Since the successful launch of ESA's Earth Explorer mission Aeolus in August 2018, atmospheric wind profiles from the ground to the lower stratosphere are being acquired on a global scale, deploying the first-ever satellite-borne wind lidar system ALADIN (Atmospheric LAser Doppler INstrument). ALADIN provides one component of the wind vector along the instrument's line-of-sight (LOS) with a vertical resolution of 0.25 km to 2 km depending on altitude. The wind accuracy is better than 1 m/s, while the random error ranges from 3 to 6 m/s. The near-real-time wind observations contribute to improving the accuracy of numerical weather prediction and advance the understanding of tropical dynamics and processes relevant to climate variability.
Already several years before the launch of the Earth Explorer mission, an airborne prototype of the Aeolus payload - the ALADIN Airborne Demonstrator (A2D) - was developed at the DLR (German Aerospace Center). Like the direct detection Doppler wind lidar on-board Aeolus, the A2D is composed of a frequency-stabilized ultra-violet laser, a Cassegrain telescope and a dual-channel receiver to measure LOS wind speeds by analyzing both molecular and particulate backscatter signals. Thanks to the complementary design of the A2D receiver, broad vertical and horizontal coverage across the troposphere is achieved. In addition to the A2D, DLR's research aircraft carries a well-established coherent Doppler wind lidar (2-µm DWL). It is equipped with a double-wedge scanner which allows for the determination of the wind vector with accuracy of better than 0.1 m/s and precision of better than 1 m/s. Hence, both wind lidars represent key instruments for the calibration/validation activities during the Aeolus mission.
After the launch of Aeolus, the A2D and 2-µm DWL were deployed during three airborne validation campaigns between November 2018 and September 2019. 20 coordinated flights along the satellite swath were conducted in Central Europe and the North Atlantic region, yielding a large amount of wind data from the troposphere under various atmospheric conditions in terms of cloud cover and dynamics. The high accuracy of the 2-µm DWL allowed to precisely assess the Aeolus systematic and random errors, and thus enabled a comprehensive evaluation of the satellite's wind data product quality. Due to the high degree of commonality of the A2D with the satellite instrument, the comparative wind results delivered valuable information on potential error sources as well as on the optimization of the Aeolus wind retrieval and related quality-control algorithms. Beyond the airborne campaigns, the A2D has been serving as testbed to explore new measurement strategies and algorithm modifications which cannot be readily implemented in the Aeolus operation modes and processors, respectively
