The Water Vapour Phase II (WAVAS II), a SPARC activity, started in 2008 (SPARC Newsletter No. 30 (2008) p. 16: SPARC Water Vapour Initiative, by C. Schiller et al.). The activity includes satellite assessment and in situ comparison components. This international activity encompasses:

1) Providing a quality assessment of upper tropospheric to lower mesospheric satellite records since the early 1990s
2) Providing, as far as possible, absolute validation against ground-truth instruments
3) Assessing inter-instrument biases, depending on altitude, location, and season
4) Assessing the representation of temporal variations on various scales
5) Including data records on isotopologues
6) Providing recommendations for usage of available data records and for future observation systems


The main objective of WAVAS II is to assess and extend our knowledge and understanding of measurements of the vertical distribution of water vapor in the upper troposphere and middle atmosphere (UT/MA), where water has small concentrations, but significant radiative impact. This is a follow-up of the SPARC WAVAS activity, whose report was published in 2000 (SPARC Report No. 2 (2000) Upper Tropospheric and Stratospheric Water Vapour. D. Kley, J.M. Russell III, and C. Philips (eds.). WCRP-113, WMO/TD – No. 1043). Information gained from this activity will improve our ability to estimate long-term changes with associated uncertainties in UT/MA water as well as make recommendations as to what data would be most valuable for model validation and how such data should be used.

Papers will be accepted for this special issue according to the following guidelines, independent if they originate from the WAVAS II activity or other activities.

Guidelines for submissions:

• Papers covering existing UT/MA satellite water vapour measurements;
• Papers discussing comparisons of UT/MA satellite measurements, including discussion of quantities derived from these measurements, such as seasonal cycles, estimates of transport, or estimates of drifts, trends and variability.
• Papers discussing merging of water vapour measurements will be considered, although this topic is not specifically part of the WAVAS-II activity.
• Model papers that incorporate the datasets discussed and the uncertainty estimates resulting from the WAVAS-II activity will also be considered for inclusion.