Water vapor observations up to the lower stratosphere through the Raman lidar during the Ma?do Lidar Calibration Campaign

摘要

A new lidar system devoted to tropospheric and lower stratospheric watervapor measurements has been installed at the Ma?do altitude stationfacility of Réunion island, in the southern subtropics.To evaluate the performances and the capabilities of the new system with aparticular focus on UTLS (Upper Troposphere Lower Stratosphere) measurements, the Ma?do Lidar Calibration Campaign(MALICCA) was performed in April 2013.Varying the characteristics of the transmitter and the receiver components,different system configuration scenarios were tested and possible parasitesignals (fluorescent contamination, rejection) were investigated. A hybridcalibration methodology has been set up and validated to insure optimallidar calibration stability with time. In particular, the receivertransmittance is monitored through the calibration lamp method that, at themoment, can detect transmittance variations greater than 10–15%.Calibration coefficients are then calculated through the hourly values ofIWV (Integrated Water Vapor) provided by the co-located GPS. The comparison between the constantsderived by GPS and Vaisala RS92 radiosondes launched at Ma?do duringMALICCA, points out an acceptable agreement in terms of accuracy of the meancalibration value (with a difference of approximately 2–3%), but asignificant difference in terms of variability (14% vs. 7–9%, forGPS and RS92 calibration procedures, respectively).We obtained a relatively good agreement between the lidar measurements and15 co-located and simultaneous RS92 radiosondes. A relative difference below10% is measured in the low and middle troposphere (2–10 km). The uppertroposphere (up to 15 km) is characterized by a larger spread (approximately20%), because of the increasing distance between the two sensors.To measure water vapor in the UTLS region, nighttime and monthly water vaporprofiles are presented and compared. The good agreement between the lidarmonthly profile and the mean WVMR profile measured by satellite MLS (Microwave Limb Sounder) has beenused as a quality control procedure of the lidar product, attesting theabsence of significant wet biases and validating the calibration procedure.Due to its performance and location, the MAIDO H₂O lidar will become areference instrument in the southern subtropics, insuring the long-termsurvey of the vertical distribution of water vapor. Furthermore, this systemallows the investigation of several scientific themes, such asstratosphere–troposphere exchange, tropospheric dynamics in the subtropics,and links between cirrus clouds and water vapor.