Water Vapour Assessment

1. Water Vapour Assessment Jörg Schulz, EUMETSAT Christian Kummerow, CSU Bojan Bojkov, ESA

2. Water Vapor - GRP basis for discussion • T, q profiles are needed to study 3D atmosphere and to use it as input for ISCCP, SRB and Sfc. Fluxes, but no single source was available. • Major obstacles were: • Old GEWEX NVAP data set was based on a concept that merged several sources of satellite and in situ data without caring too much on instrument characterization or inter-calibration of satellites. • Operationally derived profile data sets, as used in ISSCP, were derived in real time and were not well inter-calibrated. • Early NWP based reanalysis data sets suffered from inadequate data assimilation techniques that prohibited the full use of satellite observations, e.g., most of the observations are only used over oceans and under clear sky conditions. • Recent new reanalysis have much improved data assimilation techniques but exhibit artificial trends • Old sounders need background information in the retrieval to be competitive with models. • New sounders (AIRS, IASI) appear to outperform NWP models. • Many comparisons between NWP reanalysis, satellite products and radiosonde and GPS data exist in the literature but an organized product assessment has not been done. • Instrument characterization and inter-calibration is critical but difficult.

3. GRP Outlook • GSICS is tackling the calibration and inter-calibration of current satellite data streams • Data stewardship at NOAA and EUMETSAT have gained traction for calibration and inter-calibration of historical data. • HIRS data is being reprocessed with uniform calibration • GEO water vapour channels will also be reprocessed and homogenised using HIRS • NVAP activity is again active under NASA MEASURES program. • Reanalyses getting better but trends might still be artificial • Short term: Evaluate new HIRS product for use in combined data set • Longer term: Consider restarting a water vapor assessment. This should consider TPW, UTH and water vapor profiles separately over ocean and land.

4. A First Workshop • The GEWEX/ESA DUE GlobVapour workshop on long term water vapour data sets and their quality assessment was hosted by the European Space Agency ESRIN centre in Frascati (Rome), Italy from March 8-10, 2011 • 35 participants from research and space agencies representing data set producers, retrieval developers and experts on ground-based observations • Presentations on total column water vapour, profiles and upper tropospheric humidity data sets and their validation • Two breakout groups one focussing on total column estimates the other dealing with profiles and UTH

5. The Participants

6. GEWEX Needs GEWEX needs to consider candidates for homogeneous, long time series (1980 – present) products including TPW, Profiles and UTH GEWEX needs to consider shorter term (2000 – present) best possible product for process studies using co-variance among W-E products GEWEX needs to develop an assessment strategy that evaluates what we know, what we don’t know, and how we move forward. GEWEX products are not fixed, but rather get reprocessed in a cycle of approximately 5 years. Assessments need to work on those time frames also.

7. Which Products Shall be Assessed? Workshop consensus that total column water vapour, as well as water vapour profiles and their related temperature profiles should be assessed. Stand-alone temperature profiles or deep layer temperature data sets such as derived from MSU/AMSU will not be looked at. SST/LST and 2m temperature or humidity are also not included unless these are integral parts of the Vapour profile. Important is to clearly describe the data set objectives in the introduction to the assessment in order to place each product in the proper perspective relative to climate needs.

8. Preliminary list of identified products and producers The assessment may include products from passive individuals or groups if the product is generally used, fully published, and freely available without restrictions. This, in particular, applies to existing data records coming from various NWP model-based re-analysis activities.

9. Validation Data First sketch from the workshop Products shall be compared among themselves but also to independent reference data (hard to find). Workshop Participants agreed to built up a validation data base consisting of ground-based remote sensing data, quality controlled radiosondes, as well as BSRN and CERES radiation flux data.

10. Radiosonde, GPS distribution GEWEX/ESA DUE GlobVapour Workshop March 8-10, 2011, Frascati, Italy Courtesy of J. Bytheway, CSU

11. Radio Occultation to identify bias error in RS Ho et al., 2010 Remote Sensing Courtesy of B. Ho, UCAR

12. (as of 2009) Since 2009:Six Water Vapor Lidars provisionally in NDACC “Provisionally” because (more) validation is required Courtesy of T. Leblanc, JPL

13. Comparisons at what level? Start with Level 1 (FCDRs) datasets used to produce water vapour products. While this is not generally viewed as part of the assessment itself, the L1 stability and usefulness for climate data records should be documented via the data providers and verified as best as possible. Level 2 data should be compared to the validation data described above. These comparisons should allow for the characterization of product uncertainties as well as a product’s strengths and weaknesses.

14. Comparisons at what level? Level 3, monthly global and regional comparisons were thought most useful to elucidate differences among products. Level three comparisons should focus on parameters such as pdfs in addition to means. Radiation reacts to the pdf of water vapour rather than the mean. Level 3 validation data may be added to the data base if deemed useful at these space/time scales.

15. Two Phases Assessment shall be stratified in two phases Phase 1 begins with three years of recent data – where more satellite data sets and more validation data sets are available. Phase 2 then looks to longer data sets with less validation data sets. While the actual assessment experiment design was left to an assessment organizing committee that is in the formation stages, the timetable needed to have an impact on the GEWEX Radiation Panel objectives is to finish phase 1 in 2-3 years. This needs some more thinking as it might be beneficial to know the level-3 differences of the long term data in the beginning.

16. Who and Resources • Identified two leaders: • Lei Shi (NOAA/NCDC) leading UTH and profile comparisons • Marc Schröder (CMSAF, DWD) leading total column and organises technical work such as data bases • Both have institutional support to perform the work. • Within Europe the CMSAF has a budget to put a ¾ scientist for the next two years on this starting in 2012. • ESA has also access to resources and is willing to support the assessment; ways how these resources will be used best need to be explored. • Activities on level-1 data need to be supported by space agencies directly.

17. Next steps • Appoint the two leaders • Engage the participants and develop a work plan for the first two years that makes efficient use of the available resources • Start with a description of the data products that will be assessed • Develop common metric to be applied in comparisons • Circulate the plan with the GRP for comments • Plan for a second workshop in spring/summer 2012 to make agreements on the work plan and keep momentum