1. Proposed THORPEX/HEPEX Hydrologic Ensemble Project (THEPS) Presentation for 3 rd THORPEX Science Symposium September 14-18, 2009 Prepared by John Schaake, Jutta Thielen, Florian Pappenberger and Hannah Cloke Presented by John Schaake

2. Motivation HEPEX is an international R&D program aimed at developing reliable hydrologic ensemble prediction systems mainly for flood forecasting and water management applications on the short-, medium- and long term. Weather forecasts are critical to the success of HEPEX HEPEX is an ideal partner for THORPEX because it can help demonstrate the value of THORPEX and provide important feedback HEPEX can help WWRP and WCRP to develop a seamless approach to weather, water and climate prediction THORPEX and HEPEX have a natural interface in the land surface thus can integrate at a relatively low cost

3. Proposed Goals for a THORPEX/HEPEX Hydrologic Ensemble Prediction System (THEPS) Assess how TIGGE data sets and THORPEX science plan can meet requirements of the HEPEX community Provide feedback to the THORPEX community on the information content of TIGGE data sets Help the HEPEX community to use TIGGE data sets and THORPEX research results within Hydrological Ensemble Prediction Systems

4. Some Hydrological Perspectives Hydrological scales range from a few sq. km. to continental – Hydrology integrates the effects of many surface variables (e.g. precipitation, temperature, evaporation, radiation, etc.) Hydrology acts as a low-pass non-linear filter of atmospheric forcing Hydrology can help assess predictive skill and potential predictability of atmospheric forecasts over a wide range of spatial and temporal scales

5. Science Issues How can the hydrological user community take advantage of ensemble forecasts? What are the requirements for meteorological ensemble forecasts to support HEPEX? What is the trade-off between NWP model resolution and the number of ensemble members? How well do NWP ensemble members account for the total true uncertainty? Which models seem to do best? Why? What is the relative importance of small vs large scale phenomena in influencing the total uncertainty? What downscaling methods (statistical or dynamical) are available and which ones are the most suitable ones for hydrological applications? How important is the feedback of hydrological processes for the atmospheric circulation? Identify high-impact weather forecasts and assess the impact of improved forecast systems and develop new user-specific weather products

6. Avenues for THORPEX-HEPEX Cooperation Use TIGGE data sets to evaluate their value for hydro forecasts, and compare against reforecast-based techniques. Use THORPEX-HEPEX interaction as a vehicle for the dynamical- statistical downscaling comparisons that we know need to be done. Use the existing HEPEX test bed data in these comparisons. Validation / verification tools can be shared between the communities, leading to greater trustworthiness of results and comparisons; expand hydro tools to be able to do grid or weather obs based verification, or share weather tools. TIGGE data format (grib; netCDF) not easily usable in hydro community, also because of file size; can folks who pioneer their use feed back their algorithms to the TIGGE archive centers to make it easier for others to use?

7. Summary of 1 st HEPEX Workshop on Post- Processing and Downscaling Atmospheric Forecasts for Hydrological Applications Hosted by Meteo-France and SCHAPI at Toulouse, June 15-18, 2009 100+ participants from 20+ countries Recommendations for HEPEX to develop strong collaborations with WWRP/THORPEX and WCRP/GEWEX Proposal for THEPS was reviewed and endorsed Specific action items proposed by each of 3 working groups: –Short Range (0 – 2 days) –Medium Range (3 days – 2 weeks) –Long Range (beyond 2 weeks)

10. Proposed Next Steps Establish Joint Steering Group Review THEPS document to reflect THORPEX/TIGGE perspectives Identify Participants Create initial Pilot Projects Establish initial shared Data Sets Plan Workshop to review results from initial Pilot Projects and develop future plans

11. Thank You (for more details about HEPEX, please visit http://www.hepex.org)