Philip Lorenz

Evaluation of the precipitation for South-western Germany from high resolution simulations with regional climate models

Precipitation data from long-term high-resolution simulations with two regional climate models (CLM and REMO) are evaluated using a climatology based on observations for south-western Germany. Both models are driven by a present day climate forcing scenario from the global climate model ECHAM5. The climatological evaluation shows a strong seasonal dependence of the model deficiencies. In spring and summer there are relatively small differences between simulation results and observations. But during winter both the regional models and ECHAM5 strongly overestimate the precipitation. The frequency distributions of the model results agree well with observed data. An overestimation of the precipitation at the upwind sides of mountainous areas occurs in the regional simulations. We found that the coupling of the regional models to the driving model is stronger in winter than in summer. Therefore, in winter the large scale model have a larger impact on the performance of the regional simulations. During summer the benefit of regional climate simulations is higher.

Will dry events occur more often in Hungary in the future

Dry years and dry summers in Hungary have been analyzed using the regional climate model REMO for the time periods 1961–2000 and 2001–2100. Dry periods were determined and classified by intensity, considering modeled and observed precipitation and temperature data. The intensity of dry events was defined according to the negative precipitation deviation and positive temperature deviation from the climate period 1961–90. The proportion of dry years and dry summers is equivalent in the model and observations in the past. On average, the intensity of dry years simulated by the regional climate model REMO is the same as observed, whereas dry summers have more extreme conditions in the model. Based on the results of three IPCC scenario simulations (B1, A1B, A2), the probability of dry events will be higher in the second half of the 21st century. In the scenarios A1B and A2 a dry summer may happen every second year and the consecutive dry periods will last longer. For 2051–2100 the intensity of dry events increases significantly in all scenarios compared to the control period. From the analyzed scenarios B1 has the lowest future greenhouse gas emission rates, so that the smallest changes are also projected for the second half of the 21st century.

Determination of precipitation return values in complex terrain and their evaluation.

To determine return values at various return periods for extreme daily precipitation events over complex orography, an appropriate threshold value and distribution function are required. The return values are calculated using the peak-over-threshold approach in which only a reduced sample of precipitation events exceeding a predefined threshold is analyzed. To fit the distribution function to the sample, the L-moment method is used. It is found that the deviation between the fitted return values and the plotting positions of the ranked precipitation events is smaller for the kappa distribution than for the generalized Pareto distribution. As a second focus, the ability of regional climate models to realistically simulate extreme daily precipitation events is assessed. For this purpose the return values are derived using precipitation events exceeding the 90th percentile of the precipitation time series and a fit of a kappa distribution. The results of climate simulations with two different regional climate models are analyzed for the 30-yr period 1971–2000: the so-called consortium runs performed with the climate version of the Lokal Modell (referred to as the CLM-CR) at 18-km resolution and the Regional Model (REMO)–Umweltbundesamt (UBA) simulations at 10-km resolution. It was found that generally the return values are overestimated by both models. Averaged across the region the overestimation is higher for REMO–UBA compared to CLM-CR.