The European Space Agency (ESA)’s wind mission, Aeolus, was launched on 22 August 2018. Aeolus carries a single instrument, the Atmospheric Laser Doppler Instrument Aladin, which is a high spectral resolution Doppler wind lidar operated at 355 nm. Aladin is the first Doppler wind lidar in space, and is a member of the ESA Earth Explorer family. It shall demonstrate the potential of Doppler wind lidars is space for improving weather forecasting and the understanding of atmospheric dynamics and its role in climate variability. Aeolus provides profiles of single line-of-sight winds (primary product), and profiles of atmospheric backscatter and extinction (spin-off product), which will be used for weather forecasting, atmospheric physics studies, air quality modelling and for process studies relevant to climate change. The instrument samples the atmosphere from about 30 km down to the Earth’s surface, or down to optically thick clouds. The required accuracy of the wind observations is 2 m/s in the PBL, 2-3 m/s in the free troposphere, and 3-5 m/s in the lower stratosphere. The mission spin-off product allow the detection of aerosol and cloud layers. The satellite flies in a polar dusk/dawn orbit (6 am/pm local time), providing ~16 orbits per 24 hours with an orbit repeat cycle of 7 days. The wind and optical properties products are delivered near-real-time (NRT) for direct ingestion by operational numerical weather prediction (NWP) and potentially also air quality models.
The status of the Aeolus mission will be provided, including the outcome of the instrument commissioning and early mission calibration and validation activities. An assessment of the mission and instrument performance will be given, together with an overview of potentially remaining activities leading to the official data release. The status and schedule for the mission exploitation will be given, including upcoming scientific studies and the coordination and schedule for the international Numerical Weather Prediction (NWP) impact activities. Finally, plans for the exploitation of the Aeolus mission data in synergy with other ESA and third-party missions will be presented.

Already within the first weeks after the launch of ESA´s Earth Explorer mission Aeolus on 22 August 2018, the spaceborne wind lidar ALADIN (Atmospheric LAser Doppler INstrument) provided atmospheric backscatter measurements on 5 September and wind profiles on 12 September. This swift availability of observations from ALADIN after launch is considered as a great success for ESA, space industry and algorithm and processor developer teams. These teams from scientific institutes, numerical weather prediction (NWP) centres, companies and from ESTEC and ESRIN continuously improved and tested the retrieval algorithms and processors using sophisticated end-to-end simulation tools and experience gained with the airborne demonstrator for Aeolus. ALADIN as the first wind-lidar in space is not only based on new technologies for lasers, optical interferometers and detectors, but also the retrieval algorithms had to be developed without any heritage from earlier space-borne wind lidars.

This cooperation from the development phase of Aeolus will be continued and extended within a new framework for exploitation activities of Earth Explorer missions named Data Innovation and Science Cluster (DISC) during the phase E2 starting after completion of the commissioning phase activities. The Aeolus DISC activities range from instrument monitoring including calibration and characterization to algorithm refinement, and towards a continuous evolution of the processors for all product levels. Product quality monitoring plays an essential role including validation of products from external validation teams, whose results will be synthesised by DISC experts and eventually will lead to further algorithm updates. The DISC consortium will provide support to ESA ranging from anomaly resolving for the payload data ground segment to support in decisions for instrument operation, calibration and providing support to the Aeolus Quality Working Group (QWG). Also support to Aeolus users from the validation teams, science and NWP communities will be provided by DISC experts.

The aim of the Aeolus mission is to provide global observations of vertical profiles of one component of the horizontal wind vector with sufficient accuracy and precision to demonstrate a positive impact on NWP analysis and forecasts. All activities within the DISC are targeted towards supporting this overall objective. Therefore impact experiments with NWP models are building a major activity within DISC. These impact experiments, the product monitoring activities, or the feedback from the user community will eventually guide further recommendations for algorithm and processor evolution. Both forward-engineering (or bottom-up) approaches as well as complementary reverse-engineering (or top-down) approaches will be practiced.

The monitoring of the Aeolus data product quality and instrument performance, the refinement of retrieval algorithms or the evolution of the processors strongly benefit from monitoring of Aeolus data with NWP models. This NWP monitoring provides temporally continuous comparisons of Aeolus observations with NWP models for all altitudes and over all geographical regions, and this with almost immediate feedback on the data quality. The large potential of this approach was already successfully demonstrated during the commissioning phase, where a systematic error in Rayleigh winds for one specific range gate was spotted by comparison of several orbits of Aeolus data with the ECMWF model. This bias in Rayleigh winds could be traced back to one suspicious pixel on the Rayleigh ACCD. Investigations for correcting these effects on pixel level are on-going. This clearly demonstrates the successful approach of combining the expertise on the instrument, retrieval algorithms, and NWP monitoring.

An overview of the DISC activities as well as first results from instrument performance and data product quality will be discussed.

ECMWF are, and have been for over two decades, closely involved in the Aeolus Doppler Wind Lidar mission. For example: we provide the operational processing for the official mission Level-2B (L2B) horizontal line-of-sight (HLOS) wind products (DAMI); we are part of the DISC and hence are involved in developing the L2B processor (with KNMI) and the assessment of the quality of the data and its impact in operational Numerical Weather Prediction (NWP). This presentation will focus on a detailed assessment of the quality (e.g. statistics of random and systematic errors) of the Aeolus L2B winds with use of the ECMWF model short-range forecasts as a reference. The ECMWF short-range wind forecasts (without assimilating Aeolus) are sufficiently accurate (typical random errors of 2 m/s for HLOS wind) to assess the quality of Aeolus L2B winds, and they have been found to be particularly useful for detecting discrepancies in the data. Discrepancies such as temporally variable wind biases for specific range-bins, biases related to geophysical conditions e.g. cloudiness, issues with height assignment and many more. The innovations, observation minus background (O-B) statistics can be used to assess Aeolus specific calibration issues, such as Range Dependent Bias and Harmonic Bias.
Some discussion of improvements made in the Ground Segment processing based on these assessments will be given.
Initial NWP impact assessment (Observing System Experiments, with data assimilation of Aeolus L2B winds) will be presented. There are expected to be many reprocessing rounds for the Aeolus winds as problems are diagnosed (with the CAL/VAL) and the processing chain is improved. With the improvements and hence reprocessed data, it will be shown how the impact of Aeolus improves relative to previous datasets. Particular attention will be given to assessing how assimilation Aeolus L2B winds changes the fit of short-range forecasts to other observation types e.g. radiosondes, satellite radiances, and aircraft data. Finally it will be discussed how NWP forecasts improve, as it can be estimated from the OSEs.

Aladin, Aeolus lidar instrument, features with two detection channels that make it a High Spectral Resolution Lidar (HSRL). A narrow-band Fizeau interferometer provides information mostly on Mie scattered light (i.e. light backscattered by aerosol or cloud particles) and a pair of Fabry-Pérot étalons sample both sides of the Rayleigh-Brillouin spectrum backscattered by the molecular atmosphere.
In order to precisely retrieve aerosol optical properties, backscatter and extinction coefficients, a precise characterization of the radiometric and spectral behaviour of the instrument is required. This is the role of the calibration processors that work on a subset of data acquired during special operations of the satellite.
The calibration, along with observations themselves, is one of the important inputs to the L2A processor. The algorithm first separates the Mie (particle) and Rayleigh (molecules) contributions in a “cross-talk correction” step and, from there, is able to derive the optical properties of the aerosols without a priori hypothesis on their nature. The backscatter-to-extinction ratio can thus be estimated, that gives an information on the type of the particles.
The L2A is limited by several factors: Aladin features only a single polarization measurement and the multiple scattering is neglected. Multiple scattering is estimated to be small thanks to the very small angular field-of-view of the receiver. The depolarization limitation, however, must be taken into account when comparing Aeolus products to other sources of data. The UV light is emitted by the laser in a circular left polarization and processed in the receiver as linearly polarized after going through a quarter wave plate. Light is then routed by a polariser cube to the interferometers. This means that only the co-polar component of backscattered light is detected. Hence, for highly depolarizing targets such as ice crystals and dust, the backscatter coefficient will be largely under-estimated. The extinction is not affected by this problem because scattering by molecules does not affect the polarization.
Some comparison of the L2A product extinction to AERONET data for total aerosol optical depth is foreseen to validate the product. More in depth validation will also be produced during the CalVal campaign from comparison to ground based and airborne lidars.
The presentation will recall how aerosol optical products are retrieved by the L2A, what calibration coefficients are required and how they are estimated, what products are available (nature, spatial resolution), and give some hint about their accuracy.

Soon after its successful launch in August 2018, the spaceborne wind lidar ALADIN (Atmospheric LAser Doppler INstrument) on-board ESA’s Earth Explorer satellite Aeolus has demonstrated to provide atmospheric wind profiles on a global scale. Being the first ever Doppler wind lidar instrument in space, ALADIN contributes to the improvement in numerical weather prediction by measuring one component of the wind vector along the instrument’s line-of-sight (LOS) from ground throughout the troposphere up to the lower stratosphere with a vertical resolution of 0.25 km to 2 km depending on altitude.
Already in 2005, an airborne prototype of the Aeolus payload, the ALADIN Airborne Demonstrator (A2D), was developed at DLR (German Aerospace Center). Owing to its representative design and operating principle, the A2D has since provided valuable information on the wind measurement strategies of the satellite instrument as well as on the optimization of the wind retrieval and related quality-control algorithms. Hence, it represents one of the key instruments for the calibration and validation activities during the Aeolus mission.
Over the past years, the A2D was deployed in several field experiments, yielding a comprehensive wind dataset from diverse atmospheric scenes including strong wind shear and varying cloud conditions. Broad vertical and horizontal coverage across the troposphere was achieved thanks to the complementary design of the A2D receiver, which, like ALADIN, comprises a Rayleigh and Mie channel for analysing both molecular and particulate backscatter signals. Validation of the instrument performance and the wind retrieval algorithms was conducted by comparison with DLR’s coherent Doppler wind lidar (2-µm DWL) which was operated in parallel on-board the same aircraft. For the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) conducted Iceland in 2016, the systematic error of the A2D LOS wind speeds was determined to be less than 0.5 m/s for both receiver channels, while the random errors range from 1.5 m/s (Mie) to 2.7 m/s (Rayleigh).
In autumn of 2018, the first airborne campaign after the launch of Aeolus was carried out from the airbase in Oberpfaffenhofen, Germany. Aside from extending the existing dataset of wind observations, this field experiment aimed to perform several underflights of Aeolus in Central Europe in order to provide first comparative results between the two airborne wind lidars (A2D and 2-µm DWL) and the satellite instrument. At the same time, the campaign served to optimize the operational procedures, particularly in terms of flight planning, to be applied during the forthcoming Cal/Val campaigns in the operational phase of Aeolus. This work will provide an overview of the most recent airborne wind lidar campaigns and present their results both from an instrument and a meteorological point-of-view.

French ground-based Rayleigh Doppler lidars deployed at Observatoire de Haute Provence (OHP) in southern France (44° N, 6° E) and Observatoire du Maido (La Reunion island, tropical Indian Ocean, 21° S, 55° E) are among the primary instruments within ESA Aeolus Cal/Val programme. The ground-based lidars are designed to measure vertical profiles of wind velocity in the altitude range 5 - 60 km with an accuracy better than 1 m/s up to 30 km. The instruments make use of an injection-seeded Nd:YAG laser operating at 532 nm with pulse repetition rate of 30 Hz and 800 mJ per pulse energy. The horizontal wind components are obtained by measuring Doppler shift between emitted and backscattered light by means of double-edge Fabry-Perot interferometer. This technique, pioneered by French Service d’Aeronomie in 1989, is implemented in Aeolus ALADIN instrument.
We present the first results of validation of Aeolus line-of-sight wind profiles using the French Doppler lidars. The validation exercise relies on the routine lidar measurement as well as on the dedicated measurement campaigns in January and April 2019 involving simultaneous lidar acquisitions and radiosonde ascents coincident with the nearest Aeolus overpasses. The orbital configuration of Aeolus satellite enables four overpasses per week above OHP within 100 km range and twice-weekly overpasses above Maido, one of which within 10 km range. Evaluation of Aeolus wind profiles is done in consideration of the expected mesoscale variability of wind field inferred from previous lidar-radiosonde intercomparison campaigns carried out at both stations.
In addition to a quantitative validation of Aeolus wind profiles, we attempt to evaluate the capacity of Aeolus observations in resolving fluctuations of stratospheric wind field induced by atmospheric gravity waves. The spatial distribution of gravity wave activity inferred from Aeolus wind measurements is compared against climatological data sets based on GNSS radio occultation temperature observations. The potential of Aeolus wind observations for providing the add-on value constraints on gravity waves parameters for climate models is discussed.