Humidity measurements in the free troposphere using Raman backscatter

Abstract

A lidar system has been developed to measure the mixing ratio of atmospheric water vapour using Raman scattering from water vapour and nitrogen. The system uses the third harmonic of a neodymium-YAG laser at 354.7 nm, with interference filters to isolate the vibrational Raman bands. Humidity profiles extending from about 2 km to the tropopause have been obtained, with a resolution of 30 m vertically and 20 minutes in time below 5 km, and > 300m vertically and several hours in time above 10 km. Good agreement was found between the lidar profiles and those of coincident radiosondes. Distinctive layers in the humidity profiles are presented, and measurements in the vicinity of cirrus clouds indicate sharp transitions from supersaturation to saturation with respect to ice at the cloud boundary.