ISSI Working Group on
Atmospheric Water Vapour

The ISSI working group on atmospheric water vapour

Atmospheric water plays a key role in climate. Water vapour is the most important greenhouse gas, and its condensed forms (liquid and ice) exert a profound influence on both incoming solar and outgoing infrared radiation. Climate models find that predictions of climate change are very sensitive to water vapour and cloud feedback. A better understanding of the distribution of atmospheric humidity, and the processes that control it above the boundary layer, is therefore of primary importance in climate research.

Unfortunately, accurate, height-resolved global-scale measurements of atmospheric humidity are difficult to obtain. In the troposphere, the water vapour distribution is extremely variable on almost all scales, and the change in concentration of five orders of magnitude from the ground to the stratosphere means there is no standard instrument that will measure everywhere. This has led to different measuring techniques, all with strengths and weaknesses. Routine measuring instruments, such as radiosondes, suffer from a variety of systematic errors but they do provide a good coverage. Remote sensing can offer both accuracy and coverage, but at the price of reduced resolution which in the upper troposphere is a serious drawback.

In the frame of an ISSI working group the different measurement techniques shall be assessed with a particular emphasis on accuracy, calibration issues, retrieval concepts and the potential of combining and merging information from several techniques and platforms.