Vb cyclones and associated rainfall extremes over Central Europe under present day and climate change conditions
Nissen, Katrin M.; Ulbrich, Uwe; Leckebusch, Gregor C.
Cyclones moving on a track from the Mediterranean region towards Central Europe can transport humid air masses into the Central European region. Cyclones on this track are of particular interest, as they have historically often caused major river flooding during the summer season. Such systems are also known as Vb cyclones. In this study, an objective identification algorithm is presented, which detects the systems. The algorithm has been applied to the summer half year (April?September) of the ERA40 reanalysis data set. It is able to capture most documented historic hazardous events in the data set. The amount of precipitation affecting the study region is found to increase with the residence time of the cyclone within the region of interest. About 41% of the Vb cyclones are associated with precipitation exceeding the daily mean 95th percentile over Central Europe. In order to detect possible changes in the frequency and severity of Vb cyclones under rising greenhouse gas forcing, the algorithm was applied to an ensemble of 3 simulations with the ECHAM5/OM1 model covering the period from 1971?2100, which were forced with 20th century and A1B scenario greenhouse gas concentrations. For present day forcing, the model overestimates the number of Vb cyclones. The ratio between systems associated with extreme daily precipitation over the area and systems producing less impact in terms of rainfall is, however, close to the number found in observations. For the future scenario period the simulations produce a decrease in the total number of Vb cyclones, due to an eastward shift in the most common northbound cyclone tracks. At the same time, the mean amount of precipitation associated with the Vb cyclones increases by 16%. Due to long term climate variability this increase is not continuously statistical significant. The number of Vb cyclones associated with extreme precipitation remains stable throughout the 21st century.