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Abstract

Regarding weather and climate, water vapor is one of the most important atmospheric constituents. A precise knowledge of its highly variable distribution is therefore crucial for many applications such as climate monitoring or weather prediction. Water vapor lidars are the only instruments that can deliver continuous
measurements of humidity profiles of high spatial and temporal resolution. As opposed to the Raman method, differential absorption lidar (DIAL) offers the advantages of good daytime performance and self-calibration.

In the course of this work, measurements were collected with an existing alexandrite laser based DIAL system during several field experiments. Among them was the Nauru99 campaign where a water vapor DIAL system was operated on board a ship for the first time. The data from the various campaigns yielded many interesting results, for example the confirmation of the development of an internal thermal boundary layer over the island of Nauru.

It became apparent however, that the quality of the measured data was not satisfactory. During the
postprocessing of the data, many sources of systematic errors could be identified and partly eliminated. Such sources were errors in the calculation of the water vapor absorption cross section, errors in the determination of the signal baseline, and errors caused by an insufficient spectral quality of the emitted laser pulses.

Because of the quality problems, the high level of maintenance, and the large space and energy requirements associated with the old system, a new laser system was constructed. The best concept was found to be an injection seeded, gain-switched Ti:Sa ring laser. This new laser operates in the 820 nm wavelength region at a repetition rate of 50 Hz. Special features are the newly developed active stabilization scheme, the continuous monitoring of crucial parameters on a single shot basis, and the fact that no optical elements inside the laser cavity are needed, except the laser crystal itself. Achieved performance parameters are: pulse energy of 15 mJ, spectral purity of 99.97 %, and shot-to-shot energy fluctuations of 1.6 %.

A preliminary DIAL system was constructed based on the new laser system and tested extensively during a field experiment in May/June 2003 at the German Weather Service's Lindenberg Observatory. The laser performance and the quality of the measured data were very satisfactory. Uninterrupted operation of up to 11.5 h could be achieved.