Volosciuk, Claudia D. (2016) Variability of Extreme Precipitation on Different Scales in Statistical and Numerical Models. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 154 pp.

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Abstract

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.

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