Water vapour column abundance retrieval using multispectral radiometer measurements in Sydney, Australia

Water vapour plays a crucial role in atmospheric processes, from global climate to micrometeorology. It is the most variable major constituent of the atmosphere and the largest contributor to the greenhouse effect. It affects the global climate system directly, by absorbing and radiating energy from the sun, and indirectly, by its effect on cloud formation, aerosol growth, and the chemistry of the lower atmosphere. It plays a critical role in many of the chemical reactions that occur in the atmosphere. For many years, radiosondes were the main means of measuring the water vapour column throughout the atmosphere, however they are labour intensive and, because of the high costs, are only launched twice a day thus seriously under-sampling temporal variability. Sun photometers or radiometers, which are usually used for aerosol measurements, offer an alternative to radiosondes for retrieving column abundances of water vapour, when an appropriate filter is used, such as the 0.940 μm band. They are easily operated and provide continuous measurements during the daytime, with a minimum cost. The disadvantages of these instruments are that they can provide data only under clear sky conditions, and they do not recover vertical profile information. In recent times, radiometers increasingly have been used to measure atmosAust. Met. Mag. 51 (2002) 165-172