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Rainfall forecasts with a very high resolution version of HIRLAM

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At KNMI a numerical weather model is daily run and the gridpoint distance is about 55 kilometer.This model HIRLAM calculates the weather for Europe two days ahead. With such a model you can't produce very detailed forecasts, but when the resolution is increased, it becomes possible.

At KNMI research is ongoing in the development of high resolution models. The research is embedded in the international HIRLAM project. KNMI is co-operating with 9 countries to develop and improve a numerical weather prediction model. One of the partners is the Irish Meteorological Institute (Met Éireann) and Met Éireann was participating in an EU research project entitled Telflood . Telflood has as objective the investigation of methods of forecasting flash floods on steep catchments upstream from major urban areas.

Accurate forecasting of precipitation is essential for successful flood warnings. Traditional flood warning systems are inadequate because of the fast response time of steep catchments. The proposed system consists of a High Resolution Limited Area Model ( HIRLAM ) together with a hydrological catchment model and a hydraulic channel network model. All the models have existed separately but their refinement and integration into a flood forecasting system for steep mountainous catchments needed to be undertaken.

At Met Éireann experiments were conducted with nested versions of the HIRLAM model with grid resolutions varying between 55 and 5 km. The nested version had to be adapted slightly to ensure stable integration's. Several experiments were carried out and it became clear that a model with a grid size of approximately 10 km would give the best results. Also attention was paid to possible sources of noise in a high resolution model. Unwanted small scale features caused by lateral boundary conditions and the toward scale orography (mountains) were eliminated by a filtering procedure.The results of a case study on Hurricane Charley (25/26 August, 1986) are given in the following example. Fig. 1 is a 27 hour HIRLAM forecast, resolution is 55 km, showing the 24 hour rainfall to 0900 UTC on August 26.

coarse grid fine grid

The broad features with maximum rainfall in the east and less rainfall in the rest of Ireland are represented, but the rainfall amount is underestimated. The 10 km model in Fig. 2 gives a maximum rainfall of 184 mm, which is located in the mountains at the east coast of Ireland (Wicklow mountains), and this approaches the 24 hour maximum of 190 mm as measured by a climatological station in that area.

For the Telflood project the rainfall sum for the catchment of the Dodder river is important. The Dodder originates in the Wicklow Mountains and flows towards Dublin. The size of the catchment is 115 square kilometres which is rather small. When we average 9 climatological stations in the catchment of the Dodder the observations give 115 mm of rain. The predicted amounts averaged over the catchment area of the 55 and 10 km models give respectively 60 and 98 mm.

The study has shown that with increased model resolution, mountains are described in greater detail and, if the rainfall is orographically forced, the predicted rainfall is better positioned and of the right magnitude.At present a 10 kilometre version of HIRLAM is run daily for research purposes.