Towards an assessment of Aeolus' Mie radiometric performance

Kurzfassung

The intensity of the return signal acquired by Aeolus depends on numerous factors such as the output laser energy, the state of the atmosphere along the line of sight, the characteristics of the target, the optical elements of the instrument and the alignment of laser beam and telescope. Already at an early stage of the mission, it was found that a significant part of the atmospheric backscatter signal was missing on the Rayleigh channel. Whereas the properties of the transmission and reception path of the Rayleigh channel can be simulated to a reasonable extent by the Aeolus End-To-End Simulator (E2S), it is much more difficult to align the simulation to the actual characteristics of the Mie channel, in particular to the transmission function of the Fizeau spectrometer. As a first attempt to assess the radiometric performance of Aeolus Mie channel, we are trying to derive a ratio between simulated and actual Aeolus signals. Therefore, we compare useful signals obtained with the E2S against measurements made with Aeolus in aerosol-laden atmospheric scenes. In this context, the backscatter and extinction measurements of portable ground-based Raman lidar systems from PollyNET as well as temperature and pressure information from external sources represent essential inputs for the simulation. Elevated, optically thick, vertically extended and preferably homogeneous aerosol layers are considered as the most suitable target. With the Raman lidars sensing a drifting aerosol layer from a fixed location and Aeolus as a mobile instrument sampling a quasi-fixed layer, optimisation is needed concerning the match of geolocation between the ground-based and space-borne measurements. Present ratios derived from scenes over Leipzig (Germany) and Al Dhaid (United Arabic Emirates) range from 0.6 to 0.9 (less measured signal than simulated). These factors show relative uncertainties of at least ±20% with expected error contributions based on the differences between Aeolus measurements, ground based measurements and simulation, i.e. location, heterogeneity of aerosol layers, E2S input parameters, assumptions in the handling of depolarised signals, potential cloud cover at altitudes higher than the measurement range, additional noise sources, etc. Reducing the number of contributors as well as their magnitude poses the biggest challenge for a reliable assessment of the Mie radiometric performance, which might only be achievable via statistical analyses on a larger number of cases.