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

摘要

The WMO regularily distributes data from the upper-airnballoon-sonde network made up of sites in participating countries aroundnthe world, but the quality varies significantly from country to country.nSatellite observations of global water vapor hold the promise to meetnthe needs of the scientific community. At present passive sensorsnoperating in the infrared and microwave are the only data source fromnwhich to derive atmospheric moisture information. A ground-based Ramannlidar is an ideal sensor to obtain ground-truth data to compare with thensatellite data. The Raman lidar provides vertical profiles of waternvapor mixing ratio which can be used to directly compare with both thensatellite derived data and aircraft versions of future satellite passivensensors. The lidar profiles can provide an important measure ofnatmospheric moisture variability. Knowledge of moisture variability isnabsolutely essential for a proper validation of passive sensors becausenofthe errors associated with beam-filling. In general the measurementnbeam of a satellite-based IR or a microwave instrument is several 10s ofnkilometers at the Earth's surface. The upwelling radiation within thisnfootprint is influenced not only by the total concentration of thenspecies being measured but also by the distribution of that species innthe instantaneous field-of-view. We must know both to perform a validninversion. The Raman lidar developed at the Goddard Space Flight Centernconsists of an xenon fluoride excimer laser and a 0.75 meter telescope.nThe lidar in able, through the use of beamsplitters, to simultaneouslynmeasure laser scattering from aerosols, nitrogen, oxygen and waternvapor. The ratio of the water vapor signal to the nitrogen signal afterna small differential attenuation correction is proportional to waternvapor mixing ratio. The ratio measured versus the time of flight of thenlaser pulse is easily converted into an altitude profile of moisture. Andetailed description of the system along with data showing moisture andnit's variability are given