Climate Change

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Off considerable relevance for the performance of sewer systems are extreme, locally heterogeneous rainfall events. Special challenges and difficulties result from the alpine environment with strong orographic effects.
In the project an ensemble of new RCM runs with a grid size of 10kmx10km have been used as basis for an empirical statistical downscaling procedure.


Sub-daily extreme precipitation mainly occurs in the summer season as a consequence of strong convection often related to thunderstorms. For urban hydrology heavy rain events with high precipitation intensities and short durations up to one hour are specifically relevant. The atmospheric processes which are involved in the formation of these rain events have too small spatial and temporal scales to be represented by dynamical climate models. Therefore, in order to assess the effects of climate change on sub-daily heavy precipitation regional climate projections cannot directly be used. However, theoretical considerations based on the Clausius-Clapeyron Equation indicate an increase of convective heavy precipitation with rising temperatures. In this project we use a statistical downscaling method to transfer data from regional climate projections on smaller scales and make it usable for analysis of subdaily heavy precipitation. The statistical downscaling method is based on statistical relations between different scales which are derived from observations.

Downscaling procedure

Regional climate projections have been produced for Austria within the ACRP project reclip:century by AIT, Wegener Center, Boku and ZAMG. The regional climate simulations are calculated on a 10 x 10 km2 grid with the model CLM driven by two global climate models (GCMs): Echam5 and HadCM3. The data consists of two 20 th century runs and four scenario runs based on the greenhouse-gas emission scenarios: A1B, A2 and B1. A comparison of the reclip:century projections to a greater ensemble of RCMs from the ENSEMBLES project shows that the reclip:century temperature trends are in the mean range of the ensemble, partly covering the ensemble spread. In this project we use the reference period 1971-2000 and the future period 2021-2050 from the reclip:century runs to evaluate climatic trends in the study region. The precipitation regime in Innsbruck and Page 6 / 39ACRP – Calls for Proposals the surrounding area is substantially determined by the Alpine topography and the flow direction. Yearly precipitation sums in the close vicinity of Innsbruck (Patscherkofel and Birkkarspitze) are significantly higher than yearly precipitation sums in the city area.


Three of the four model projections indicate a significant future increase of subdaily extreme precipitation which is relevant to consider for urban hydrology.

Relative changes of extreme precipitation sums from 1971-2000 to 2021-2050. The statistical return period is 5 years. Bold letters denote statistically significant changes.