Abstract : 1C.9
An objective Alpine weather pattern classification

Reinhold Steinacker, Nicola Biermair
University of Vienna

Weather pattern classifications are presently experiencing a revival. This is caused on the one hand by the availability of homogenous long term reanalysis data and on the other hand by objective means of classification by digital evaluation. Long time series of weather pattern classifications are an interesting means of investigating the global change with respect to the atmospheric circulation. Weather and climate over mountainous terrain is specifically affected by the flow direction. Luv and lee effects strongly affect precipitation and the channelled flows in the valleys and across passes. Therefore long time ago a simple weather type classification for the Eastern Alps was developed considering only the basic flow directions (8 directional classes and one class with negligible basic flow) at low tropospheric levels. The disadvantage of this classification was the relatively large number of days with a variable flow direction, e. g. due to frontal passages. The availability of three hourly Alpine reanalysis fields of the VERACLIM project opened the possibility to classify the flow direction not only on a daily basis but to consider also short term changes. Some evaluations of recent trends in Alpine flow directions and the frequency of frontal passages are shown and discussed.