The ALADIN Airborne Demonstrator & 2 µm Doppler Wind Lidar Team: Aeolus Support from Mission Preparation and Validation to Reprocessing

Kurzfassung

The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt e.V., DLR) executed four airborne validation campaigns during the first three years of ESA’s Earth Explorer mission Aeolus. The Atmospheric LAser Doppler INstrument (ALADIN) on-board Aeolus was the first Lidar in space, providing wind profile measurements on a global scale for almost five years after its launch in August 2018. One of the major mission goals was achieved by improving the Numerical Weather Prediction (NWP) quality with its data, backed by an extensive validation strategy. After three campaigns in Central Europe (2018, 2019) and the North Atlantic region around Iceland (2019), DLR carried out the Aeolus VAlidation Through Airborne LidaRs in the Tropics (AVATAR-T) on Sal Island, Cape Verde, in September 2021 as a part of the ESA-NASA Joint Aeolus Tropical Atlantic Campaign (JATAC). These efforts built on more than a decade of pre-launch ground and airborne campaigns that had been aimed at supporting Aeolus operations and processor development. The campaigns deployed two lidar instruments: a scanning, high-accuracy heterodyne Doppler wind lidar (DWL) system operating at 2-µm wavelength, which served as a reference by providing wind vector profiles for the Aeolus validation, and the ALADIN Airborne Demonstrator (A2D). The A2D, as a prototype for the direct-detection DWL ALADIN, consists of a frequency-stabilised ultra-violet laser, a Cassegrain telescope and the same dual-channel optical and electronical receiver design. Like ALADIN, it measures wind speeds along the single instrument’s line-of-sight by analysing particulate (Mie) and molecular (Rayleigh) backscatter signals. Operating these two lidar instruments in parallel onboard the DLR Falcon research aircraft in a downward looking configuration allowed for detailed exploration of ALADIN’s wind measurement technology under various atmospheric backscatter conditions. Over the course of the four post-launch airborne validation campaigns, the team completed 190 flight hours covering 26,000 km along the Aeolus track during 31 coordinated underflights. Different operational states of the mission and geographical regions were probed including atmospheric conditions with high wind speeds and variability in and around the Jet Stream in the vicinity of Iceland and the tropical dynamics of the Saharan air layer, the African Easterly Jet, the Subtropical Jet and the Intertropical Convergence Zone around Cape Verde. Thanks to the close similarity with the satellite instrument in design and measurement principle, the collocated A2D and 2-µm wind observations acquired during the campaigns enabled valuable insights into the optimization of the Aeolus wind retrieval and related quality control algorithms. For example, the A2D, unlike ALADIN, provided broad vertical and horizontal coverage of Mie winds within the Saharan air layer, while filtering out Rayleigh winds affected by Mie contamination through crosstalk. These findings supported the refinement of the Aeolus wind processor to enhance data coverage and accuracy already during the mission, and continue with evaluating the re-processed data quality after the mission end in July 2023. An overview is provided ranging from mission-relevant results obtained with pre-launch campaigns to comparative wind observations of Aeolus and the DLR airborne DWLs extending to the current re-processed data evaluation. With the follow-on generation of an Aeolus-DWL being developed by ESA and EUMETSAT, a similarly successful mission support based on an accordingly upgraded second-generation airborne demonstrator can build on this legacy.