Calibrations and Wind Observations of an Airborne Direct-Detection Wind LiDAR Supporting ESA’s Aeolus Mission

Abstract

The Aeolus satellite mission of the European Space Agency (ESA) has brought the first wind LiDAR to space to satisfy the long-existing need for global wind profile observations. Until the successful launch on 22 August 2018, pre-launch campaign activities supported the validation of the measurement principle, the instrument calibration, and the optimization of retrieval algorithms. Therefore, an airborne prototype instrument has been developed, the ALADIN Airborne Demonstrator (A2D), with ALADIN being the Atmospheric Laser Doppler Instrument of Aeolus. Two airborne campaigns were conducted over Greenland, Iceland and the Atlantic Ocean in September 2009 and May 2015, employing the A2D as the first worldwide airborne direct-detection Doppler Wind LiDAR (DWL) and a well-established coherent 2-µm wind LiDAR. Both wind LiDAR instruments were operated on the same aircraft measuring Mie backscatter from aerosols and clouds as well as Rayleigh backscatter from molecules in parallel. This paper particularly focuses on the instrument response calibration method of the A2D and its importance for accurate wind retrieval results. We provide a detailed description of the analysis of wind measurement data gathered during the two campaigns, introducing a dedicated aerial interpolation algorithm that takes into account the different resolution grids of the two LiDAR systems. A statistical comparison of line-of-sight (LOS) winds for the campaign in 2015 yielded estimations of the systematic and random (mean absolute deviation) errors of A2D observations of about 0.7 m/s and 2.1 m/s, respectively, for the Rayleigh, and 0.05 m/s and 2.3 m/s, respectively, for the Mie channel. In view of the launch of Aeolus, differences between the A2D and the satellite mission are highlighted along the way, identifying the particular assets and drawbacks.

Rayleigh wind retrieval for the ALADIN airborne demonstrator of the Aeolus mission using simulated response calibration

“…The first approach to obtain a response calibration function is based on measurements during which the laser beam is pointed into zenith direction while assuming that the vertical velocity of the probed atmospheric volume is negligible, i.e., no Doppler frequency shift is induced. Then, either the frequency of the laser transmitter is scanned with a constant FPI cavity length (Reitebuch et al, 2018;Lux et al, 2018;Marksteiner et al, 2018) or the cavity length of the FPIs is 448 X. Zhai et al: Rayleigh wind retrieval for the ALADIN airborne demonstrator scanned while keeping the laser frequency locked (Dou et al, 2014).…”

“…In order to avoid such modifications, the A2D tries to conduct instrument response calibrations in a preferably pure Rayleigh atmosphere. Furthermore, the characteristics of the ground, such as high albedo and preferably flat terrain, and low ground elevation, should be considered to improve the SNR, to facilitate the deduction of a ground return response curve and to maximize the vertical coverage of the atmosphere (Marksteiner, 2013;Weiler, 2017;Lux et al, 2018;Marksteiner et al, 2018). In some cases, A2D calibrations were performed over terrain with high elevation (e.g., Greenland).…”

“…This is accomplished by flying curves with a roll angle of 20 • , which corresponds to the installation angle of the A2D telescope in the DLR Falcon 20 aircraft. Regions showing gravity wave activity or strong convection are avoided as they cross the assumption of negligible vertical wind velocity (Lux et al, 2018;Marksteiner et al, 2018). Overall, the reliability and repeatability of A2D MRRCs is a main limitation for accurate wind retrieval.…”

“…In addition, the A2D is expected to contribute to the optimization of the wind measurement strategies for the satellite instrument as well as to the improvement of wind retrieval and quality control algorithms during satellite operation (Durand et al, 2006;. As the first ever airborne direct-detection wind lidar, A2D has been deployed in several ground and airborne campaigns over the last 12 years (Li et al, 2010;Marksteiner, 2013;Weiler, 2017;Lux et al, 2018;Marksteiner et al, 2018).…”

“…Different instrument response calibration approaches have been studied using both measured and simulated response calibration to characterize and calibrate the ALADIN Rayleigh channel (Tan et al, 2008;Dabas et al, 2008;Rennie et al, 2017). Currently, only measured Rayleigh response calibrations (MRRC) are used for the A2D (Marksteiner, 2013;Lux et al, 2018;Marksteiner et al, 2018). However, the atmospheric temperature affects the Rayleigh-Brillouin line shape and has a direct effect on the instrument response calibration (Dabas et al, 2008).…”