Regional Downscaling of Storm Surge Episodes

Regional climate model COSMO-CLM developed by DWD used for dynamical downscaling of observed storm events and potential storm surge events in future GHG scenarios (see www.clm-community.eu for a detailed model description). COSMO-CLM driven by reanalysis data: The model was used for dynamical downscaling of wind for the most extreme observed historic storm surge events. As this study concentrated on the atmospheric influences we choosed storm surges with the highest wind surge values. The model data was used to compare wind speed estimates over the German Bight region with the storm surge atlas from the station Cuxhaven (provided by G. Gönnert, LSBG Hamburg). COSMO-CLM driven by ECHAM5/MPI-OM model data: To estimate possible future changes of storm surge risk due to increased GHG concentrations, the model was used to downscale events (detected in GCM model data) with high effective wind speeds (wind projected on the direction 295°) over the German Bight region. A comparison between the most intense events detected in GCM data under recent climate conditions (20C) and under possible future climate conditions (A1B) was used to estimate changes in storm surge intensity and frequency under future climate conditions. For most potential storm surge events (driven by GCM data) and for observed storm surge events (driven by ERA-Interim reanalysis data) each event is simulated by a five member mini ensemble. These members differ due to their model domain, which is shifted by 8 grid boxes to north, south, east and west. The results of this ensemble can be used to estimate the uncertainty of the storm surge event.