Airborne all-solid-state DIAL for water vapour measurements in the tropopause region: system description and assessment of accuracy
A novel airborne all-solid-state DIAL system for water vapour measurements in the upper troposphere and lower stratosphere has been developed at DLR. Its transmitter is based on an injection-seeded KTP optical parametric oscillator (OPO) pumped by the second harmonic of a Q-switched, diode-pumped single-mode Nd:YAG laser at a repetition rate of 100 Hz. The OPO is optimised for operation in the spectral region between 920–950 nm with an average output power of 1.2 W and a spectral purity higher than 99%. The latter is controlled on a pulse-by-pulse basis during in-flight operation. The system is also designed to perform simultaneous polarisation-sensitive backscatter measurements at 532 nm and 1064 nm for aerosol detection. For the first time, daytime measurements of two-dimensional water vapour cross sections with high vertical (500–750 m) and horizontal (6–20 km) resolution were performed in the tropopause region. Very low mixing ratios (10–50 ppmv) and strong gradients were observed inside stratospheric intrusions. The sensitivity (1-σ error) of the DIAL in the centre of the stratospheric intrusions ranges from 3% (0.6 ppmv) in the near field to 12% (5 ppmv) in the far field (∼4 km). First comparisons with in situ measuring instruments show very good agreement.
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