Figure 1. The IIHR Wind Sounding lidar.

The transverse wind component is measured by projecting a series of beams which are evenly spaced in the transverse direction. An aerosol structure traveling through the beam will produce a modulation in the lidar signal at that range having a frequency, f = v / d where d is the distance between each of the beams and v is the component of velocity in the direction of the line of beams. An example of the signal from the system is shown in figure 2. The resulting Fourier transform of the signal is shown in figure 3.

Figure 2. A sample lidar signal showing periodic signals as the atmospheric structures travel through the series of laser beams.

Figure 3. The fast Fourier Transform of the signal in figure 2 showing a peak at 5 Hz.

The technique provides instantaneous velocities as well as mean velocities. Thus some turbulence quantities ( e.g., turbulent intensities, Reynolds stresses, and higher moments or statistics) can be derived. In addition, particulate-related quantities can also be measured to obtain such quantities as cloud height and optical depth / reflectivity or boundary layer height and relative particulate loading with altitude. The vertical wind velocity is found from a correlation analysis. The current system can provide wind measurements every 1.5 meters in altitude throughout the depth of the boundary layer (generally 1 to 3 km in altitude). Wind velocities can be determined on time scales as short as 2.5 seconds. Longer term averaging is also possible resulting in more precise wind measurements.

More Information

For more information about the IIHR wind sounding lidar or atmospheric dynamics, please contact Prof. Bill Eichinger.