Comparison of measurements of water vapor by a microwave radiometer and Raman lidar

In radiometric remote sensing of the atmosphere, the ability to calculate radiances from underlying state variables is fundamental. Traditionally, such “forward model” calculations have coupled radiosonde observations of the state variables with detailed absorption and radiative transfer models to compare with radiance observations. However, for a variety of reasons, radiosondes are not always satisfactory, especially during low humidity conditions, or when there are large horizontal or temporal gradients in the humidity structure. The use of Raman lidar is an alternative method of measuring humidity profiles, and has the added advantage of providing knowledge that the atmosphere above the instruments is clear. In November-December 1991, a substantial number of remote sensor and in situ instruments were operated together in Coffeyville, Kansas, USA, during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE II). Included in the suite of instruments were (a) the NOAA Environmental Technology Laboratory´s (ETL) three channel microwave radiometer; (b) the NASA/Goddard Space Flight Center´s (GSFC) Raman lidar; (c) ETL´s Radio Acoustic Sounding System (RASS) (7); and (d) frequent research-quality radiosondes. The authors present results of simultaneous microwave radiometer measurements with collocated Raman lidar measurements of water vapor. Information on temperature profiles was obtained from composite data from radiosondes and RASS. The Raman lidar soundings of humidity routinely reach 8.5 km during clear nighttime conditions, but reach only to 3-4 km during the day. During the presence of liquid-bearing clouds, the Raman lidar may not penetrate much beyond cloud base. However, a suite of ground-based sensors, such as Raman lidar, RASS, and microwave radiometers, could perhaps provide soundings during both day and night and during cloudy conditions that are also useful for meteorological remote sensing. Such an array of sensors has been operated on an experimental basis by the Department of Energy´s Atmospheric Radiation Program, and further deployment by the ARM program is also planned