Brief description of the measuring principle
The measurement principle is based on the emission of an optical signal in the near infrared (1.5 µm). Due to its characteristic wavelength, the signal is scattered by aerosol particles and cloud droplets as it passes through the atmosphere. Part of the backscattered infrared signal is received by the Doppler lidar detector and converted by an optical superposition process ("heterodyne principle") into a measurable high-frequency signal from which the backscatter intensity and frequency change are determined.
The cause of the detected frequency shift is the natural motion of the backscattering aerosol and cloud particles (Doppler effect), from which the radial wind is derived. The vertical range depends on the aerosol content and is limited due to extinction in optically thick cloud cover.
Using radial wind measurements from different directions, the three-dimensional wind vector can be determined by suitable derivation methods. In addition, the determination of cloud cover and cloud height as well as various boundary layer parameters is a subject of current research.
Features of the Doppler lidar
The Doppler lidar in Meppen is usually configured as an optical wind profiler - with the aim of determining vertical profiles of the wind vector from about 100 m to, depending on backscatter conditions, a maximum height of 10 km. For each wind vector measurement from the radial wind measurement, five linearly independent directions are required to calculate the wind vector under assumptions about the horizontal homogeneity of the wind field. The 200s hemispherical scanner allows scanning of the entire upper half-space.
In the current measurement configuration, the 3 dimensional wind component and a horizontal section are currently performed every 15 min.
The "Leosphere 200s" Doppler lidar from Vaisala has been in testing for field use in Meppen since April 2019.