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

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.

Downscaling to high horizontal resolution

Approx. 125 km wide gridcells in global model

9 km3 km

Why is high horizontal resolution important?

27 km

3 km9 km

Mean annual precipitation in

downscaled climate at 9 km

resolution

Actual climate

Future climateControl climate

Mean annual precipitation in control climate

3 km resolution

3 km resolution

Mean annual precipitation in future climate

Histogram of 24 hour precipitation at 9 km

Fewer large events in future climate

Red - FutureBlack - Control

Histogram of 24 hour precipitation at 3 km

Red - FutureBlack - Control

Fewer medium size events in future climate

No change in large events!

Spatial distribution of 24 hour precipitation at 3 km

Control climate

50 mm bins in histogram

Future climate

Increase in Southeast-Iceland,

decrease elsewhere

Decrease everywhere

Spatial distribution of 24 hour precipitation at 3 km

Control climate

Spatial distribution of 24 hour precipitation at 9 km

Future climate

Decrease throughout

Iceland

Control climate

Temporal distribution of 24 hour precipitation at 3 km

Future climate

Shift in precipitation from winter and spring to summer

and autumn

Temporal distribution of 24 hour precipitation at 3 km

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

Different regional models give different results

Nawri and Björnsson, 2010

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.

Control climate

Temporal distribution of 24 hour precipitation

at 9 km

Future climateFewer

windstorms

Actual climate