[Dutch]

The HIRLAM model at mesoscales: XHIRLAM

n order to investigate to what extent it is possible to improve the spatial detail of weather analyses and forecasts for the Netherlands, the HIRLAM atmospheric model has been tested extensively at high horizontal resolutions, using mesh sizes of 5-11km. This socalled mesoscale version of the HIRLAM model has become known as HIRLAM-11.

In the course of 1999, the HIRLAM-11 model was first set up and tested extensively in the context of the ASWAN project. In this project the quality of the mesoscale analyses and predictions has been studied; furthermore it was determined to what extent high resolution observations from wind profilers can contribute to this quality.

Performance and examples:

It has been proven conclusively that HIRLAM-11 is able to represent all sorts of weather phenomena in a much better and more detailed way than the coarser HIRLAM weather prediction model (which has a mesh width of 22km and covers Europe and the North Atlantic Ocean). Strong gradients at land-sea transitions, small-scale weather systems such as squall lines, and extreme weather events such as storms or heavy precipitation, generally are described more accurately by the HIRLAM-11 model. Because of this, it has been decided to upgrade the operational HIRLAM model in KNMI's automatic production suite to a 11km grid. This is expected to take place in October 2006.

An example of a small-scale weather phenonmenon (a sea breeze circulation) as represented by HIRLAM-11 can be found here. Animations of recent XHIRLAM model fields for various meteorological quantities can be found here.

Possibilities of use

The Dutch water authorities (Rijkswaterstaat) use the weather information from the HIRLAM model for coastal defense and for the control and maintenance of coastal waters and the access ways to the Dutch harbours. Wind and pressure information from HIRLAM are used as input to the hydrodynamical models used operationally by Rijkswaterstaat. At their request, KNMI has studied the consequences of the increased spatial detail of wind and pressure prognoses. More information on this can be found here (Dutch only).

Towards higher resolution

For several important users of HIRLAM weather information, such as Rijkswaterstaat and aviation authorities, a spatial resolution of 10km is still insufficient. A higher level of detail is required in particular for wind prognoses. For this reason, a module has been developed by which HIRLAM-11 can be transformed to grids with a mesh width of approximately 1km, using very detailed topographical information. This so-called downscaling module has been implemented experimentally, and will be made operational in the course of 2006.

In the mean time, much research is spent on the development of a mesoscale version of HIRLAM, at 2-3 km horizontal resolution. At such resolutions, it is necessary to use a non-hydrostatic version of the model. High resolution observations from satellites and weather radars will be used to initialize this model. Also, the physics parametrizations used in the model will be adapted to realistically describe km-scale processes, in particular the development of clouds.New methods are being developed for the objective assessment of the forecast quality of such detailed models. All of this research is done in the context of the HIRLAM-A research programme.

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Jeanette Onvlee