A New Airborne Doppler Lidar System for Boundary-Layer Research — a First LES Study

Airborne Doppler Lidar (ADL) measurements offer new insights into atmospheric flow phenomena. We present a new ADL system for use aboard the Dornier 128-6 (D-IBUF) research aircraft of the Institute of Flight Guidance, TU Braunschweig. The system is developed in a joint project of KIT and TU Braunschweig. A Lockheed Martin WindTracer WTX Doppler lidar measuring at 1.6 mm wavelength, with a pulse duration of 300 ns and 5,4 kW peak power, is operated at a pulse repetition frequency of 750 Hz and 100 ms accumulation time. It will be coupled with a new airborne scanner built by Arges GmbH. This 2-axis scanner covers ±140° elevation and ±30° azimuth at scan rates of 20° s^-1. The system allows for flexible scanning geometries and automatic correction of aircraft movement. It yields high spatial resolution due to the slow flying aircraft (65 m s^-1) and high measurement frequency.

As a part of this study, a Doppler lidar simulator has been developed based on large eddy simulations. The simulator is capable of emulating the full measurement process and allows for an evaluation of the measurement quality and errors. Therefrom, we are able to analyze the skills of different possible scanning geometries and flight pattern. The simulation results show that the new system will be able to provide high resolution wind data for boundary-layer research (<50 m for vertical wind and approximately 1 km for horizontal wind, depending on distance from aircraft). More than 30000 simulations allow for a statistical evaluation of currently used error propagation characteristics in Doppler wind measurements for the horizontal wind speed. One of the results illustrates that without accounting for the autocorrelation in measurements, due to the spatial coherence of the measured wind field, the uncertainty estimates of the retrieved horizontal wind speed can be erroneous.