ESA'S Wind Mission Aeolus - Overview, Status and Outlook

The European Space Agency's (ESA) wind mission, Aeolus, hosts the first space-based Doppler Wind Lidar (DWL) world-wide. Its scientific objectives are to improve weather forecasts and to advance the understanding of atmospheric dynamics and its interaction with the atmospheric energy and water cycle. The primary data product is profiles of horizontally projected line-of-sight winds from the surface up to about 30 km, and spin-off products are profiles of cloud and aerosol optical properties. Aeolus was launched on 22 August 2018, and the Atmospheric LAser Doppler INstrument (ALADIN) switch-on was completed with first high energy output in wind mode on 4 September 2018. The on-ground data processing facility worked excellent, allowing L2 product output in near-real-time (within 3 hours of sensing) from the start of the mission. During the first 20 months in orbit, ESA, the Aeolus Data Innovation and Science Cluster (DISC), Aeolus calibration and validation (CAL/VAL) teams and industry worked on the instrument commissioning, calibration, validation and product improvements, leading to the public release of the Level 2B (L2B) wind product in May 2020. The Aeolus L2B wind data were then of such good quality that four weather centers started to use the product in their daily forecasts during 2020, and more have plans to follow in 2021. As of May 2020, the biases of the near-real-time data were on average close to the mission requirements, although the random errors were higher than expected due to lower than expected atmospheric return signal on orbit. Other issues encountered in-flight were for example drifts in the instrument alignment and nonoptimal telescope focus and temperature control, leading to drifts in the laser emitted output energy and/or internal and atmospheric path signal strength and long term evolutions on the wind product bias. A further issue encountered was a gradually increasing number of “hot pixels”, i.e. pixels with elevated background signals, appearing on the ALADIN ACCDs over time. Mitigation strategies for these issues arebeing implemented, or are considered for a potential follow-on mission. The Aeolus optical properties spin-off product (L2A) has been further improved after launch, including methods for improved aerosol and cloud backscatter discrimination. The product has been used to study smoke emissions, e.g. from the early 2020 Australian fires, and has been successfully experimentally assimilated in Copernicus Atmosphere Monitoring Service (CAMS) model C-IFS. After two and a half years in orbit, and despite of some performance issues related to the ALADIN instrument, Aeolus has already achieved most of its scientific objectives. Finally, the mission was recently extended until end 2022.