Airborne lidar observations supporting the ADM-Aeolus mission for global wind profiling

The Atmospheric Dynamics Mission ADM-Aeolus of ESA will be the first lidar mission to sense the global wind field from space. The instrument is based on a direct-detection Doppler lidar operating at 354.9 nm with two spectrometers for aerosol/cloud and molecular backscatter. In order to assess the performance of the Doppler lidar ALADIN on ADM-Aeolus and to optimize the retrieval algorithms with atmospheric signals, an airborne prototype – the ALADIN Airborne Demonstrator A2D – was developed. The A2D was the first airborne direct-detection Doppler lidar with its maiden flight on the DLR Falcon aircraft in 2005. Three airborne campaigns with a coherent-detection 2-μm wind lidar and the direct-detection wind lidar A2D were performed for pre-launch validation of Aeolus from 2007-2009. Furthermore, a unique experiment for resolving the Rayleigh-Brillouin spectral line shape in the atmosphere was accomplished in 2009 with the A2D from a mountain observatory at an altitude of 2650 m. Results of this experiment and the latest airborne campaign in the vicinity of Greenland and Iceland will be discussed

EUROPÄISCHE LIDAR-MISSIONEN ZUR ERDBEOBACHTUNG - STATUS UND AUSBLICK

Europäische Lidarmissionen zur Erdbeobachtung – Status und Ausblick Mittels satellitengestützer Lidarsysteme lassen sich Profile atmosphärischer Größen und Bestandteile auf globaler Skala messen. Ein Meilenstein der europäischen Erdbeobachtung und der Weltraum-Lidarentwicklung war der Start der ESA Earth Explorer Mission – Aeolus – im August 2018. Aeolus ist das erste europäische Lidar und das weltweit erste Windlidar im All. Aeolus liefert präzise, hochaufgelöste Daten des globalen Windfeldes von Boden bis in die Stratosphäre, und trägt damit maßgeblich zur Verbesserung der numerischen Wettervorhersage bei. Zur Fernerkundung des Windes werde ultraviolette Laserpulse in die Atmosphäre gesandt, deren Photonen teilweise an Molekülen und Aerosolen zum Instrument zurückgestreut werden. Abhängig von der Windgeschwindigkeit hat das zurückgestreute Licht eine leicht veränderte Frequenz (Dopplereffekt), welche mit Hilfe hochpräziser Spektrometer analysiert wird, um das atmosphärische Windprofil abzuleiten. Trotz des relativ einfachen Messprinzips ist dessen Umsetzung gerade im Weltall herausfordernd, da besonders frequenzstabile und effiziente Laserquellen sowie hochpräzise Spektrometer eingesetzt werden müssen. Die technologisch wie wissenschaftlich herausfordernde Entwicklung und Missions-vorbereitung wurde durch das DLR im Rahmen von Messkampagnen mit dem flugzeuggetragenen Demonstrator-Instrument, die Prozessor- und Simulatorentwicklung sowie mit ausgiebigen Tests kritischer optischer Komponenten maßgeblich unterstützt. Nach der erfolgreichen Fertigstellung des Instruments konnte demonstriert werden, dass die Technologie im Orbit funktioniert und alle Missionsziele erreicht werden können. Aeolus entwickelte sich im operationellen Betrieb zu der drittwichtigsten Quelle satellitengestützter Wetterdaten für das ECMWF Wettermodell. Neben Aeolus stehen derzeit zwei weitere europäische Lidarmissionen in den Startlöchern. 2024 plant ESA die EarthCARE Mission zu starten, um mit komplementären Lidar- und Radarmessungen den Einfluss von Aerosolen und Wolken auf den globalen Strahlungshaushalt der Atmosphäre zu untersuchen. Das Lidarsystem auf EarthCARE verwendet eine bauähnliche Laserquelle wie Aeolus und einen spektral hochauflösenden Empfänger. 2027 soll die deutsch-französische Klimamission MERLIN starten um Methankonzentrationen in der Erdatmosphäre zu vermessen und so Aufschluss über die Quellen und Senken des Treibhausgases geben. Des Weiteren werden derzeit intensive Diskussionen über eine Aeolus Nachfolgemission geführt welche für die Jahre 2030+ vorgesehen ist. In diesem Vortrag werden die einzigartigen Vorteile von satellitengestützen Lidar-Systemen und deren Nutzen für die Erdbeobachtung aufgezeigt, sowie die Herausforderungen bei der Entwicklung dieser Instrumente diskutiert

Airborne measurements and large-eddy simulations of small-scale gravity waves at the tropopause inversion layer over Scandinavia

Gravity waves are an important coupling mechanism in the atmosphere. Measurements by two research aircraft during a mountain wave event over Scandinavia in 2016 revealed changes of the horizontal scales in the vertical velocity field and of momentum fluxes in the vicinity of the tropopause inversion. Idealized simulations revealed the presence of interfacial waves. They are found downstream of the mountain peaks, meaning that they horizontally transport momentum/energy away from their source

Airborne Coherent Doppler Wind Lidar measurements of vertical and horizontal wind speeds for the investigation of gravity waves

Gravity waves are well known phenomena in the atmosphere, but there is still a lack of knowledge of their life cycle including excitation, propagation and dissipation mechanisms. In order to investigate these topics, DLR’s coherent Doppler wind lidar system was recently deployed during 3 airborne campaigns on the Falcon F20 research aircraft, namely the GW-LCYCLE I campaign (Kiruna, Sweden, December 2013), the DEEPWAVE campaign (Christchurch, New Zealand, June/July 2014) and the GW-LCYCLE II campaign (Kiruna, Sweden, January/February 2016). In this paper, a case study based on a research flight performed during GW-LCYCLE I is discussed and a method for correcting horizontal wind contribution in the vertical wind retrieval based on ECMWF data is introduced. The remaining systematic error of the retrieved vertical wind is estimated to be less than 10 cm/s. A measurement of a flight leg across the Scandinavian mountain ridge is used to characterize gravity waves during strong forcing conditions. The measured vertical wind reaches amplitudes of larger than ± 3 m/s and horizontal wavelengths of 10 km to 20 km. A comparison with WRF-model calculations shows a quite good representation of the horizontal structure of the vertical wind. The amplitude however is obviously underestimated by a factor of 2 and shows maximum wind speeds of ± 1.5 m/s

ADM-Aeolus pre-launch activities and recent advances in spaceborne and airborne Wind Lidar Systems

The first space-borne wind lidar mission ADM-Aeolus from ESA is currently scheduled for launch by mid-2017. For the preparation of the Aeolus validation, an airborne field experiment was performed during 3 weeks in May 2015 with the DLR Falcon and the NASA DC-8 aircraft. For the first time 4 wind lidars were deployed during an airborne campaign including two coherent and two direct-detection wind lidars at a wavelength of 2μm and 355 nm. A total of 7 coordinated flights of the Falcon and DC-8 yielded an extensive dataset. Additionally, DLR’s airborne coherent Doppler Wind Lidar was recently deployed in 3 coordinated airborne campaigns aiming to investigate the life cycle of gravity waves from ground up to the mesosphere. The horizontal and vertical wind measurements of the lidar provide valuable data for characterizing tropospheric gravity waves and background wind conditions

Airborne coherent wind lidar measurements of the momentum flux profile from orographically induced gravity waves

In the course of the GW-LCYCLE II campaign, conducted in Jan/Feb 2016 from Kiruna, Sweden, coherent Doppler wind lidar (2 µm DWL) measurements were performed from the DLR Falcon aircraft to investigate gravity waves induced by flow across the Scandinavian Alps. During a mountain wave event on 28 January 2016, a novel momentum flux (MF) scan pattern with fore and aft propagating laser beams was applied to the 2 µm DWL. This allows us to measure the vertical wind and the horizontal wind along the flight track simultaneously with a high horizontal resolution of ≈800 m and hence enables us to derive the horizontal MF profile for a broad wavelength spectrum from a few hundred meters to several hundred kilometers. The functionality of this method and the corresponding retrieval algorithm is validated using a comparison against in situ wind data measured by the High Altitude and Long Range (HALO) aircraft which was also deployed in Kiruna for the POLSTRACC (Polar Stratosphere in a Changing Climate) campaign. Based on that, the systematic and random error of the wind speeds retrieved from the 2 µm DWL observations are determined. Further, the measurements performed on that day are used to reveal significant changes in the horizontal wavelengths of the vertical wind speed and of the leg-averaged momentum fluxes in the tropopause inversion layer (TIL) region, which are likely to be induced by interfacial waves as recently presented by Gisinger et al. (2020).</p

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

Verification of different Fizeau fringe analysis algorithms based on airborne wind lidar data in support of ESA's Aeolus mission

The Aeolus mission by the European Space Agency was launched in August 2018 and stopped operations in April 2023. Aeolus carried the direct-detection Atmospheric LAser Doppler INstrument (ALADIN). To support the preparation of Aeolus, the ALADIN Airborne Demonstrator (A2D) instrument was developed and applied in several field campaigns. Both ALADIN and A2D consist of so-called Rayleigh and Mie channels used to measure wind from both molecular and particulate backscatter signals. The Mie channel is based on the fringe-imaging technique, which relies on determining the spatial location of a linear interference pattern (fringe) that originated from multiple interference in a Fizeauspectrometer.The accuracy of the retrieved winds is among others depending on the analytic algorithm used for determining the fringe location on the detector. In this paper, the performance of two algorithms using Lorentzian and Voigt fit functions is investigated by applying them to A2D data that were acquired during the AVATAR-I airborne campaign. For performance validation, the data of a highly accurate heterodyne detection wind lidar (2-µm DWL) that was flown in parallel are used as a reference. In addition, a fast and non-fit-based algorithm based on a four-pixel intensity ratio approach (R4) is developed. It is revealed that the Voigt-fit-based algorithm provides 50% more data points than the Lorentzian-based algorithm while applying a quality control that yields a similar random error of about 1.5 m/s. The R4 algorithm is shown to deliver a similar accuracy as the Voigt-fit-based algorithms, with the advantage of a one to two orders of magnitude faster computation time. Principally, the R4 algorithm can be adapted to other spectroscopic applications where sub-pixel knowledge of the location of measured peak profiles is needed

Airborne temperature profiling in the troposphere during daytime by lidar utilizing Rayleigh–Brillouin scattering

The airborne measurement of a temperature profile from 10.5 km down towards ground (about 1.4 km above sea level) during daytime by means of a lidar utilizing Rayleigh-Brillouin (RB) scattering is demonstrated for the first time, to our knowledge. The spectra of the scattered light were measured by tuning the laser (Lambda=354.9 nm) over a 11 GHz frequency range with a step size of 250 MHz while using a Fabry Perot interferometer as a spectral filter. The measurement took 14 min and was conducted over a remote area in Iceland with the ALADIN Airborne Demonstrator on-board the DLR Falcon aircraft. The temperature profile was derived by applying an analytical RB line shape model to the backscatter spectra, which were measured at different altitudes with a vertical resolution of 630 m. A comparison with temperature profiles from radiosonde observations and model temperatures shows reasonable agreement with biases of less than +/-2K. Based on Poisson statistics, the random error of the derived temperatures is estimated to vary between 0.1 K and 0.4 K. The work provides insight into the possible realization of airborne lidar temperature profilers based on RB scattering