The Structure and Phase of Cloud Tops as Observed by Polarization Lidar

Abstract

High-resolution observations of the structure of cloud tops have been obtained with polarization lidar operated from a high altitude aircraft. Case studies of measurements acquired from cumuliform cloud systems are presented, two from September 1979 observations in the area of Florida and adjacent waters and a third during the May 1981 CCOPE experiment in southeast Montana. Accurate cloud top height structure and relative density of hydrometers are obtained from the lidar return signal intensity. Correlation between the signal return intensity and active updrafts was noted. Thin cirrus overlying developing turrets was observed in some cases. Typical values of the observed backscatter cross section were 0.1?0.5 (km sr?1) for cumulonimbus tops. The depolarization ratio of the lidar signals was a function of the thermodynamic phase of cloud top areas. An increase of the cloud top depolarization with decreasing temperature was found for temperatures above and below ?40°C. The observed values of depolarization from water clouds were greater than reported by previous studies. Increased multiple scattering due to a larger range from the receiver to scattering medium is thought to have given rise to the greater water cloud depolarization for the cloud top measurements.