1. Precipitation over Iceland simulated in a future climate scenario at various horizontal resolutions Hálfdán Ágústsson, Ólafur Rögnvaldsson, Haraldur Ólafsson and Einar Magnús Einarsson Institute for Meteorological Research, University of Iceland, University of Bergen and Icelandic Meteorological Office

2. Downscaling to high horizontal resolution Dynamical downscaling is done with the state of the art WRF numerical atmospheric model, which is widely used for research and operational forecasting. Three datasets are downscaled to 27, 9 and 3 km resolution:  Actual climate of 1961-1990 based atmospheric reanalysis from the ECMWF.  Control climate of 1961-1990 based on atmospheric climate simulations with the Arpege model.  Future climate of 2021-2050 based on atmospheric climate projections with the Arpege model. The Arpege model is run by the Bergen group on a T159c3 irregular grid. The scenario chosen is the SRES A1B. Here we focus on precipitation and the relevant physics are parameterized using the WSM3-scheme.

3. Downscaling to high horizontal resolution Approx. 125 km wide gridcells in global model 9 km 3 km

4. Why is high horizontal resolution important? 27 km 3 km9 km

5. Mean annual precipitation in downscaled climate at 9 km resolution Actual climate Future climateControl climate

6. Mean annual precipitation in control climate 3 km resolution

7. Mean annual precipitation in future climate

8. Histogram of 24 hour precipitation at 9 km Fewer large events in future climate Red - Future Black - Control

9. Histogram of 24 hour precipitation at 3 km Red - Future Black - Control Fewer medium size events in future climate No change in large events!

10. Spatial distribution of 24 hour precipitation at 3 km Control climate 50 mm bins in histogram

11. Future climate Increase in Southeast-Iceland, decrease elsewhere Decrease everywhere Spatial distribution of 24 hour precipitation at 3 km

12. Control climate Spatial distribution of 24 hour precipitation at 9 km Future climate Decrease throughout Iceland

13. Control climate Temporal distribution of 24 hour precipitation at 3 km

14. Future climate Shift in precipitation from winter and spring to summer and autumn Temporal distribution of 24 hour precipitation at 3 km

15. Control climate Temporal distribution of 24 hour precipitation at 9 km Future climate Same as at 3 km. Shift from winter and spring to summer and autumn

16. Different regional models give different results Nawri and Björnsson, 2010

17. Conclusions and final remarks Resolution is important!!! A major shift in precipitation regime can occur with improved resolution of simulations of local climate change. Time of largest precipitation events may shift from winter to autumn. Extreme precipitation events may become more frequent in Southeast-Iceland but less frequent in other parts of Iceland.

18. Control climate Temporal distribution of 24 hour precipitation at 9 km Future climate Fewer windstorms Actual climate