Continuous Water Vapor Profiling with a Diode-Laser-Based Differential Absorption Lidar (DIAL)

The atmospheric sciences research and forecasting communities have a clearly-stated need to obtain improved measurements of water vapor. Accurate, high-resolution, continuous measurements of water vapor are a key observational gap for the mesoscale weather and climate process studies communities. Local soundings of high vertical resolution atmospheric state parameters via radiosondes measurements combined with global coverage of low vertical resolution state parameters via satellite-based measurements form the backbone of observations used for weather forecasting. But the limited spatial and temporal resolution of the current technology prohibits observations of key atmospheric features required for accurate forecasting of mesoscale high-impact weather events like thunderstorms. Passive remote sensors such as infrared and microwave radiometers are useful at low ranges close to the surface but in general provide low vertical resolutions and GPS receivers provide only integrated -- or column measurements -- of the total precipitable water vapor. Active remote sensors, such as Raman lidars provide the high spatial and temporal measurements of water vapor that are needed by the observational and modeling communities. They are, however, generally expensive instruments to build, operate and maintain. Montana State University (MSU) has pioneered an alternative low-cost active remote sensing capability which has the potential to help fill the observational gap for range resolved measurements of atmospheric water vapor. This technology employs the well-known differential absorption lidar (DIAL) technique and uses diode-laser-based technologies for the transmitter which significantly reduces the initial and operational costs.

Since June of 2011, MSU and the National Center for Atmospheric Research (NCAR) have worked together to expand and evaluate the capability of this new technique. In 2012 the MSU prototype water vapor DIAL was modified to allow for unattended operations with a completely eye-safe beam. The modified instrument was field tested over a wide range of atmospheric conditions alongside other instrumentation to evaluate its performance. The evaluation indicated that the technology was well-suited for autonomous, long-term measurement of water vapor over a wide range of concentrations and atmospheric conditions. However, significant engineering modifications were required to make the instrument capabilities useful for the atmospheric science community. The revised design, now being constructed and tested, will be discussed. It should allow measurements closer to ground level, improve performance in the presence of clouds and during daytime, and improve the system's stability and reliability. Its completion and the necessary subsequent testing and intercomparisons would ideally create a field prototype that would be available to the broader National Science Foundation (NSF) user community. Furthermore, the prototype instrument would have the potential to form the basis of a ground-based network of eye-safe autonomous instruments.