The ARCADE Raman Lidar and atmospheric simulations for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is the next generation of ground-based very high energy gamma-ray Imaging Atmospheric Cherenkov Telescopes (IACTs). Since observations with this technique are affected by atmospheric conditions, an accurate knowledge of the atmospheric properties is fundamental to improving the precision and duty cycle of the CTA. Measurements of absorption and scattering properties of the atmosphere, due to aerosols and molecules, can be used either in the event reconstruction software or in a detailed atmospheric radiative transfer model such as MODTRAN, an analytical code designed to model the propagation of electromagnetic radiation. The output of the MODTRAN software is then used as an input for the air shower simulation and Cherenkov light production, giving the optical depth profiles that together with the refractive index allow the proper simulation of the gamma-ray induced signals and a correct measurement of the primary energy from the detected signals. The ARCADE Raman Lidar is part of the CTA baseline for the on-site characterization of the aerosol attenuation profiles of the UV light. The collected data will be used in preparation for the full operation of the array, providing nightly information about the aerosol properties on site at 355 nm, such as the vertical aerosol optical depth and the water vapour mixing ratio with an altitude resolution better than 100 m from about 400 m to 10 km above ground level. These measurements will help to define the needs for Monte Carlo simulations of the shower development and of the detector response. This instrument will also be used for the intercalibration of the future Raman Lidars that are expected to operate at the CTA sites. This contribution includes a description of the ARCADE Lidar, that includes two Raman channels (nitrogen and water vapour) and one elastic channel, and the characterization of the performance of the system and test results in L’ Aquila. The system is expected to be shipped to the northern site of the CTA (La Palma) before the end of 2017, to acquire data locally for 1 year before being moved to the southern site (Chile).