Why is regional climate modelling needed

Regional climate modelling is needed firstly, because regional climate is determined by the interaction of large-scale processes, e.g. travelling cyclones, and regional scale processes. Large scale circulation determines the statistics of weather events that characterize the climate of a certain region. But also regional and local scale forcings and regional special circulations, like valley winds, modulate the regional climate change signal, which could even feed back into the large scale.

Secondly, in order to simulate climate change at the regional scale it is necessary to simulate processes at a wide range of spatial and temporal scales. The more so if strong orographic features and land use differences exist.

However, it must be stated here that systematic errors in the large scale forcing fields cannot be corrected. Hence global model biases remain at regional scales. In addition, the parameterizations of unresolved physical processes should be adapted to the different scales and feedbacks into the global scale should be taken into account. At present only first attempts to perform both tasks exist.

Figure 3.10: Average winter and summer temperature change at the end of the 21st century (2071-2100) in Central Europe, for the emission scenario A1B (MPI, 2006b)

The following examples are taken from a new regional modelling attempt for the IPCC scenarios at the Max Planck Institute for Meteorology in Hamburg where double nesting down to horizontal scales of 10 km has been performed for several scenarios using the global model results as lateral forcing. As

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Cw- A10: J/2I"0 i- ill Chi- A1J j(j7t/jl00 I- in ' 9 * ' /' 9 5 C

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Figure 3.11 Average winter and summer precipitation change at the end of the 21st century (2071-2100) in Central Europe for the emission scenario A1B

clearly visible in figure 3.10, both central European summer and winter become much warmer at the end of the 21st century, with a strong tendency for strongest warming in the south. When looking at the precipitation change (figure 3.11) for the same period in scenario A1B winter time wetness increases as well as summer dryness. In addition, the changes are more pronounced in the hilly terrain than in the flat lands. It is clear from many more results (not shown here) for days with snow or frost, highest precipitation events, etc. that many new weather extremes would result and known extremes in precipitation become more frequent. Our security related infrastructure becomes certainly less adapted.

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