Noemi Franco

The Raman lidar technique to measure atmospheric temperature profiles is based on the dependence on temperature of the intensity of the atmospheric N2 and O2 rotational Raman lines [1]. The technique requires very good stability of the laser wavelength, or frequent recalibrations, to avoid errors in the retrieved temperature produced by wavelength drifts. Frequency doubled or tripled Nd:YAG lasers are usually employed to implement this technique. To achieve laser wavelength stability, injection-seeded lasers are used that transfer the wavelength stability of the seeder to the high-power laser [2]; this has also the consequence of narrowing the spectrum of the transmitted radiation. Temperature profiling using free-running lasers are also reported in the literature [3]. In this case wavelength stability must be obtained by keeping the laser operating conditions, and in particular the Nd:YAG rod temperature, very stable.We have assessed the effects on the atmospheric temperature retrieval of the spectral width and temperature-induced wavelength drift of the 3rd harmonic of a free-running Nd:YAG laser. We have found that the spectral width has a negligible effect, as compared with the negligible spectral width of an injection-seeded laser, in the receiving filters that are part of the lidar. However, slight temperature-induced drifts on the central wavelength of the laser emitted spectrum entail small changes in the filter responses that impair the calibration and cause an uncertainty in the retrieved atmosphere temperature. We have estimated that to keep the retrieved temperature uncertainty below 1 K, the rod temperature must also to be kept within a ±1 K range. This is also the temperature stability that would be needed in the seeder of an injection seeded laser, as changes of temperature in the seeder will also cause wavelength drifts, hence uncontrolled biases in the atmosphere temperature measurements that would add to their uncertainty. [1] J. Cooney, Measurement of Atmospheric Temperature Profiles by Raman Backscatter, J Appl Meteorol Climatol. 11 (1972) 108–112. https://doi.org/10.1175/1520-0450(1972)0112.0.CO;2[2] E. Hammann, A. Behrendt, F. Le Mounier, V. Wulfmeyer, Temperature profiling of the atmospheric boundary layer with rotational Raman lidar during the HD(CP)2 Observational Prototype Experiment, Atmos Chem Phys. 15 (2015) 2867–2881. https://doi.org/10.5194/acp-15-2867-2015.[3] P. Di Girolamo, R. Marchese, D.N. Whiteman, B.B. Demoz, Rotational Raman Lidar measurements of atmospheric temperature in the UV, Geophys Res Lett. 31 (2004) 1–5. https://doi.org/10.1029/2003GL018342.

The Cloud and Aerosol Lidar for Global Scale Observations of the Ocean-Land-Atmosphere System (CALIGOLA) is an advanced multi-purpose space lidar mission with a focus on atmospheric and oceanic observation aimed at characterizing the Ocean-Earth-Atmosphere system and the mutual interactions within it. This mission has been conceived by the Italian Space Agency (ASI) with the aim to provide the international atmospheric and ocean science communities with an unprecedented dataset of geophysical parameters capable to increase scientific knowledge in the areas of atmospheric, aquatic, terrestrial, cryospheric and hydrological sciences. The mission is planned to be launched in the time frame 2030-2031, with an expected lifetime of 3-5.Exploiting the three Nd:YAG laser emissions at 354.7, 532 and 1064 nm and the elastic (Rayleigh-Mie), depolarized and Raman lidar echoes from atmospheric constituents, CALIGOLA will carry out 3λ profile measurements of the particle backscatter coefficient and depolarization ratio and 1-2λ (354.7 and 532 nm) profile measurements of the particle extinction coefficient from aerosols and clouds. These measurements allow for aerosol typing and the determination of aerosol size and microphysical properties. Furthermore, measurements of the elastic and depolarized backscattered echoes from the sea surface and the underlying layers will allow characterizing the optical properties of the marine surface (ocean color) and the suspended particulate matter in terms of oceanic particulate backscattering coefficient, while diffuse attenuation for downwelling irradiance at 1-2λ will be determined from the H2O roto-vibrational Raman signals. These measurements will allow characterizing phytoplankton seasonal and inter-annual dynamics. Additionally, fluorescent scattering measurements at 1-3λ (450, 685 and 735 nm) from marine chlorophyll and atmospheric aerosols will be exploited to characterize ocean primary production and for atmospheric aerosol typing, respectively. CALIGOLA will also allow for accurate measurements of the small-scale variability of the earth's surface elevation, primarily associated with variations in the ice and snow, terrain, vegetation and forest canopy height.The space mission CALIGOLA is explicitly included in the on-going ASI 2021-2023 Activity Plan. A Phase-A study focusing on the technological feasibility of the major sub-systems is on-going, commissioned by ASI to Leonardo S.p.A. (LDO). Scientific studies in support of the mission are also on-going, with the University of Basilicata (UNIBAS) being the leading scientific institution. In September 2023 NASA-LARC initiated a pre-formulation study to assess the feasibility of a possible contribution to CALIGOLA based on the development of the receiver detection chain and data down link capabilities. In September 2024 NASA will decide if proceed or not with the cooperation.This conference contribution aims at illustrating the different atmospheric and ocean sciences’ objectives and a preliminary assessment of the mission observational requirements in terms of observable quantities, their vertical/horizontal resolution and precision/accuracy. The contribution also aims at illustrating the technical and technological solutions identified in the design of the instrument during the pre-feasibility and feasibility studies carried out by LDO, in cooperation with UNIBAS and other Italian research institutions. Expected system performance in a variety of environmental conditions will be provided based on the application of an end-to-end simulator developed at UNIBAS.

The crucial parameter for characterizing the energy exchange between the Earth's surface and the atmosphere within the Atmospheric Boundary Layer (ABL) is the latent heat flux (LHF). This represents the speed at which energy stored as latent heat in water vapor molecules is transported into the ABL due to the turbulent convective movement of the air.The integration of both lidar measurements provides a comprehensive perspective on atmospheric processes related to latent heat flux, significantly contributing to improving the understanding of the water cycle and associated meteorological phenomena. During the WaLiNeAs campaign (Water vapor Lidar Network Assimilation), a consortium of French, German, Italian, and Spanish research groups deployed a network of 6 autonomous Water Vapor (WV) Lidars in the French territory. This network delivers measurements with high vertical resolution and accuracy throughout the Western Mediterranean, starting in the fall of 2022 and addressing critical gaps in water vapor observations in the lower troposphere from current operational networks and satellites.As part of the WaLiNeAs initiative, a Lidar system developed by the University of Basilicata was positioned near a Wind Lidar with the goal of collecting measurements of heat flux and turbulent kinetic energy (TKE). These two systems operated continuously for three months starting from the end of September 2022, covering the most favorable period in southern France and acquiring high-resolution measurements (10 seconds, 30 meters).AcknowledgmentThe authors acknowledge Next Generation EU Mission 4 “Education and Research” - Component 2: “From research to business” - Investment 3.1: “Fund for the realization of an integrated system of research and innovation infrastructures” - Project IR0000032 – ITINERIS.This work was supported by the Agence Nationale de la Recherche (WaLiNeAs, Grant ANR-20-CE04-0001). This research was also funded by the Italian Ministry for Education, University and Research (grants STAC-UP and FISR2019-CONCERNING) and the Italian Space Agency (grants As-ATLAS and CALIGOLA).References[1] Flamant, C., Chazette, P., Caumont, O. et al. (2021) A network of water vapor Raman lidars for improving heavy precipitation forecasting in southern France: introducing the WaLiNeAs initiative. Bull. of Atmos. Sci.& Technol. 2, 10.[2] Kiemle, W. A. Brewer, G. Ehret, R. M. Hardesty, A. Fix, C. Senff, M. Wirtg, G. Poberaj and M. A. Lemone. (2007) Latent Heat Flux Profiles from Collocated Airborne Water Vapor and Wind Lidars during IHOP_2002. American Meteorological Society pp:627-639.[3] Behrendt , V. Wulfmeyer1 , C. Senff , S. K. Muppa, F. Späth , D. Lange , N. Kalthoff , and A. Wieser. (2020). Observation of sensible and latent heat flux profiles with lidar Atmos. Meas. Tech., 13, 3221–3233.

We assess the temperature stability requirements of unseeded Nd:YAG lasers in lidar systems for atmospheric temperature profiling through the rotational Raman technique. Taking as a reference a system using a seeded laser assumed to emit pulses of negligible spectral width and free of wavelength drifts, we estimate first the effect of the pulse spectral widening of the unseeded laser on the output of the interference filters, and then we derive the limits of the allowable wavelength drift for a given bias in the temperature measurement that would add to the noise-induced uncertainty. Finally, using spectroscopic data, we relate the allowable wavelength drift to allowable temperature variations in the YAG rod. We find that, in order to keep the bias affecting atmospheric temperature measurements smaller than 1 K, the Nd:YAG rod temperature should also be kept within a variation range of 1 K.

We assess the temperature stability requirements of unseeded Nd:YAG lasers in lidar systems for atmospheric temperature profiling through the rotational Raman technique. Taking as a reference a system using a seeded laser assumed to emit pulses of negligible spectral width and wavelength-drift free, we estimate first the effect of the pulse spectral widening of the unseeded laser on the output of the interference filters, then we derive the limits of the allowable wavelength drift for a given bias in the temperature measurement that would add to the noise-induced uncertainty. Finally, using spectroscopic data, we relate the allowable wavelength drift to allowable temperature variations of the YAG rod. We find that, in order to keep the bias affecting atmospheric temperature measurements smaller than 1 K, the Nd:YAG rod temperature should also be kept within 1 K.