ALBERT

Description: The beginning of the 21st century was marked by a number of severe summer floods in Central Europe associated with extreme precipitation (e.g., Elbe 2002, Oder 2010 and Danube 2013). Extratropical storms, known as Vb-cyclones, cause summer extreme precipitation events over Central Europe and can thus lead to such floodings. Vb-cyclones develop over the Mediterranean Sea, which itself strongly warmed during recent decades. Here we investigate the influence of increased Mediterranean Sea surface temperature (SST) on extreme precipitation events in Central Europe. To this end, we carry out atmosphere model simulations forced by average Mediterranean SSTs during 1970–1999 and 2000–2012. Extreme precipitation events occurring on average every 20 summers in the warmer-SST-simulation (2000–2012) amplify along the Vb-cyclone track compared to those in the colder-SST-simulation (1970–1999), on average by 17% in Central Europe. The largest increase is located southeast of maximum precipitation for both simulated heavy events and historical Vb-events. The responsible physical mechanism is increased evaporation from and enhanced atmospheric moisture content over the Mediterranean Sea. The excess in precipitable water is transported from the Mediterranean Sea to Central Europe causing stronger precipitation extremes over that region. Our findings suggest that Mediterranean Sea surface warming amplifies Central European precipitation extremes.

Description: Much of our knowledge about future changes in precipitation relies on global (GCM) and/or regional climate models (RCM) that have resolutions which are much coarser than typical spatial scales of precipitation, particularly extremes. The major problems with these projections are both climate model biases and the gap between gridbox and point scale. Wong et al. developed a model to jointly bias correct and downscale precipitation at daily scales. This approach, however, relied on pairwise correspondence between predictor and predictand for calibration, and thus, on nudged simulations which are rarely available. Here we present an extension of this approach that separates the downscaling from the bias correction and in principle is applicable to free running GCMs/RCMs. In a first step, we bias correct RCM-simulated precipitation against gridded observations at the same scale using a parametric quantile mapping approach. To correct the whole distribution including extreme tails we apply a mixture distribution of a gamma distribution for the precipitation mass and a generalized Pareto distribution for the extreme tail. In a second step, we bridge the scale gap: we predict local variance employing a vector generalized linear gamma model (VGLM gamma) with the bias corrected time series as predictor. The VGLM gamma model is calibrated between gridded and point scale (station) observations. For evaluation we adopt the perfect predictor experimental setup of VALUE. Precipitation is in most cases improved by (parts of) our method across different European climates. The method generally performs better in summer than in winter and in winter best in the Mediterranean region with a mild winter climate and worst for continental winter climate in mid & eastern Europe or Scandinavia. A strength of this two-step method is that the best combination of bias correction and downscaling methods can be selected. This implies that the concept can be extended to a wide range of method combinations.

Description: To investigate the influence of atmospheric model resolution on the representation of daily precipitation extremes, ensemble simulations with the atmospheric general circulation model ECHAM5 at different horizontal (T213 to T31) and vertical (L31 to L19) resolutions and forced with observed sea surface temperatures and sea ice concentrations have been carried out for 01/1982 - 09/2010. All results have been compared with the highest resolution, which has been validated against observations. Resolution affects both the representation of physical processes and the averaging of precipitation across grid boxes. The latter, in particular, smoothes out localized extreme events. These effects have been disentangled by averaging precipitation simulated at the highest resolution to the corresponding coarser grid. Extremes are represented by seasonal maxima, modeled by the generalized extreme value distribution. Effects of averaging and representation of physical processes vary with region and season. In the tropical summer hemisphere, extreme precipitation is reduced by up to 30% due to the averaging effect, and a further 65% owing to a coarser representation of physical processes. Towards mid- to high latitudes, the latter effect reduces to 20%; in the winter hemisphere it vanishes towards the poles. A strong drop is found between T106 and T63 in the convection dominated tropics. At the lowest resolution, northern hemisphere winter precipitation extremes, mainly caused by large scale weather systems, are in general represented reasonably well. Coarser vertical resolution causes an equatorward shift of maximum extreme precipitation in the tropics. The impact of vertical resolution on mean precipitation is less pronounced; for horizontal resolution it is negligible.

Description: In this doctoral thesis, extreme precipitation as simulated by climate models is investigated by applying state-of-the art statistical extreme value models. The representation of extreme precipitation in such simulations is evaluated, a statistical post-processing method to improve climate model simulated precipitation is developed, and the sensitivity of extreme precipitation to Mediterranean sea surface temperatures (SSTs), as well as the responsible mechanisms, is investigated. The impact of atmospheric model resolution on the representation of extreme precipitation is studied in simulations with the atmosphere general circulation model (AGCM) ECHAM5 at different horizontal (from T213 to T31) and vertical (from L31 to L19) resolutions. The resolution dependence of daily extreme precipitation is studied globally, i.e. beyond the limited areas with high-quality gridded observations, by taking the highest resolution T213 as reference. Resolution affects both the representation of physical processes and the averaging of precipitation across gridboxes. These effects are disentangled and quantified by averaging simulated precipitation in the highest resolution to the coarser grids before computing the return levels. In the convection-dominated tropics and in the extratropics during summer, a horizontal resolution of at least T106 is required to represent return levels comparable to those found at the highest resolution (T213). In the mid and high latitudes in winter, only marginal differences between return levels at the different resolutions are found. Coarser vertical resolution causes an equatorward shift of maximum extreme precipitation in the tropics. For mean precipitation, the impact of vertical resolution is less pronounced; for horizontal resolution it is negligible. These findings provide guidance to modellers in choosing an appropriate AGCM resolution for studies of extreme precipitation. To improve the representation of climate model simulated precipitation, including extremes, a combined statistical bias correction and stochastic downscaling model is developed and evaluated across different European climates. Precipitation is in most cases improved by (parts of) our method. The general concept of combining two methods, and thereby separating bias correction and downscaling into two steps, is a powerful approach as it benefits from the respective methodological advantages. The method generally performs better in summer than in winter, and in winter best in the mild winter climate of the Mediterranean region and worst for the continental winter climate in mid and eastern Europe, or Scandinavia. A strength of this two-step approach is that the best combination of methods can be selected. This implies that the concept can be extended to a wide range of method combinations. By employing a sensitivity study with the AGCM ECHAM5, the impact of the recent increase in Mediterranean SSTs on Central European summer precipitation extremes has been studied. Higher Mediterranean SSTs of the recent decades amplify the magnitude of extreme precipitation events associated with cyclones that originate over the Mediterranean Sea, such as Vb-cyclones. This intensification exceeds the Clausius-Clapeyron scaling. The responsible physical mechanism is increased evaporation from, and enhanced atmospheric moisture content over, the Mediterranean Sea. The excess in precipitable water is transported from the Mediterranean Sea to Central Europe, causing stronger precipitation extremes over that region. Our atmosphere model sensitivity experiment suggests that the projected intensification of precipitation related to Vb-cyclones can be attributed to the rise in Mediterranean SSTs, which are projected to continue throughout the 21st century.