By dr. Louise Nuijens
In the past few months over summer, TU Delft and KNMI have been bundling efforts towards making extensive wind measurements at Cabauw with as many as three wind lidars. The broad objective is to provide high-resolution 3D wind profiles that are used to understand the air motions (horizontal and vertical winds) in the boundary layer and to validate large-eddy simulations and weather forecasts. Operating three wind lidars at the same site might seem redundant, but the opposite is true! Not only do the complementary measurements lead to higher resolution wind products, they guarantee continuous measurements when an instrument fails. The preliminary images offer an intriguing look into the wind variance that is present throughout the convective and cloud-topped boundary layer.
A single wind lidar can perform various scanning strategies. By pointing vertically at nadir, the lidars can measure vertical wind speed (updrafts and downdrafts). By making a slanted (off-nadir) 360-degree scan, the lidars can measure horizontal wind speed and direction. If a single lidar alternates between these strategies, the horizontal and vertical wind measurements are not taken simultaneously, and a lower temporal resolution is reached. To study turbulent and coherent convective plumes in the boundary layer, a high temporal resolution and a measurement of the three wind components simultaneously is ideal.
Helpful redundancy in July
The two WindCubes and the Skiron3D started performing complementary measurements in June 2023. The KNMI WindCube is set to measure the vertical wind at 50 m range resolution and 1s temporal resolution, while the TU Delft WindCUbe measures the horizontal wind at 50 m range and 2s mean temporal resolution. The Skiron3D is a more powerful lidar with greater range and sensitivity, which was set to measure the horizontal wind at 25 m range and 15s temporal resolution. It is a great addition to help cross-validation the other wind measurements, but will only be at Cabauw temporarily before moving to Schiphol. A few days into July, the KNMI WindCube decided to take vacation and stopped measuring. An unfortunate event! To guarantee 3D wind measurements, the TU Delft WindCube switched to vertical wind measurements, making the Skiron3D an essential addition.
In July, the dual-frequency CLARA radar also joined the experiment by performing two complementary scan modes. One mode is slanted, where CLARA retrieves the radial velocities at the same elevation as the wind lidars, allowing wind retrievals within clouds. The other is zenith pointing, allowing to identify clouds and precipitation and estimate the vertical wind velocity component within the clouds. For both scan modes, the range resolution spans from 22 – 30m (surface up to 12km), while the temporal resolution of the horizontal wind profiles is 72 seconds and of the vertical wind is 1s.
First look at wind evolution and variability
Several weeks into the operations we realized that July was an unusually cloudy, rainy and windy month. The frequent frontal passages led to strong winds and often gusty conditions and plenty of vigorous cumulus convection. Before that, in June, some more summertime fair-weather cumulus conditions were captured. In the below figure, a compilation of quicklooks gathered on June 27 reveal how convective plumes influence winds. The growth of the boundary layer starting at sunrise can be seen in the general ceilometer backscatter signals that increase with height from approximately 6 AM onwards. Within the boundary layer winds appear well-mixed at wind speeds less than about 5- 7 m/s. From 10AM vertical updrafts and downdrafts strengthen and just before 11AM the first cumulus clouds start forming (as seen in the ceilometer data). At about the same time horizontal wind speed variance is increasing, with wind speeds regularly reaching 10 m/s.
Figure 1. Quicklooks from the ceilometer and three wind lidars on June 27, 2023
Quicklooks from all instruments and cloud satellite snapshots can be accessed through the following quicklook browsers implemented by Jose Dias Neto:
- CMTRACE quicklook browser
- Cloud satellite snapshots from MODIS Aqua, June 2023
- Cloud satellite snapshots from MODIS Aqua, July 2023
- Cloud satellite snapshots from MODIS Aqua, August 2023
Supporting projects, people and funding
TUD WindCube: In March 2023, TU Delft acquired a Vaisala WindCube 200s, funded by the CMTRACE VIDI project of dr. Louise Nuijens, the Ruisdael project and the Civil Engineering and Remote Sensing faculty. The CMTRACE project aims to vizualise and understand air motions (horizontal and vertical winds) in the sub-cloud layer and into clouds, and to study the momentum transport by the convective and mesoscale circulations tied to clouds. The WindCube was installed and tested in the Green Village at the TU Delft campus with the support of Rob McKenzie and dr. Jose Dias Neto, where the EWI building served as a hard target in the alignment calibration of the instrument, and then moved to Cabauw.
Figure 2. Testing and alignment calibration of TUD Windcube at the Green Village
The KNMI WindCube, operated by dr. Steven Knoop, was purchased through the Ruisdael program and installed at Cabauw in April 2021 (see earlier new item here). So far its measurements were mainly focused on vertical profiles of the horizontal wind in the boundary layer. During the summer of 2022 dedicated measurements on the vertical velocity were performed to study the up- and downdrafts in the boundary layer (together with dr. Natalie Theeuwes).
KNMI has recently purchased the Skiron 3D Windlidar from Leonardo. The purchase was funded by KNMI’s aviation innovation program RP3+. At present the Skiron 3D is operated at Cabauw for commisioning, training, testing and atmospheric research. The main purpose besides atmospheric research is product development for operational use in aviation. At a later stage the instrument will be moved to Schiphol Airport to determine airport wind climate and aviation winds. Principal researcher for the Skiron 3D is dr. Tiemo Mathijssen.
Figure 3. The two scanning Windcubes (left) and the Skiron 3D (right) at Cabauw