Evolution of Aeolus Atmospheric and Solar Background Data

Abstract

Since the launch of the Aeolus satellite end of August 2018, billions of laser pulses in the ultraviolet (UV) have been emitted into the Earth’s atmosphere. The laser signals that are backscattered by air molecules, cloud particles, and aerosols and received by the ALADIN instrument onboard of Aeolus are primarily used to determine globally the speed of the wind in the lowermost 30km of the atmosphere. They are also applied to derive operationally cloud and aerosol properties along the orbit. Moreover, the behaviour of the backscattered signals over time represents an indicator of the performance of the ALADIN instrument including its two redundant lasers FM-A and FM-B. The latter aspect is of particular importance since Aeolus is an Earth Explorer mission to demonstrate the feasibility of the Doppler wind lidar technique for measuring wind profiles from space, intended for assimilation in Numerical Weather Prediction (NWP) models in near-real-time (within three hours of sensing). Various instrument parameters such as those of the lasers, optics, and telescope, have been changed and tested several times to find their optimal setpoints. The same holds for the processor parameters to finally meet the mission requirements with respect to the accuracy of the wind measurements. The outcome of these investigations paves the way for the operational follow-on mission Aeolus-2. Therefore, the backscattered atmospheric signals have been regularly monitored within the Aeolus DISC, in particular those coming from the air molecules in the so-called Rayleigh channel. In our presentation, the approach to derive these signals for clear-sky conditions, avoiding interfering backscattered signals from cloud particles and aerosols, is outlined. The long-term evolution of these signals is presented and discussed for both FM-A and the FM-B periods. In doing so, comparisons with the UV emit energy are provided in order to facilitate the understanding of their evolution. After each laser shot, the atmospheric background signal level is also measured. It is used to correct the laser return signals and determines their signal-to-noise ratio. Consequently, this background level has an influence on the winds and wind errors derived from the backscattered laser signals. Thus, the so-called solar background signals have been regularly monitored since the launch of Aeolus, too. They show orbital and seasonal variations, and are influenced by different changes of the instrument settings and processor parameters. In our contribution, results of the short-term and long-term evolution of the background signals measured by Aeolus are additionally presented and discussed.