provided the opportunity to deploy short-range wind lidars offshore. The current network consists of 7 stations, within 4 wind farms. The measured wind profiles are made available to key users, including the wind farm operators and KNMI forecasters, in near-real time. At Cabauw, a long-range scanning Doppler lidar provides vertical profiles of horizontal wind and vertical velocity in the boundary layer, from which for instance low-level jets, mixing height and gravity waves are detected. It is part of the European DWL network within E-PROFILE. More recently, KNMI deployed a long-range scanning Doppler lidar for research and operational deployment at Amsterdam Schiphol Airport, supporting KNMI’s aviation services. Research includes the development of aircraft wake vortex persistence monitoring or alerting systems for dangerous wind phenomena such as wind shear or gust fronts.Here we will present an overview of KNMI’s ground-based lidar activities, along with an outlook on future developments and opportunities.

We present observations of a gravity wave (GW) event over the Netherlands and Belgium in the night and early morning of June 30, 2022. This event was captured by instruments that are part of the meteorological observational network of The Netherlands and Belgium, including the operational weather radars and automatic lidar ceilometers, and by the (tower) in-situ and Doppler lidar at the Cabauw atmospheric research station. Together they provide 3D information on the GW event, with detailed information on the vertical profiles from Cabauw, and the spatial extent and evolution from the observational synoptic network. Two distinct GW trains were observed, both interpreted as ducted GW that are trapped in the lowermost 500 m of the stable nocturnal boundary layer. The GWs showed large vertical velocity amplitudes up to 3 m/s, resulting in strong modulations of wind, temperature, humidity and pressure. By comparing our observations to the Copernicus European Regional ReAnalysis (CERRA) wind and temperature fields, we interpret that these GWs were generated by convergence lines over Belgium and the North Sea that was linked to an occlusion front.

We present observations of a gravity wave (GW) event over the Netherlands and Belgium in the night and early morning of 30 June 2022. This event was captured by instruments that are part of the meteorological observational network of The Netherlands and Belgium, including the operational weather radars and automatic lidar ceilometers, and by the (tower) in situ and Doppler lidar at the Cabauw atmospheric research station. Together they provide 3D information on the GW event, with detailed information on the vertical profiles from Cabauw, and the spatial extent and evolution from the observational synoptic network. Two distinct GW trains were observed, both interpreted as ducted GW that are trapped in the lowermost 500 m of the stable nocturnal boundary layer. The GWs showed large vertical velocity amplitudes up to 3 m/s, resulting in strong modulations of wind, temperature, humidity and pressure. By comparing our observations to the Copernicus European Regional ReAnalysis (CERRA) wind and temperature fields, we interpret that these GWs were generated by convergence lines over Belgium and the North Sea that was linked to an occlusion front.

We present observations of atmospheric gravity waves (AGW) on the Dutch North Sea, using short-range wind lidars that are deployed within offshore wind farms for operational wind measurements. This weather phenomenon can lead to high-amplitude oscillations in the wind speed and wind direction with a period of a few minutes, and therefore impact wind energy. As an example we show wind lidar data for the AGW event of 2025-05-02, with observations from six stations within four wind farms ( Borssele, Hollandse Kust Zuid, Hollandse Kust Noord and Hollandse Kust West ). We also show backscatter profiles from collocated automatic lidar ceilometers that shed light on the vertical extent of the AGW. Our observations can contribute in the understanding of the impact of AGWs on wind farm performance.

Over last few years we have collected tens of atmospheric gravity wave (GW) events, using a long-range scanning Doppler lidar at Cabauw atmospheric research station in the Netherlands, and our North Sea wind lidar network, in which short-range wind lidars are deployed on platforms within offshore wind farms. These events are ducted GWs that are trapped in the stable boundary layer and propagate horizontally, characterized by a near-monochromatic wave with vertical velocity amplitude up to a few m/s, a period of a few minutes, and duration of a hour or more. The origin of those GWs are non-orographic, and likely linked to fronts and convergence lines. These GWs can lead to a strong modulation of wind in the lower 100m’s of the atmosphere, and are therefore relevant for wind energy. Vertical velocity data from Doppler lidars, from either continuous vertical stare measurements or derived from wind profiling scans, provide a direct way to observe and characterize these GWs. We also consider other observations, including tower in-situ measurements at Cabauw, weather radars and our nationwide automatic lidar ceilometers (ALC) network. Together they provide 3D information on the GW event, with detailed information on the vertical profiles from Cabauw, and the spatial extent and evolution from the observational synoptic network.Here we present two GW events. Firstly, we show observations of a GW event over the Netherlands and Belgium in the night and early morning of June 30, 2022 [1]. Two distinct GW trains were observed, both interpreted as ducted GW that are trapped in the lowermost 500 m of the stable nocturnal boundary layer. The GWs showed large vertical velocity amplitudes up to 3 m/s, resulting in strong modulations of wind, temperature, humidity, and pressure. Secondly, we present observations from the GW event over the North Sea and the Netherlands in the night and early morning of May 2, 2025. This event was also captured by multiple stations in our North Sea wind lidar network.This work provides a starting point to further explore the occurrence and properties of (anomalous) boundary-layer gravity wave events in the Netherlands, including the North Sea. These comprehensive sets of observations may serve as a testbed for high resolution weather models that aim to capture these type of GW events and the effect they have on the (lower) atmosphere. The particular siting of our North Sea wind lidars, i.e., in the middle of large offshore wind farms, provides the possibility to study the effect of GWs on wind farm performance.[1] Knoop S, Assink J D, Leijnse H, Tijm S, de Haij M J, Bosveld F C, Theeuwes N E, Evers L G, Unal C and Laffineur Q 2025 “High-resolution observations of a mesoscale gravity wave event in the nocturnal boundary-layer over The Netherlands and Belgium”, submitted to Journal of Geophysical Research: Atmospheres, preprint on https://doi.org/10.22541/essoar.176478826.60490095/v

The Royal Netherlands Meteorological Institute (KNMI) operates several ground-based Doppler lidars, which are laser-based remote sensing instruments to measure wind. We deploy long-range scanning Doppler lidars at our atmospheric research site Cabauw (Vaisala Windcube200S) since 2021 and at Amsterdam Schiphol Airport (Leonardo Skiron3D) since 2025, after a two-year deployment at Cabauw. Together with Rijkswaterstaat (RWS) we operate a network of short-range Doppler lidars (ZX lidars ZX300M) on the North Sea, on TenneT substations within offshore wind farms. Our Doppler lidars are based on infrared laser light and rely on aerosol scattering. As such, the wind measurements are typically bounded to the atmospheric boundary layer and are limited by (optically dense) clouds. The short-range Doppler lidars provide wind profiles up to a height of 300m.The Doppler lidar instruments allow for unattended and continuous 24/7 operation and data is collected in near-real time. This enables the use of this data to forecasters for nowcasting, for data assimilation into numerical weather prediction models and for research purposes. These instruments can operate during severe storms, in which they can provide information of the wind field below the cloud base. Furthermore, Doppler lidars can provide boundary layer wind profiles prior to such storms, which could lead to better forecasting of those storms.Here we present an example of Doppler lidar measurements during a severe convective weather event of 9th of July 2024, a day at which KNMI issued an orange weather warning (“code oranje”) for the whole of the Netherlands. At the time both long-range scanning Doppler lidars were operational at Cabauw. The Skiron3D was performing Velocity Azimuth Display (VAD) scans providing vertical profiles of horizontal wind speed and direction with a range resolution of 25m and 100m respectively with an update frequency of approximately 3 minutes. The Windcube200s was performing continuous vertical velocity measurements with a temporal resolution of 1 s and a vertical resolution of 75m. On radar imagery and lightning detection system is observed that the showers and lightning arrives at 17:10 UTC. Signs of approaching severe weather are already visible in the Doppler lidar data starting at 15:00 UTC, indicated by a growing layer with a differing wind direction. Strong updrafts are observed shortly before the arrival of the showers. Heavy rainfall obscured the lidar measurements only for a very short period, such that valuable information can be derived from the lidar measurements before, during and after the storm.

Kelvin–Helmholtz (K-H) or shear instability may occur in continuously stratified flows and along interfaces between fluids associated with strong vertical shear. This process is suspected to be a major source of clear air turbulence and a contributor to vertical mixing of momentum and scalars in the atmospheric boundary layer. It is believed that it plays a substantial role in intermittent turbulence generation and vertical mixing in a stable boundary layer. Occasionally, a K-H instability becomes visible as K-H billow clouds. Here we present an observation of a K-H instability on the early morning of July 25, 2022 at the Cabauw atmospheric research station in The Netherlands. The K-H instability is observed in Doppler lidar vertical stare measurements (with 1s temporal resolution) as a prominent wavelike pattern in the vertical velocity centered at altitude of 1000m, in which the upper and lower halves are shifted in phase. These K-H waves have a period a few minutes, with vertical velocity amplitudes up to 2m/s, and last for about 15 minutes. Maximum eddy dissipation rate is found to be 2x10-3m2s-3. The residual layer that night was up to 2000m, providing sufficient aerosol backscatter signal for Doppler lidar measurements to observe these K-H waves.The Doppler lidar wind profile measured just before the appearance of the K-H instability showed a strong wind shear (0.04s-1) with a shear layer depth of about 300m. The Richardson number (Ri) profiles, derived from combining the vector wind shear (Doppler lidar) and Brunt-Vaisala frequency profiles (microwave radiometer), shows Ri

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/ems2023-27

In the current changing climate, national meteorological services are focussing more of their efforts on the prediction of weather extremes at local scales. Forecasting the weather where people live. In order to use the state-of-the-art numerical weather prediction (NWP) models at the scales of cities or individual neighbourhoods, a step-change in resolution is required. Where we currently run NWP models at tens of kilometres globally and a few kilometres regionally, modelling weather extremes in cities would require hectometric resolutions. Here, we highlight the development and research required to go towards these scales. These scales not only require changes in the urban canopy parameterisations and more detailed land-use characteristics, but they also require development in other parts of the model. For example, the way clouds and convection are parameterised are important for the triggering of resolved convection. These parameterisations also influence near-surface variables. Additionally, turbulence parameterisations should be adjusted to consider the grey zone of turbulence at 100-m scale resolution. We also show how to use detailed (remote sensing) observations combined with idealised large eddy simulation (LES), to verify whether the NWP is resolving the right processes. The combination or LES and wind and turbulence profiles from remote sensing at an urban (Rotterdam) and a rural site (Cabauw) in the Netherlands is used to further develop NWP models, like HARMONIE-AROME, at urban scales.