4. October 2022 | Magazine:

Picture of the month: Five seconds of fog Precision wind measurements of the Physikalisch-Technische Bundesanstalt

From left to right: five seconds of dense fog. Our picture of the month shows a section of the wind lidar project of the Physikalisch-Technische Bundesanstalt (PTB). In this project, the team around Dr. Michael Eggert and Paul Wilhelm is constantly developing a system that sets new standards in remote wind measurements: PTB’s bistatic wind lidar. Although the fog image is rather a curious special case (the lidar can be used even at very low particle densities and clear visibility): In fog, the novel measuring instrument demonstrates its performance and resolution particularly impressively.

Wind lidar image of five seconds of fog. On the left of the image, even a turbulence caused by a neighbouring measuring mast appears. Picture credits: Paul Wilhelm/PTB

“Lidar works similarly to radar, except that lidar works in the infrared spectrum: We send light into the atmosphere and observe how it comes back to us. Using the Doppler effect, we can precisely calculate how fast individual aerosol particles are,” says Dr Michael Eggert, explaining the device, which measures the wind at any location on its own. “This precision determines where electricity can be sensibly generated from the wind. Every breath of air can make the difference between efficiency and a bad investment here.”

However, this also requires complex signal processing. PhD student Paul Wilhelm has an eye on precisely this mass of data. Our picture of the months shows a glimpse of this: A spectrogram that impressively shows the Doppler effects of a dense fog at an altitude of 200 metres measured by the lidar system. Each of the countless aerosol particles makes its contribution to the image. Even turbulence caused by a neighbouring measurement mast comes to light.

Matchmaker Summer School

The fact that the Carolo-Wilhelmina can now get into the mood for autumn with a bit of fog is owed to the B-IGSM Summer School, a joint graduate school of PTB and TU Braunschweig. During a week of intensive exchange on metrological (and in this case even meteorological) topics, the images from the wind lidar project came to the fore and ensured lively discussion in the context of a poster session.