Raman lidar measurements of water vapor as ground-truth for passive remote sensors

The WMO regularily distributes data from the upper-air balloon-sonde network made up of sites in participating countries around the world, but the quality varies significantly from country to country. Satellite observations of global water vapor hold the promise to meet the needs of the scientific community. At present passive sensors operating in the infrared and microwave are the only data source from which to derive atmospheric moisture information. A ground-based Raman lidar is an ideal sensor to obtain ground-truth data to compare with the satellite data. The Raman lidar provides vertical profiles of water vapor mixing ratio which can be used to directly compare with both the satellite derived data and aircraft versions of future satellite passive sensors. The lidar profiles can provide an important measure of atmospheric moisture variability. Knowledge of moisture variability is absolutely essential for a proper validation of passive sensors because ofthe errors associated with beam-filling. In general the measurement beam of a satellite-based IR or a microwave instrument is several 10s of kilometers at the Earth´s surface. The upwelling radiation within this footprint is influenced not only by the total concentration of the species being measured but also by the distribution of that species in the instantaneous field-of-view. We must know both to perform a valid inversion. The Raman lidar developed at the Goddard Space Flight Center consists of an xenon fluoride excimer laser and a 0.75 meter telescope. The lidar in able, through the use of beamsplitters, to simultaneously measure laser scattering from aerosols, nitrogen, oxygen and water vapor. The ratio of the water vapor signal to the nitrogen signal after a small differential attenuation correction is proportional to water vapor mixing ratio. The ratio measured versus the time of flight of the laser pulse is easily converted into an altitude profile of moisture. A detailed description of the system along with data showing moisture and it´s variability are given