WINTER STORMS IN SWITZERLAND NORTH OF THE ALPS 1864 1865-1993/1994/

In the framework of the Swiss National Research Program 31 ''Climate c hanges and natural disasters'' the question was brought up whether a g lobal warming of the atmosphere would have an influence on the frequen cy and/or intensity of the extratropical storms of the Swiss winter se ason. In order to investigate a possible trend, time series of days wi th a minimum wind speed estimate or measurement equivalent to Beaufort 7, 8 and 9 were established. The longest being a record of the mesone t station Zurich from the period 1864 until 1993 (130 years). Slightly shorter time series for three additional stations in Northern Switzer land were compiled to control the behavior of the longest record and t o verify the observed temporal trend. From the location of the four in vestigated wind records the observational domain was restricted to the part of Switzerland north of the Alps whereas for further meteorologi cal considerations the whole North-Atlantic-European area was included . A negative regional trend in the number of storm days has been obser ved during the last century. In particular, the period before 1940 has to be interpreted as windier than the following decades. The duration of a storm event also decreased on average. In spite of an increase i n cyclonic westwind situations since about 1960 over Europe, Switzerla nd was hit by fewer storms during the same period. One explanation cou ld be that the whole westwind belt has moved slightly further north wh ere a deepening of the cyclones was observed in recent times. Switzerl and is usually situated at the southernmost edge of the particular sto rm fields and is therefore less influenced by strong gales. Rare excep tions are the cases when a secondary depression directly hits Central Europe, e.g. as happened in February 1990 (storm ''Vivian'').