Ackermann, Ulrike (2012) Darstellung von Wetterlagen im ECHAM5-Atmosphärenmodell. (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 77 pp.

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Abstract

In this thesis the capability of the ECHAM5 atmosphere model in simulating Sea Level Pressure fields in the European/Northeast Atlantic region realistically at a temporal scale of a few days are investigated. Therefore a high resolution run of the model with prescribed Sea Surface Temperature from an ocean model will be compared with the ERA40 reanalysis data for the period from 1959 to 1990 using automated Lamb weather types. The model is able to reproduce the patterns of the weather types with regard to the exact place and intensity of the pressure fields. In comparison with a previous study which used the coupled ECHAM5 ocean atmosphere model at a lower resolution the period of significant high mean difference decreased from Mai-September to August, but in August the difference is now higher. In summer the model has more problems in simulating the circulation correctly than in winter which confirms previous studies. There is a slight improvement if specific weather types are defined for the seasons. In winter the frequency distribution is independent of the use of yearly or seasonal weather types whereas in summer there can be seen large differences. Partially that is due to the fact that the yearly patterns are more similar to the winter patterns than to the summer patterns. The high pressure ridge between Middle Europe and Russia is a winter phenomenon, it can be seen in nearly all winter and yearly weather types. In summer there is instead a pronounced ridge to the Azores high which often is the dominant high pressure system in the chosen region in summer. With the seasonal weather types there is no annual cycle in contrast to the yearly weather types. This means that the seasons do not differ in the direction of the flow but in the distribution and characteristics of the relevant pressure systems. The weather types that occur most frequently are the westerly types in summer as well as in winter which is confirmed by the Großwetterlagen catalog of the German Weather Service. In addition some relationships are shown. A significant anti-correlation between westerly and easterly respectively northerly and southerly weather types is shown. Furthermore the number of days with westerly weather types and the NAO-index are significant positive correlated in winter. Finally the occurrence of westerly weather types can explain one quarter of the variability of the maximum temperature at the
station of Hamburg-Fuhlsb ̈uttel in autumn and winter.

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