Measurement geometry of Aeolus
The Aeolus mission
The knowledge of wind speed is crucial for both weather forecasting and the understanding of atmospheric dynamics. Therefore, experts of the World Meteorological Organization (WMO) have repeatedly stated that the global measurement of wind profiles is of highest priority. In August 2018, after a development period of 16 years, the European Space Agency (ESA) has launched the Earth Explorer Satellite Aeolus into space. Named after the Greek god of the winds, the satellite carries the revolutionary instrument ALADIN - the first European lidar and the world's first Doppler wind lidar in space. It essentially consists of a pulsed, ultraviolet laser, a reflecting telescope with a diameter of 1.50 metres and a highly sensitive optical spectrometer.
Wind measurements of Aeolus over Germany during storm Chiara on 10/02/2020. Aeolus observed wind speeds above 300 km/h at 10 km altitude
The emitted laser pulses are scattered in the atmosphere by air molecules, aerosols and cloud particles that move with the wind, so that a small portion returns to the satellite. The wind speed can be derived from the frequency difference between emitted and backscattered pulses which is caused by the Doppler effect, while the duration of the pulses contains the altitude information. In this manner, global wind profiles can be created from the ground up to altitudes of 30 kilometers with a resolution of 0.5 km to 2 km and an accuracy of about 2 m/s to 5 m/s. The wind data supplied by the satellite are directly incorporated into weather forecasting models, where they close large data gaps in the global observation system, especially over the oceans and in the tropics.
Consortium of the Aeolus Data Innovation and Science Cluster (DISC)
The algorithms and processors required to derive wind profiles from the raw measurement data of ALADIN were developed by an international team including the DLR institutes IPA and IMF, the companies DoRIT, ABB, S&T and Serco, and several European weather services (ECMWF, Météo-France, KNMI). Within the Aeolus Data Innovation and Science Cluster (DISC), this international consortium under the lead of DLR-IPA is responsible for monitoring and improving the data quality. Besides the further development of processors, also studies on the influence of Aeolus data on weather forecasting belong to the responsibilities of Aeolus DISC. Already one year after launch of the satellite, Aeolus DISC scientists have brought the data quality to such a high level that the European Centre for Medium-Range Weather Forecasting (ECMWF) has been able to demonstrate a positive impact of Aeolus data on the weather forecasts. Therefore, since 9 January 2020, ECMWF exploits Aeolus data in their daily weather forecasting. Also other weather services, such as the German Weather Service (DWD), are currently preparing the use of the Aeolus data for their daily weather forecasts.
The technical and scientific functionality of the satellite instrument was already demonstrated in advance of the Aeolus mission with a prototype instrument, the ALADIN airborne demonstrator (A2D). This direct-detection Doppler wind lidar, developed by DLR and Airbus, was deployed in several campaigns aboard the research aircraft Falcon to accurately measure, for example, the North Atlantic jet stream. This allowed to validate the measurement principle, to optimize the operation procedures and to improve the wind retrieval algorithms already before the start of the satellite.
Instrumentation of the DLR Falcon with the coherent 2-µm Doppler wind lidar and A2D
Since the launch of Aeolus in August 2018, already three flight campaigns have been conducted to validate the satellite instrument in different phases of the mission. Besides the A2D, also the 2-µm Doppler wind lidar was used aboard the DLR Falcon. While the first two campaigns were conducted from Oberpfaffenhofen in November 2018 and May 2019, the base for the third validation campaign was located in Keflavík, Iceland. A total of 20 satellite underflights were carried out during the three campaigns, with wind measurements being taken over a distance of more than 15,000 kilometres along the satellite measurement track. Using this extensive data set under various conditions with respect to season, geographical region as well as ground albedo, cloud types and atmospheric dynamics, the quality of the Aeolus data products was assessed in detail and processor updates were proposed. The validation campaigns thus make a significant contribution to the refinement of the data processors and to improving the quality of the satellite data. Further campaigns are planned during the mission life time, in which not only wind data but also aerosol parameters, which Aeolus provides as a by-product, will be validated.
Flight tracks of Aeolus underflights with the DLR Falcon during the validation campaign AVATARI (Aeolus validation through airborne lidars in Iceland) in autumn 2019 in Keflavík, Iceland