Raman Water Vapour Retrieval Technique and Lidar Design

Scattering in the atmosphere includes a large variety of processes. Each process is the result of a specific interaction between the incident radiation and the molecule or particle. In the case of Raman scattering, the wavelength of the scattered light is shifted by an amount that depends on the energy change between its vibrational and/or rotational states. In a water vapour Raman lidar, two receiving channels are set up, one receiving the lidar signals at a wavelength Raman-shifted by water vapour, and one at a wavelength Raman-shifted by a reference molecule having a well known mixing ratio throughout the altitude range considered. For altitudes below 80 km, the well-mixed gas typically used is Nitrogen. After several corrections typical of lidar-signal processing, the ratio of the corrected signals collected by the water vapour channel to that collected by the nitrogen channel is proportional to water vapour mixing ratio. Though an absolute calibration of the entire lidar system is theoretically possible, the signal ratio is usually scaled to a concurrent water vapour measurement (for example from radiosonde, microwave, or GPS) in order to deduce water vapour mixing ratio. Current water vapour lidar systems typically measure water vapour from the ground to an altitude of 12-16 km. The goal within the NDACC network is to provide calibration stability to 10% or better in order for the technique to detect long-term variability and changes.

The inclusion of water vapour Raman lidars in NDACC is very recent. This lidar technique was included in NDACC in order to measure the long-term changes in water vapour in the upper troposphere and lower stratosphere (10-20 km). Due to the extreme dryness of this region of the atmosphere, the lidars operate at their detection capability limits. There are currently ten NDACC water vapour Raman lidar instruments. Seven systems are at fixed locations, three systems are mobile. The seven fixed systems are located at Observatoire de Haute-Provence (France), Table Mountain Facility (California), Mauna Loa Observatory (Hawaii), Payerne (Switzerland), London (Canada) and Rome and Potenza (both in Italy). All three mobile systems are based at the NASA Goddard Space Flight Center in Greenbelt, Maryland (USA).

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