Validation of Envisat Measurements by Ground-based Nighttime Lidar Soundings (AOID9100)

A Rayleigh/Mie/Raman lidar system located in the Rome Tor Vergata site participates in the Atmospheric Chemistry Validation Team of ENVISAT for the validation of the MIPAS GOMOS and SCIAMACHY measurements [1]. During the commissioning phase, first set of inter-comparisons has been performed with the measurements available at the present status of the lidar system setup. The lidar system and the data features are briefly reviewed and the lidar error budget is described. Preliminary inter-comparisons between the lidar and ENVISAT temperature and water vapor profiles during commissioning phase are presented. 1. SYSTEM OVERVIEW The lidar utilizes two laser beams from a Nd:YAG pulsed laser (second and third harmonics: 532 nm, VIS, and 355 nm, UV, respectively) that are vertically sent into the atmosphere. The VIS radiation is used to detect the elastic backscattering signal from air molecules and aerosol. The UV is used to generate Raman backscattering signals from nitrogen and water vapor separately. Three orders of collectors of different size are utilized to collect the backscattered radiation from the atmosphere. This scheme is necessary to record signals from three separate altitude ranges and to get useful measurements from the boundary layer to the mesosphere, overcoming the problem of the high dynamical range of the signal (about 13 orders of magnitude in the interesting range). At present the largest collector is working with two 50 cm-diameter telescopes that are used to collect UV returns from the upper atmospheric layers; while for the VIS radiation, a single similar telescope is used. Simultaneously, a medium telescope (30 cm diameter) is employed to collect the VIS and UV returns from the lower atmospheric layers and a smaller telescope (15 cm diameter) is utilized to collect the VIS from the PBL. In the definitive configuration, which should be working by the next month, an array of nine 50 cm-diameter telescopes will be employed as largest collector for both UV and VIS echoes. The separation of the various interesting wavelength is made in two steps: a dichroic mirror, near the focus of each telescope, divides the VIS from the UV radiation, in the whole. Thereafter all signals from the telescopes are brought to different channels of photodetection through optical fibers. Then, in each UV channel, a second dichroic system separates the N2 Raman signal (386.7 nm) and the water vapor Raman signal (407.5 nm), and transmits them toward different photomultipliers. The UV elastic backscattering is also separated in a different channel, but still not detected. Narrow band (2–4 nm) interference filters are used to limit the contribution due to the sky background. Photomultipliers are used for the signal detection. The signal from the photomultipliers is acquired, both by A-D conversion and by photoncounting techniques, by means of a multiple-input transient recorder. This records the signal variation as a function of time delay from the laser pulse and performs and a time integration of the various profiles acquired, before transferring the resulting ones to a PC where they are stored. The system is assembled in two standard containers that can be transported by truck. Detailed description of the system can be found in [2, 3]