THE WINTERSTORM VIVIAN OF 27 FEBRUARY 1990 - ABOUT THE METEOROLOGICALDEVELOPMENT, WIND FORCES AND DAMAGE SITUATION IN THE FORESTS OF SWITZERLAND

During the months January and February 1990 a series of severe cyclone s were responsible for enormous wind-induced damage in Europe. The fin al of this series, on 27 February 1990, cyclone ''Vivian'' mainly affe cted the alpine valleys of Switzerland. 5 Millions m3 of timber were f elled by the severe winds, a record number in this century. A complete damage survey of the deforested areas offers in combination with mete orological data an unique data set for a detailed case study of this e xtreme event. This paper describes the general meteorological developm ent from the synoptic scale down to the mesoscale of Switzerland and p resents a general overview of the damage situation. The main results s how that a rare situation of a straight frontal zone stretching over t he whole Atlantic Ocean and showing a strong gradient in temperature p ointed directly toward Central-Europe. Two waves formed along this elo ngated polar front and deepend rapidly to depressions. The first low t ravelled on the southernmost trajectory of the whole storm series and affected Switzerland most. North of the Alps the prefrontal warm air w as blocked to the east by the arriving coldfront and had to escape int o the complex terrain of the alpine valleys. There, the stormy winds w ere strengthened by channelizing and ''Fohn'' effects. The large tempe rature gradient between the prefrontal and the incoming air masses ind uced thunderstorm activity which vortices and downdrafts might have en hanced locally. As a result most of the damaged forested areas were fo und between 1200 and 1600 m MSL on slopes, which were mainly exposed t oward the prevailing NW-winds. A comparison of extreme wind speeds for the period 1978-1992 revealed that this event's extreme high speed of 74.5 m/s, measured at a high elevated pass station in the mountains, was exceptional. For lower elevated stations the wind speeds were high but in the range of other observed extreme values. In addition to the severe wind forces the duration of sustained high wind speed was exce ptionally long during February 1990.