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Abstract
At Adelaide we are developing a low-cost differential absorption lidar (DIAL) for profiling water vapour in the lower part of the atmosphere. Water vapour concentrations in the lower atmosphere are highly variable; they can change significantly on timescales of 15 minutes, and on horizontal scales of tens of kilometres. Low-cost eye-safe DIAL than can be readily replicated would help to improve the spatial and temporal resolution, by supplementing existing hygrometry, or replace techniques with high operating costs. Such an instrument is very desirable for several reasons, including quantitative precipitation forecasting, fog prediction and assessment of bushfire danger.
Our system uses diode lasers in the 825nm wavelength region, and differs from previously constructed water DIAL systems in having two master lasers with active stabilisation of both laser wavelengths. The transmitted energy of 500 nJ, in 1 µs pulses, is easily rendered eyesafe.
Measurements of the backscatter coefficient of the urban aerosols that predominate over Adelaide have been used in a sensitivity analysis. With a pulse energy of 500 nJ, random uncertainties in the measured water concentration at 4 km height will be of order 10%, assuming favourable aerosol and water profiles, and an appropriately chosen water absorption line. Our initial DIAL measurements, with a water line that is too strong, agree reasonably well with the sensitivity analysis in giving a maximum range of about 800 m. We will also report on our efforts to improve the absolute accuracy of the retrieved water mixing ratios and extend the range.
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