A COMPARISON OF WATER-VAPOR MEASUREMENTS MADE BY RAMAN LIDAR AND RADIOSONDES

This paper examines the calibration characteristics of the NASA/GSFC R aman water vapor lidar during three field experiments that occurred be tween 1991 and 1993. The lidar water vapor profiles are calibrated usi ng relative humidity profiles measured by AIR and Vaisala radiosondes. The lidar calibration computed using the AIR radiosonde, which uses a carbon hygristor to measure relative humidity, was 3%-5% higher than that computed using the Vaisala radiosonde, which uses a thin film cap acitive element These systematic differences were obtained for relativ e humidities above 30% and so cannot be explained by the known poor lo w relative humidity measurements associated with the carbon hygristor. The lidar calibration coefficient was found to vary by less than 1% o ver this period when determined using the Vaisala humidity data and by less than 5% when using the AIR humidity data. The differences betwee n the lidar relative humidity profiles and those measured by these rad iosondes are also examined. These lidar-radiosonde comparisons are use d in combination with a numerical model of the lidar system to assess the altitude range of the GSFC lidar. The model results as well as the radiosonde comparisons indicate that for a lidar located at sea level measuring a typical midlatitude; water vapor profile, the absolute er ror in relative humidity for a 10-min, 75-m resolution profile is less than 10% for altitudes below 8.5 km. Model results show that this max imum altitude can be extended to 10 km by increasing the averaging tim e and/or reducing the range resolution.