An End-to-end simulator for the performance assessment of space Raman Lidar systems

One of the most pressing and widely discussed topics today is climate change. This topic transcends geographical boundaries, impacting ecosystems, economies and societies around the world and requiring increasingly interdisciplinary research, involving different fields with a clear focus on atmospheric science, oceanography, geophysics, ecology. In the face of escalating environmental challenges, a deep understanding of the planet's climatic processes and the underlying mechanisms driving these transformations has never been more necessary. A comprehensive monitoring of the atmosphere is essential to increase our comprehension of the Earth system. A systematic and accurate collection of atmospheric data with a global coverage allows to study processes and to improve models and tools used in forecasting, allowing to anticipate and promptly respond to extreme events with more precision and timeliness. Over the past decade, the Lidar technique has become more and more competitive thanks to the advancement in many technological fields, including the development of high-power stable laser, with short pulse duration and narrow linewidth; the increased sensitivity and speed of photodetectors; the advancement in optical filters' manifacturing; as well as the miniaturization of components and the reduction of production costs. Unlike traditional passive methods, Lidars directly provide vertically resolved profiles, bypassing the complexity of inversion algorithms. The high temporal and spatial resolution allows to retrieve vertical gradients in water vapor mixing ratio and temperature profile and is crucial to determine the vertical distribution of aerosols, clouds and other atmospheric constituents. Moreover, the acquisition of measurements requires short time interval, making near real-time monitoring possible. The need to fill gaps that are still present in our observational capabilities - especially in the low troposphere - together with new possibilities brought by improved instrument and technologies, converge to the idea of exploiting the space-borne Raman Lidar technique. The availability of high vertically resolved thermodynamic and/or optical profiles would bring an entirely new globally-sampled information, also in under-observed parts of the planet and atmosphere. The contribution of these new data would have strong benefits in several research areas, such as radiative transfer models, land-atmosphere feedbacks, mesoscale circulations, data assimilation etc. In this context, it's important to have a tool capable to assess the performance of a space-borne Raman lidar, to define its experimental parameters, as well as to estimate the impact of retrieved products, in order to have a useful support for the development phase and feasibility studies. For this purpose, an end-to-end simulator has been developed. The simulator allows to simulate the lidar signals, taking into account all the experimental and environmental conditions and then to analyze them in order to retrieve thermodynamic and optical parameters, together with their statistical and systematic uncertainties. The purpose of this thesis is to describe the procedures on which the simulator is based, providing a detailed description of the strategies used and the theoretical background. Specifically, the first chapter describes the structure of lidar systems, with a brief overview on the main components. A section is also dedicated to two ground based Raman lidar - Concerning and Marco - designed and developed during the second year of the PhD course. The development of these systems was useful to better understand all the mechanisms that had to be simulated and also to have a reference to test the theoretical model included in the simulator. Moreover, many of the technological solutions used for the systems are a starting point for the development of the space-borne prototype. Concerning and Marco were also used in the field campaign WaLiNeAs, during which they operated in a network of lidars located in the south of France starting from October 2022. The aim was to provide near real-time water vapor profile to be assimilated in a NWP model, in order to improve the capability to predict extreme precipitation events. Concerning operated continuously until January 2023, providing very accurate measurements during both day and night and in all weather conditions. Marco is based on a micro-pulse lidar, so it is less performing than Concerning, but with more than a year of continuous operations has demonstrated the capability to provide useful products with an ultra compact and economic design. The second chapter is focused on the end-to-end simulator. The model is divided into two separate modules: the forward module estimates the lidar signals in terms of detected photons, by simulating the propagation of the laser beam in the atmosphere, its interaction with the atmospheric constituents (molecules and particles) and the behavior of all the devices included in the receiving system. The structure of the simulator allows to consider both space-borne and ground-based lidar systems and to specify all the experimental parameters, so that sensitivity studies can be easily conducted. Specifically, it is possible to simulate the elastic signal, i.e., the amount of radiation elastically backscattered by atmospheric constituents at the same wavelength of the incident radiation, the Raman N2 and H2O roto-vibrational signals and pure-rotational N2-O2 signals. Simulated signals include a background term that depends on the solar radiation received together with the backscattered signal. An improved theoretical model is proposed to better evaluate this contribution with sun zenith angles near 90 degrees, values with which traditional models, based on parallel-plane approximation, fail. The improved algorithm is particularly suited to evaluate the performance of space-borne lidar on dawn-dusk orbits. To simulate the statistical fluctuation of the number of photons typical of the shot-noise, the signals are perturbed using the Poisson statistics. Finally, a section is dedicated to simulation in presence of clouds. Clouds contribute attenuating the signal, but also increasing the background due to cloud reflectivity. Moreover, in case of discontinuous clouds, the simulation consider the capability of the lidar to exploit shots between clouds, using weights linked to cloud distribution and the combination of possible paths. The second part of the chapter is dedicated to the retrieval module, used to analyze the lidar signals in order to retrieve vertically resolved profiles of water vapor mixing ratio, temperature, backscatter and extinction coefficient, using consolidated techniques, a description of which is provided. The end-to-end structure allows to estimate both the statistical and systematic uncertainties, by comparing the retrieved profiles with the data used as input. The third chapter shows the results used in two ongoing space lidar projects - Atlas and Caligola - obtained through the simulator. Atlas is a space-borne Raman lidar able to measure atmospheric temperature and water vapor mixing ratio with high temporal and spatial resolution. The mission concept was proposed in the frame of "Earth Explorer-11 Mission Ideas" and was also submitted in an improved version to the call "Earth Explorer-12". The simulator was used in this context to verify the performance of the lidar in different conditions, e.g., changing some experimental parameters, in different illumination conditions, using different data as input. The simulation were performed considering both atmospheric models and data extracted from the NASA's GEOS-5 dataset in order to evaluate the performance along multiple orbits around the Earth. On the other hand, Caligola is a three-wavelength Raman lidar with mission founded by the Italian Space Agency (ASI), with objectives that includes both atmospheric and oceanic observations. The project involves the collaboration of several organization: the scientific studies are lead by the University of Basilicata together with ISMAR-CNR, technological feasibility studies are on-going at the Leonardo S.p.A and lately NASA has showed a strong interest in the mission and has initiated a pre-formulation study at Langley with the possibility to be maintained within the Earth Systematic Mission Program Once at NASA Goddard Space Flight Center. Within this project, a strong cooperation with Leonardo required several simulations to define the experimental setup of Caligola, including the selection of channels, sensitivity studies for the choice of the field-of-view, considerations about the expected performance depending on the local passage time etc. The end-to-end simulator is therefore a powerful instrument that can be used in different contexts and for different purposes, from conceptual design to verification and validation. It has certainly been a useful tool for missions and projects that, if achieved, would make a contribution to research and knowledge.