A new lidar design for operational atmospheric wind and cloud/aerosol survey from space

Abstract

Abstract. Global wind profile measurement has, for a long time, been a first priority for numerical weather prediction. The demonstration, from ground-based observations, that a double-edge Fabry–Pérot interferometer could be efficiently used for deriving wind profiles from the molecular scattered signal in a very large atmospheric vertical domain has led to the choice of the direct detection technique in space and the selection of the Atmospheric Dynamics Mission (ADM)-Aeolus by the European Space Agency (ESA) in 1999. ADM-Aeolus was successfully launched in 2018, after the technical issues raised by the lidar development had been solved, providing the first global wind profiles from space in the whole troposphere. Simulated and real-time assimilation of the projected horizontal wind information was able to confirm the expected improvements in the forecast score, validating the concept of a wind profiler using a single line-of-sight lidar from space. The question is raised here about consolidating the results gained from ADM-Aeolus mission with a potential operational follow-on instrument. Maintaining the configuration of the instrument as close as possible to the one achieved (UV emission lidar with a single line of sight), we revisit the concept of the receiver by replacing the arrangement of the Fizeau and Fabry–Pérot interferometers with a unique quadri-channel Mach–Zehnder (QMZ) interferometer, which relaxes the system's operational constraints and extends the observation capabilities to recover the radiative properties of clouds. This ability to profile wind and cloud/aerosol radiative properties enables the meeting of the two highest priorities of the meteorological forecasting community regarding atmospheric dynamics and radiation. We discuss the optimization of the key parameters necessary in the selection of a high-performance system, as based on previous work and development of our airborne QMZ lidar. The selected optical path difference (3.2 cm) of the QMZ leads to a very compact design, allowing the realization of a high-quality interferometer and offering a large field angle acceptance. Performance simulation of horizontal wind speed measurements with different backscatter profiles shows results in agreement with the targeted ADM-Aeolus random errors, using an optimal 45∘ line-of-sight angle. The Doppler measurement is, in principle, unbiased by the atmospheric conditions (temperature, pressure, and particle scattering) and only weakly affected by the instrument calibration errors. The study of the errors arising from the uncertainties in the instrumental calibration and in the modeled atmospheric parameters used for the backscattered signal analysis shows a limited impact under realistic conditions. The particle backscatter coefficients can be retrieved with uncertainties better than a few percent when the scattering ratio exceeds 2, such as in the boundary layer and in semi-transparent clouds. Extinction coefficients can be derived accordingly. The chosen design further allows the addition of a dedicated channel for aerosol and cloud polarization analysis.

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

“…Like for the A2D, the vertical incidence angle, represented by the vertical spot position, is even more crucial for the systematic error of the Aeolus Rayleigh channel, but not architecture and thus makes use of a co-alignment sensor at receiver chain level for ensuring that the emitted laser beam is aligned with the receiver FOV (do Carmo et al, 2021). However, there are also other receiver designs that may be applicable in future spaceborne Doppler wind lidars, e.g., based on Michelson or Mach-Zehnder interferometers (Herbst and Vrancken, 2016;Tucker et al;Baidar et al, 2018;Bruneau and Pelon, 2021), which allow for larger acceptance angles, thus relaxing the requirements on the transmitter-receiver co-alignment.…”

“…This is especially true for the A2D's spaceborne counterpart ALADIN, despite its monostatic design where the transmit and receive path are realized by the same telescope which makes an active co-alignment, as used in the A2D, obsolete. However, the developed retrieval modifications are not applicable to DWLs that rely on other measurement principles, such as Mach-Zehnder interferometers (Baidar et al, 2018;Tucker et al, 2018;Bruneau and Pelon, 2021), particularly as they are generally less sensitive to angular variations.…”

ALADIN laser frequency stability and its impact on the Aeolus wind error

“…The spatial location of the fringe changes approximately linearly with frequency of the incident light so that a Doppler frequency shift manifests as a spatial displacement of the fringe centroid position. Derivation of the Doppler shift from the broadband Rayleigh-Brillouin backscatter spectrum is achieved by two sequential Fabry-Pérot interferometers (FPIs) that are utilized for applying the double-edge technique (Chanin et al, 1989;Garnier and Chanin, 1992;Flesia and Korb, 1999). The two FPIs act as bandpass filters with adequate width and spacing that are symmetrically placed around the frequency of the emitted laser pulse.…”

“…A modified ramp-delay-fire technique that was developed for the A2D (Lemmerz et al, 2017) allows for fast responses in the microsecond regime, thus providing high frequency stability (< 4 MHz RMS) even in harsh vibration environments. Referencing on a pulse-by-pulse basis, as performed in other wind lidar instruments (Baidar et al, 2018;Tucker et al, 2018;Bruneau and Pelon, 2021), would relax the requirements in terms of frequency stability. This could, for instance, be realized with avalanche photodiodes (APDs) working in real counting mode.…”

“…These techniques will be applied in the upcoming space missions EarthCARE (Earth Clouds, Aerosols and Radiation Explorer) (Illingworth et al, 2015), ACDL (Aerosol and Carbon Detection Lidar) (Liu et al, 2019) and MERLIN (Methane Remote Sensing Lidar Mission) (Ehret et al, 2017), which are scheduled for launch within the next 5 years. However, it should be mentioned that not all Doppler lidar and HSRL techniques require high frequency stability, especially if the referencing to the outgoing signal is performed on a pulse-by-pulse basis (Baidar et al, 2018;Tucker et al, 2018;Bruneau and Pelon, 2021).…”