Numerical weather prediction and HIRLAM project

National meteorological services are required to provide short and medium range weather forecasts, warnings and alerts for their territory. Modern-day weather forecasting is highly relying on numerical weather prediction (NWP). The medium range forecasts are produced with global models using the whole of Earth as a domain having a horizontal resolution of about 20-40 km. For short range applications it is more efficient to run the NWP systems for only a small part of the globe using a Limited Area Model (LAM) operating on smaller scale grid and using boundary conditions from global models. [Nie06] In Europe there are a number of consortia that collaborate on Limited Area Modelling. The High Resolution Limited Area Modelling project HIRLAM was the first group to be established in the Nordic Countries in 1985. It has expanded to include other countries in Western and Southern Europe and it has been a successful collaboration programme. The developments of earlier projects are carried over and further developed in new projects. In January 2006 the programme advanced to a new phase called HIRLAM-A, which inherited all the knowledge from previous HIRLAM-6 project and its predecessors. The main objectives, targets and priorities of each project are listed in a memorandum of understanding, which is available at HIRLAM webpage www.hirlam.org. In addition to developing the HIRLAM model, the programme targets to provide and maintain NWP expertise among the HIRLAM members, enhance collaboration both between scientists in the HIRLAM institutes and with co-operating consortia and with other NWP groups. The HIRLAM forecast system is at the moment used in routine weather forecasting by several European NMS including Finnish FMI. [HIR06] A NWP-system comprises two main components, which are the atmospheric model and the data assimilation system. The atmospheric model is further divided into two main parts: the model dynamics and the physical parameterization [Nie06]. In this paper, the basics of NWP modelling are presented via the High Resolution Limited Area Modelling project HIRLAM.