## 1 Introduction  
The Cherenkov Telescope Array Observatory (CTAO) [1,2] is the next-generation observatory of the ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs) class. CTAO will observe high-energy cosmic radiation (\"gamma rays\") for research in high-energy astrophysics. The observatory is composed of more than 60 telescopes at two locations: in the northern hemisphere, CTAO-N is currently under construction at the Observatorio del Roque de Los Muchachos (ORM, La Palma, Canary Islands, Spain, 28\({}^{\circ}\)N 17\({}^{\circ}\)W), and in the southern hemisphere, CTAO-S will be constructed at a site belonging to the European Southern Observatory (ESO), which is close to Cerro Paranal, Chile (24\({}^{\circ}\)S 70\({}^{\circ}\)W). The telescope arrays are spread over an area of approximately half a square kilometre in the north and three square kilometres in the south and are both located at altitudes of around 2200 m above sea level. IACTs observe the few nanosecond-long flashes of UV-blue Cherenkov light emitted by particle cascades generated in the atmosphere by cosmic gamma rays entering the Earth's atmosphere. They use, for that purpose, large reflectors of up to 28 m diameter and fields-of-view (FoV) of 4\({}^{\circ}\)-10\({}^{\circ}\) for current operating systems. By analyzing the distribution of this Cherenkov light received, IACTs reconstruct the timing, energy, and direction of the primary gamma ray. Because the Cherenkov light is emitted around 5-20 km above the ground, a dominant contribution to the systematic uncertainty in the gamma-ray energy and flux reconstruction of IACT results from an inaccurate determination of atmospheric transmittance [3,4]. For this reason, CTAO has been programmed [5] to continuously monitor the atmosphere along the line of sight of the observing telescopes and to, in particular, assess the aerosol extinction profile with Raman LIDARs (RLs) [6], together with the assessment of the Aerosol Optical Depth (AOD) across the observed FoV, with a stellar photometer specifically designed for that purpose [7]. Atmospheric conditions at world-class sites for astronomy, such as CTAO-N and CTAO-S, are characterized by low aerosol content [8,9], few episodes of dust intrusions [10], frequent absence of clouds or presence of cloud cover with negligible optical depth [11], and generally a highly dry and transparent atmosphere [12], which leads to excellent observing conditions. IACT science data cannot be reasonably analyzed with AODs larger than about 0.7, even if the optical properties of the atmosphere are well characterized [13,14], because the influence of the residual systematics on the data becomes too large. Hence, in this regime, LIDAR-based characterization is not required. On the other hand, LIDAR-based extinction profiles for AOD below 0.7 have been successfully used to recalibrate IACT data [13] and are foreseen to become an integral part of the CTAO calibration chain [15]. In such conditions, CTAO requires aerosol profiling with a range resolution better than a few hundred meters [15,16]. LIDARs at IACT installations shall characterize the entire troposphere and the lower stratosphere. Since under normal conditions, the height of the nocturnal planetary boundary layer (PBL) at the CTAO-N site reaches \(\lesssim\)800 m above ground [9,17] and the Cherenkov light from typical gamma-ray induced particle showers is emitted entirely above it [18], the fine structure of the PBL does not need to be resolved [19]. Only in the event of Saharan dust intrusions (the so-called \"calima\" on the Canary Islands [9,20]) does the PBL reach higher altitudes, up to \(\sim\)6 km above ground [9,17,21]. High-level CTAO science requirements translate into requirements for the determination of atmospheric parameters in terms of clouds and PBL properties in addition to operation requirements:  
**Cloud properties:** The RL shall detect clouds in an altitude range from 2 km to 20 km above ground within a cloud Vertical Optical Depth (VOD) range of 0.01 \(<\) VOD \(<\) 0.7.