Water vapor profiling using a widely tunable amplified diode laser differential absorption lidar (DIAL)

Abstract

Water vapor is one of he most significant constituents of the atmosphere because of its role in cloud formation, precipitation, and interactions with electromagnetic radiation, especially its absorption of longwave infrared radiation. Some details of the role of water and related feedback mechanisms in the Earth system need to be characterized better if local weather, global climate, and the water cycle are to be understood. Water vapor profiles are currently obtained with several remote sensing techniques, such as microwave radiometers, passive instruments like the Atmospheric Emitted Radiance Interferometer (AERI), and Raman lidar. Each of these instruments has some disadvantage, such as only producing column integrated water vapor amounts or being large, overly customized, and costly, making them difficult to use for deployment in networks or onboard satellites to measure water vapor profiles. This thesis work involved the design, construction, and testing of a highly-tunable Differential Absorption Lidar (DIAL) instrument utilizing an all-semiconductor transmitter.

It was an attempt to take advantage of semiconductor laser technology to obtain range-resolved water vapor profiles with an instrument that is cheaper, smaller, and more robust than existing field instruments. The eventual goal of this project was to demonstrate the feasibility of this DIAL instrument as a candidate for deployment in multi-point networks or satellite arrays to study water vapor flux profiles. This new DIAL instruments transmitter has, for the first time in any known DIAL instrument, a highly-tunable External Cavity Diode Laser (ECDL) as a seed laser source for two cascaded commercial tapered amplifiers. The transmitter has the capability of tuning over a range of ~17nm to selectively probe several available water vapor absorption lines, depending on current environmental conditions. This capability has been called for in other recent DIAL experiments, Tests of the DIAL instrument to prove the validity of its measurements are presented, Initial water vapor profiles, taken in the Bozeman, MT, area, were taken, analyzed, and compared with co-located radiosonde measurements, Future improvements and directions for the next generation of this DIAL instrument are discussed.