CO2 LIDAR TECHNIQUE FOR OBSERVING CHARACTERISTIC DROP SIZE IN WATER CLOUDS

An analytical evaluation demonstrates that a calibrated 10.6-mum-wavelength lidar can measure the mean radius and the effective radius of the drop size distribution in a water cloud. The radius parameter observed is a weighted average over the penetration depth of the pulse, with weighting factor decreasing with optical depth. In this method, the lidar signal is integrated and boundary conditions on optical depth are applied to obtain the average extinction-to-backscatter ratio. The radius parameter is determined by comparing the measured ratio with that found from Mie scatter calculations for a variety of typical drop size distributions. This extinction-to-backscatter method was originally proposed in the literature for measuring mode radius, but at 10.6-mum wavelength the current results show better accuracy for mean or effective radius. Other CO2 laser wavelengths can be used, but slightly more stable results are expected at longer wavelengths. Sources of error are discussed. A lidar probed the sides of fair-weather cumulus clouds in the first application of this method. The resulting values of effective radius were reasonable, and the expected increase of effective radius with height was observed.