Van Bebber's cyclone tracks at 700 hPa in the Eastern Alps for 1961-2002 and their comparison to Circulation Type Classifications
Hofstätter, Michael; Chimani, Barbara
In this study, a systematic tracking of atmospheric cyclones at the pressure level of 700 hPa has been performed to determine three different track types and their climatologic characteristics over Central Europe from 1961 to 2002 by using ERA-40 (ECMWF - European Centre for Medium Range Weather Forecast, 40 year Re-Analysis) and ERA-Interim data. The specific focus is on cyclone tracks of type V as suggested by Van Bebber in 1891 and congeneric types, because of their association with extreme large-scale precipitation amounts and related flooding in Central and Eastern Europe. The tracking procedure consists of detecting isolated minima of the band-pass filtered 700 hPa geopotential height field and combining them over time to continuous tracks by nearest neighbour approach. Results show that the common Vb cyclone track is a rare event (3.5/year) and the probability of occurrence is largest in April with a secondary maximum in autumn. Furthermore, there is no temporal trend of any of these track types noticeable throughout the 42-years investigation period. The new 700 hPa cyclone track catalogue is used as a reference to verify the hypothesis of a coherence of Vb-tracks with certain circulation types (CTs). Selected objective and subjective circulation type classifications (CTCs) from COST (European Cooperation in Science and Technology) action 733 as well as a manual Vb-classification from the ZAMG (Zentralanstalt für Meteorologie und Geodynamik) are utilized. This study shows both the shortcomings and the potential of CTCs to discriminate Vb cyclone tracks by certain classes of stationary circulation patterns. Subjective CTCs rank higher than objective ones in terms of Brier Skill Score, which demonstrates the power to enhance relevant synoptic phenomena instead of favouring mathematical criteria. For the first time, an objective catalogue with the famous cyclone track Vb and its climatology is now on hand for the period 1961-2002, offering a valuable basis to further improve our knowledge on the issues of Vb and related hydro-meteorological aspects.