Content - Water Vapour
WAVAS II - Water Vapour Phase II
The main aims of the Water Vapour Assessment 2 (WAVAS-2) are to update the first SPARC Water Vapour Assessment (WAVAS-1) by:
- Including recent satellite, field, and laboratory measurements
- Consolidating evaluation of current understanding concerning processes affecting stratospheric H2O and its evolution.
In the year 2000, SPARC published its Assessment of Upper Tropospheric and Lower Stratospheric (UTLS) Water Vapour (SPARC Report No. 2). The key topic addressed in this report was the analysis and assessment of long-term changes of UTLS water vapour, with an emphasis on the observed increase of stratospheric water vapour. The report had a strong focus describing and comparing relevant datasets using in situ hygrometers and remote sensing instruments from laboratories all over the world in order to create a suitable dataset, including historical data back to the 1940s. Data presented in the report are available via the SPARC Data Centre. The distribution and variability of UTLS water vapour, relevant processes, and impact of increased water vapour on radiation, dynamics, and chemistry were discussed. However, a quantitative explanation of the analysed changes was not possible in 2000.
Following the recommendations of this report, climatological measurement programmes have continued, new campaigns to investigate UTLS water vapour have been carried out, new satellite observation programmes have been launched, and many model and laboratory studies have been made since 2000. Emerging from the new observations, an additional “puzzling” question became apparent in that unexpected high relative humidities were observed, largely in the cold tropopause region both inside and outside of clouds. Data quality, in particular knowing the absolute accuracy and not simply the relative discrepancies between different sensors, has become a crucial issue if we are to assess these questions. These accuracy issues have lead to the need of cross-validation of established and recently developed hygrometers, both in the field and laboratory.
In light of these developments, it seems timely to update the SPARC water vapour assessment of 2000. In particular, there is a need to summarise the relevant results over the past decade from various field experiments, laboratories and models in a comprehensive report or review publications. The major goal of such an exercise is to assess the value and the accuracy of recent measurements and to provide new recommendations and guidelines for future research on UTS water vapour.
The major topics to be addressed during WAVAS-2 are:
- Data quality: How reliable are in situ and remote sensing field data in terms of accuracy and precision?
- Clear air and in-cloud super-saturation: Can the observations be explained within the framework of our current knowledge or do we need new theoretical concepts and new laboratory investigations, e.g. of ice growth at extreme temperatures?
- Recent observations of UTLS water vapour changes: Are these observations mutually consistent, do we understand them, and what are our abilities for future predictions?
- Impact on atmospheric chemistry and climate: What are the implications of changing UTLS water vapour for radiation, dynamics, chemistry, clouds, and climate?
The SPARC Scientific Steering Group proposed during its annual meeting in September 2007 to initiate a new water vapour initiative (WAVAS-2), which is coordinated by Gabriele Stiller, Thomas Peter, and Karen Rosenlof, with a kick-off workshop being held at the 2008 SPARC General Assembly in Bologna, Italy.