Retrieval improvements for the ALADIN Airborne Demonstrator in support of the Aeolus wind product validation

Abstract

Abstract. The realization of the European Space Agency's Aeolus mission was supported by the long-standing development and field deployment of the Atmospheric LAser Doppler INstrument (ALADIN) Airborne Demonstrator (A2D) which, since the launch of the Aeolus satellite in 2018, has been serving as a key instrument for the validation of ALADIN, the first-ever Doppler wind lidar (DWL) in space. However, the validation capabilities of the A2D are compromised by deficiencies of the dual-channel receiver which, like its spaceborne counterpart, consists of a Rayleigh and a complementary Mie spectrometer for sensing the wind speed from both molecular and particulate backscatter signals, respectively. Whereas the accuracy and precision of the Rayleigh channel is limited by the spectrometer's high alignment sensitivity, especially in the near field of the instrument, large systematic Mie wind errors are caused by aberrations of the interferometer in combination with the temporal overlap of adjacent range gates during signal readout. The two error sources are mitigated by modifications of the A2D wind retrieval algorithm. A novel quality control scheme was implemented, which ensures that only backscatter return signals within a small angular range are further processed. Moreover, Mie wind results with large bias of opposing sign in adjacent range bins are vertically averaged. The resulting improvement of the A2D performance was evaluated in the context of two Aeolus airborne validation campaigns that were conducted between May and September 2019. Comparison of the A2D wind data against a high-accuracy, coherent DWL that was deployed in parallel on board the same aircraft shows that the retrieval refinements considerably decrease the random errors of the A2D line-of-sight (LOS) Rayleigh and Mie winds from about 2.0 to about 1.5 m s−1, demonstrating the capability of such a direct detection DWL. Furthermore, the measurement range of the Rayleigh channel could be largely extended by up to 2 km in the instrument's near field close to the aircraft. The Rayleigh and Mie systematic errors are below 0.5 m s−1 (LOS), hence allowing for an accurate assessment of the Aeolus wind errors during the September campaign. The latter revealed different biases of the Level 2B (L2B) Rayleigh-clear and Mie-cloudy horizontal LOS (HLOS) winds for ascending and descending orbits, as well as random errors of about 3 m s−1 (HLOS) for the Mie and close to 6 m s−1 (HLOS) for the Rayleigh winds, respectively. In addition to the Aeolus error evaluation, the present study discusses the applicability of the developed A2D algorithm modifications to the Aeolus processor, thereby offering prospects for improving the Aeolus wind data quality.

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

“…Note that the model wind data are independent of the Aeolus winds, as they are produced before assimilation of the latter (Aeolus winds were assimilated in the previous model runs, though). The AUX_MET data (zonal and meridional wind components) were averaged onto the L2B grid by means of a weighted aerial interpolation algorithm (Marksteiner, 2013), which was also applied for the harmonization of the L2B and A2D datasets in previous airborne campaigns (Lux et al, 2018(Lux et al, , 2020a(Lux et al, , 2022. This procedure allows for a direct comparison of each L2B wind observation (O)…”

“…The WindVal-III (Lux et al, 2020a) and AVATARE (Aeolus VAlidation Through Airborne LidaRs in Europe) campaigns (Witschas et al, 2020) covered Central Europe in November 2018 and May 2019 during the early stage of the mission. The North Atlantic and polar region around Iceland was targeted with the AVATAR-I campaign in September 2019 (Lux et al, 2022) three months after the switch to the second flight-model (FM) laser (FM-B) (Lux et al, 2020b). Finally, the AVATAR-T (Aeolus VAlidation Through Airborne LidaRs in the Tropics) campaign was conducted around the Cabo Verde archipelago in September 2021 as part of the Joint Aeolus Tropical Atlantic Campaign (JATAC) (Fehr et al, 2022).…”

Validation of the Aeolus L2B wind product with airborne wind lidar measurements in the polar North Atlantic region and in the tropics

“…In addition to the 2-µm DWL measurements, in-situ observations with the Falcon nose-boom are used for comparison. A dedicted study of the Aeolus measurement principle, its calibration procedures and retrieval algorithms is performed based on A2D observations as discussed in Lux et al (2020) and Lux et al (2022a).…”

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

“…The DLR 2-µm DWL has been deployed in several airborne campaigns within the last two decades to measure for instance the optical properties of aerosols (Chouza et al, 2015(Chouza et al, , 2017 and horizontal wind speeds over the Atlantic Ocean as input data for numerical weather prediction assimilation experiments (Weissmann et al, 2005;Schäfler et al, 2018). Furthermore, it was extensively used for pre-launch (Marksteiner et al, 2018;Lux et al, 2018) and post-launch validation activities of the first space-borne wind lidar Aeolus (Witschas et al, 2020;Lux et al, 2020;Witschas et al, 2022;Lux et al, 2022). Additionally, horizontal and vertical wind speed measurements have been performed and have been used to characterize the spectral features of orographically induced gravity waves (Witschas et al, 2017;Wagner et al, 2017;Gisinger et al, 2020).…”