Quality control and error assessment of the Aeolus L2B wind results from the Joint Aeolus Tropical Atlantic Campaign

Since the start of the European Space Agency's Aeolus mission in 2018, various studies were dedicated to the evaluation of its wind data quality and particularly to the determination of the systematic and random errors in the Rayleigh-clear and Mie-cloudy wind results provided in the Aeolus Level-2B (L2B) product. The quality control (QC) schemes applied in the analyses mostly rely on the estimated error (EE), reported in the L2B data, using different and often subjectively chosen thresholds for rejecting data outliers, thus hampering the comparability of different validation studies. This work gives insight into the calculation of the EE for the two receiver channels and reveals its limitations as a measure of the actual wind error due to its spatial and temporal variability. It is demonstrated that a precise error assessment of the Aeolus winds necessitates a careful statistical analysis, including a rigorous screening for gross errors to be compliant with the error definitions formulated in the Aeolus mission requirements. To this end, the modified Z score and normal quantile plots are shown to be useful statistical tools for effectively eliminating gross errors and for evaluating the normality of the wind error distribution in dependence on the applied QC scheme, respectively. The influence of different QC approaches and thresholds on key statistical parameters is discussed in the context of the Joint Aeolus Tropical Atlantic Campaign (JATAC), which was conducted in Cabo Verde in September 2021. Aeolus winds are compared against model background data from the European Centre for Medium-Range Weather Forecasts (ECMWF) before the assimilation of Aeolus winds and against wind data measured with the 2 µm heterodyne detection Doppler wind lidar (DWL) aboard the Falcon aircraft. The two studies make evident that the error distribution of the Mie-cloudy winds is strongly skewed with a preponderance of positively biased wind results distorting the statistics if not filtered out properly. Effective outlier removal is accomplished by applying a two-step QC based on the EE and the modified Z score, thereby ensuring an error distribution with a high degree of normality while retaining a large portion of wind results from the original dataset. After the utilization of the described QC approach, the systematic errors in the L2B Rayleigh-clear and Mie-cloudy winds are determined to be below 0.3 m s−1 with respect to both the ECMWF model background and the 2 µm DWL. Differences in the random errors relative to the two reference datasets (Mie vs. model is 5.3 m s−1, Mie vs. DWL is 4.1 m s−1, Rayleigh vs. model is 7.8 m s−1, and Rayleigh vs. DWL is 8.2 m s−1) are elaborated in the text.</p

X-ray, Optical, and Radio Observations of the Type II Supernovae 1999em and 1998S

Observations of the Type II-P (plateau) Supernova (SN) 1999em and Type IIn (narrow emission line) SN 1998S have enabled estimation of the profile of the SN ejecta, the structure of the circumstellar medium (CSM) established by the pre-SN stellar wind, and the nature of the shock interaction. SN 1999em is the first Type II-P detected at both X-ray and radio wavelengths. The Chandra X-ray data indicate non-radiative interaction of SN ejecta with a power-law density profile (rho \propto r^{-n} with n ~ 7) with a pre-SN wind with a low mass-loss rate of ~2 \times 10^{-6} M_sun/yr for a wind velocity of 10 km/sec, in agreement with radio mass-loss rate estimates. The Chandra data show an unexpected, temporary rise in the 0.4--2.0 keV X-ray flux at ~100 days after explosion. SN 1998S, at an age of >3 years, is still bright in X-rays and is increasing in flux density at cm radio wavelengths. Spectral fits to the Chandra data show that many heavy elements (Ne, Al, Si, S, Ar, and Fe) are overabundant with respect to solar values. We compare the observed elemental abundances and abundance ratios to theoretical calculations and find that our data are consistent with a progenitor mass of approximately 15-20 M_sun if the heavy element ejecta are radially mixed out to a high velocity. If the X-ray emission is from the reverse shock wave region, the supernova density profile must be moderately flat at a velocity ~10^4 km/sec, the shock front is non-radiative at the time of the observations, and the mass-loss rate is 1-2 \times 10^{-4} M_sun/yr for a pre-supernova wind velocity of 10 km/sec. This result is also supported by modeling of the radio emission which implies that SN 1998S is surrounded by a clumpy or filamentary CSM established by a high mass-loss rate, ~2 \times 10^{-4} M_sun/yr, from the pre-supernova star.Comment: 14 pages, 10 figures, accepted by ApJ, includes new data (one new obs. each of SN 1999em and SN 1998S), expanded discussion of spectral fit

Rayleigh wind retrieval for the ALADIN airborne demonstrator of the Aeolus mission using simulated response calibration

Aeolus, launched on 22 August in 2018, is the first ever satellite to directly observe wind information from the surface up to 30 km on a global scale. An airborne prototype instrument called ALADIN airborne demonstrator (A2D) was developed at the German Aerospace Center (DLR) for validating the Aeolus measurement principle based on realistic atmospheric signals. To obtain accurate wind retrievals, the A2D uses a measured Rayleigh response calibration (MRRC) to calibrate its Rayleigh channel signals. However, differences exist between the respective atmospheric temperature profiles that are present during the conduction of the MRRC and the actual wind measurements. These differences are an important source of wind bias since the atmospheric temperature has a direct effect on the instrument response calibration. Furthermore, some experimental limitations and requirements need to be considered carefully to achieve a reliable MRRC. The atmospheric and instrumental variability thus currently limit the reliability and repeatability of a MRRC. In this paper, a procedure for a simulated Rayleigh response calibration (SRRC) is developed and presented in order to resolve these limitations of the A2D MRRC. At first the transmission functions of the A2D Rayleigh channel double-edge Fabry-Pérot interferometers (FPIs) in the internal reference path and the atmospheric path are characterized and optimized based on measurements performed during different airborne and ground-based campaigns. The optimized FPI transmission functions are then combined with the laser reference spectrum and the temperature-dependent molecular Rayleigh backscatter spectrum to derive an accurate A2D SRRC which can finally be implemented into the wind retrieval. Using dropsonde data as a reference, a statistical analysis based on a dataset from a flight campaign in 2016 reveals a bias and a standard deviation of line-of-sight (LOS) wind speeds derived from a SRRC of only 0.05 and 2.52 m/s, respectively. Compared to the result derived from a MRRC with a bias of 0.23 m/s and a standard deviation of 2.20 m/s, the accuracy improved and the precision is considered to be at the same level. Furthermore, it is shown that the SRRC allows for the simulation of receiver responses over the whole altitude range from the aircraft down to sea level, thus overcoming limitations due to high ground elevation during the acquisition of an airborne instrument response calibration

The impact of Aeolus wind retrievals on ECMWF global weather forecasts

Abstract Aeolus is the world's first spaceborne Doppler Wind Lidar, providing profiles of horizontal line-of-sight (HLOS) wind retrievals. Numerical weather prediction (NWP) impact and error statistics of Aeolus Level-2B (L2B) wind statistics have been assessed using the European Centre for Medium-range Weather Forecasts (ECMWF) global data assimilation system. Random and systematic error estimates were derived from observation minus background departure statistics. The HLOS wind random error standard deviation is estimated to be in the range 4.0-7.0 m/s for the Rayleigh-clear and 2.8-3.6 m/s for the Mie-cloudy, depending on atmospheric signal levels which in turn depend on instrument performance, atmospheric backscatter properties and the processing algorithms. Complex systematic HLOS wind error variations on time-scales less than one orbit were identified, most strongly affecting the Rayleigh-clear winds. NWP departures and instrument housekeeping data confirmed that it is caused by temperature gradients across the primary mirror. A successful bias correction scheme was implemented in the operational processing chain in April 2020. In Observing System Experiments (OSEs), Aeolus provides statistically significant improvement in short-range forecasts as verified by observations sensitive to temperature, wind and humidity. Longer forecast range verification shows positive impact that is strongest at the day two to three forecast range: 2% improvement in root-mean-square error for vector wind and temperature in the tropical upper troposphere and lower stratosphere, and polar troposphere. Positive impact up to 9 days is found in the tropical lower stratosphere. Both Rayleigh-clear and Mie-cloudy winds provide positive impact, but the Rayleigh accounts for most tropical impact. The Forecast Sensitivity Observation Impact (FSOI) metric is available since 9 January 2020, when Aeolus was operationally assimilated, which confirms Aeolus is a useful contribution to the global observing system, with the Rayleigh-clear and Mie-cloudy winds providing similar overall short-range impact in 2020

RAYLEIGH WIND RETRIEVAL FOR THE ALADIN AIRBORNE DEMONSTRATOR OF THE AEOLUS MISSION USING SIMULATED RESPONSE CALIBRATION

Aeolus, launched on 22 August 2018, is the first ever satellite to directly observe wind information from space on a global scale. An airborne prototype called ALADIN Airborne Demonstrator (A2D) was developed at the German Aerospace Center (DLR) for validating the Aeolus measurement principle based on realistic atmospheric signals. However, atmospheric and instrumental variability currently limit the reliability and repeatability of the A2D instrument response calibration. In this study, a simulated Rayleigh response calibration (SRRC) is presented for resolving the limitations of A2D instrument response calibration

VirES for Aeolus, a Virtual Workspace for ESA's Atmospheric Dynamics Mission

VirES is a Virtual workspace for Earth-observation Scientists, a service provided by the European Space Agency (ESA). VirES has firstly been established for ESA's magnetic field mission Swarm as "VirES for Swarm" and has been extended to ESA's atmospheric dynamics mission Aeolus, which is scheduled for launch in August 2018. The service is developed by the Austrian IT company EOX in strong collaboration with missions' scientists. VirES is a web-based service (https://vires.services) that enables scientists to discover, visualize, select and download data of Earth-observation missions through an easy to operate graphical user interface. "VirES for Aeolus" will provide access to Aeolus L1B, L2A, L2B, L2C products and auxiliary data. The first version 1.0 passed acceptance tests in April 2018 and developments towards Version 1.1 (launch version) are in progress. The service is planned to be accessible for public use as soon as the mission's commissioning phase is completed and first data products are released by ESA

DLR’S AIRBORNE SUPPORT OF ESA’S AEOLUS MISSION FROM PRE-LAUNCH CAMPAIGNS TO MISSION PERFORMANCE VALIDATION

In August 2018, the first-ever spaceborne wind lidar – Aeolus – was launched and has since then been providing global data of the horizontal wind field from the ground up to 30 km to improve numerical weather prediction. Aeolus is based on a single instrument called ALADIN (Atmospheric Laser Doppler Instrument), which comprises a single-frequency, ultraviolet solidstate laser in a master oscillator power amplifier configuration that emits nanosecond pulses into the atmosphere. High output energy and excellent frequency stability ensure a sufficient signal-tonoise ratio of the backscatter return needed for an accurate determination of the wind-induced Doppler frequency shift. To demonstrate the Aeolus measurement principle and to validate the corresponding wind data quality, the German Aerospace Center (DLR) started with airborne prelaunch validation activities already in 2007. After launch, these activities were extended by four campaigns over Europe, the North Atlantic region, and the tropics. In this talk, the challenges of the development and the operation of ALADIN are addressed and the successful accomplishment of the Aeolus mission is demonstrated using the results obtained from the post-launch validation campaign

Correction of wind bias for the lidar on-board Aeolus using telescope temperatures

The European Space Agency satellite Aeolus provides continuous profiles of the horizontal line-of-sight wind component at a global scale. It was successfully launched into space in August 2018 with the goal to improve numerical weather prediction (NWP). Aeolus data has already been successfully assimilated into several NWP models and has already helped to significantly improve the quality of weather forecasts. To achieve this major milestone the identification and correction of several systematic error sources was necessary. One of them is related to small temperatures fluctuations across the 1.5 m diameter primary mirror of the telescope which cause varying wind biases along the orbit of up to 8 m/s. This paper presents a detailed overview of the influence of the telescope temperature variations on the Aeolus wind products and describes the approach to correct for this systematic error source in the operational near-real-time (NRT) processing. It was shown that the telescope temperature variations along the orbit are due to changes of the top-of-atmosphere short- and long-wave radiation of the Earth and the response of the telescope’s thermal control system to that. To correct for this effect ECMWF model-equivalent winds are used as bias reference to describe the wind bias in a multiple linear regression model as a function of various temperature sensors located on the primary telescope mirror. This correction scheme has been in operational use at ECMWF since April 2020 and is capable of reducing a large part of the telescope-induced wind bias. In cases where the influence of the temperature variations is particularly strong it was shown that the bias correction can improve the orbital bias variation by up to 53 %. Moreover, it was demonstrated that the approach of using ECMWF model-equivalent winds is justified by the fact that the global bias of models u-component winds w.r.t to radiosondes is smaller than 0.3 m/s. However, this paper also presents the alternative of using Aeolus ground return winds which serve as zero wind reference in the multiple linear regression model. The results show that the approach based on ground return winds only performs 10.8 % worse than the ECMWF model-based approach and thus has good potential for future applications for upcoming reprocessing campaigns or even in the NRT processing of Aeolus wind products

Airborne Wind Lidar Observations for the Validation of ESA's Wind Mission Aeolus

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