Water vapor measurements by lidar: Raman and DIAL campaigns

Ground-based water vapor measurements by lidar have been performed in Potenza, Southern Italy, by the application of the Raman and the DIAL techniques. Raman measurements have been accomplished through the simultaneous detection of the backscattered radiation in the vibrational Raman bands of water vapor and molecular nitrogen as stimulated by a 355 nm beam, while DIAL measurements at 720 nm have been accomplished by means of a dye laser transmitter. Water vapor measurements in the troposphere up to approximately 10 km above station level have been obtained through the simultaneous application of the Raman and DIAL techniques. Reported measurements have been performed on 18 February 1997, both in the early evening and at night-time. The deviation between DIAL and Raman measurements is found not to exceed 25% up to 7.5 km, while the deviation between Raman and radiosonde data keeps smaller than 20% up to 7 km. The large deviation between Raman and radiosonde data above 7 km is to be attributed to the limited accuracy (dry bias) of the Vaisala humicap sensor at low humidity levels for temperatures below -30°C. A further goal of the presentation is to define a calibration procedure for Raman measurements based on the comparison with simultaneous DIAL measurements. The estimate of the Raman calibration coefficient on the basis of simultaneous and independent water vapor DIAL measurements is more reliable than the traditional approach based on the comparison with radiosonde data