Airborne forward-pointing UV Rayleigh lidar for remote clear air turbulence detection: system design and performance.

A high-performance airborne UV Rayleigh lidar system was developed within the European project DELICAT. With its forward-pointing architecture, it aims at demonstrating a novel detection scheme for clear air turbulence (CAT) for an aeronautics safety application. Due to its occurrence in clear and clean air at high altitudes (aviation cruise flight level), this type of turbulence evades microwave radar techniques and in most cases coherent Doppler lidar techniques. The present lidar detection technique relies on air density fluctuation measurement and is thus independent of backscatter from hydrometeors and aerosol particles. The subtle air density fluctuations caused by the turbulent air flow demand exceptionally high stability of the setup and in particular of the detection system. This paper describes an airborne test system for the purpose of demonstrating this technology and turbulence detection method: a high-power UV Rayleigh lidar system is installed on a research aircraft in a forward-looking configuration for use in cruise flight altitudes. Flight test measurements demonstrate this unique lidar system being able to resolve air density fluctuations occurring in light-to-moderate CAT at 5 km or moderate CAT at 10 km distance. A scaling of the determined stability and noise characteristics shows that such performance is adequate for an application in commercial air transport.

Performance evaluation for long-range turbulence-detection using ultraviolet lidar.

Clear-air turbulence could be detected at long range using a UV lidar. Because the vertical speed cannot be retrieved from Doppler shift analysis at long range, the turbulence detection is based on the measurement of molecular density fluctuation associated with the turbulent wind. After an optimization of the characteristics of the candidate UV lidar, we present an evaluation of the detection range and of the false alarm rate and missed alarm rate depending on the altitude and vertical velocity root mean square. This study shows that 96% of turbulence with vertical velocity leading to dislodging of unsecured objects in the airplane can be detected at 15 km using a 2 W laser at 355 nm with a false alarm rate of 0.18 per flight hour.