Ninth ARM Science Team Meeting Proceedings, San Antonio, Texas, March 22-26, 1999 1 Development of a Compact Lidar to Profile Water Vapor in the Lower Troposphere J. L. Machol Cooperative Institute for Research in Environmental Sciences, University of Colorado National Oceanic and Atmospheric Administration Environmental Technology Laboratory Boulder, Colorado R. M. Hardesty National Oceanic and Atmospheric Administration Environmental Technology Laboratory Boulder, Colorado J. B. Abshire and M. A. Krainak National Aeronautics and Space Administration Goddard Space Flight Center Greenbelt, Maryland Introduction The National Oceanic and Atmospheric Administration (NOAA) Environmental Technology Laboratory (ETL) in collaboration with the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) and the National Center for Atmospheric Research (NCAR) is developing a small Differential Absorption Lidar (DIAL) system to continuously profile lower tropospheric water vapor. This lidar will be ground-based and will measure water vapor to an altitude of several kilometers with ~100-m resolution and ~15-minute averaging times. Need for an Inexpensive Water Vapor Profiler In recent years, continuous measurements of wind and temperature profiles have been made with automated ground-based remote sensors, such as wind profilers with radio-acoustic sounding systems (RASSs). The information from these new instruments has improved the understanding and modeling of both synoptic and mesoscale atmospheric phenomena. To date, however, an equivalent capability for continuous and widespread measurement of water vapor profiles has not been available. Water vapor profiles are important for understanding and predicting moisture convection and horizontal transport. For example, Crook (1996) showed that the strength of convection is sensitive to the differ- ence between the surface and boundary layer water vapor concentrations. Accurate partitioning of water vapor, along with associated temperature profiles, is necessary to characterize the atmospheric radiation balance. Understanding the transport of water vapor is also important for predicting cloud formation.