Wind throw damages on forests : frequency and associated pressure patterns 1961-1990 and in a future climate scenario

Abstract: Many different simulations of the future climate have shown a probable change of the climate of the earth, mainly caused by anthropogenic emissions. A change in the climate influences many parameters. In this report the frequency of extreme wind situations in Southern Sweden and associated pressure patterns during the period 1961 to 1990 are compared to the conditions for the years 2070-2099. Different methods are tested. The first attempt is based on analog situations. Selected situations associated with observed strong winds during the period 1961-1990 are identified. Reanalyzed mean sea level pressure data from the National Centers for Environmental Predictions (NCEP) is used to find the pressure field associated to each storm situation. Analog situations to these days are then searched in the Hadley Center’s circulation model HadAM3, present day (COM) and future conditions (A2) simulations. The second attempt is based on calculated geostrophic wind. Days with a geostrophic wind exceeding 25 m/s in the NCEP dataset are divided into groups by cluster analysis. Days in the HadAM3COM and A2 simulations that are associated with geostrophic wind speeds over 25 m/s are then divided into these classes. The frequency of each group in HadAM3COM and HadAM3A2 is then compared. Principal component analysis is applied to the days in each run associated to strong geostrophic winds, and the main principal components of the present day and future simulations are compared. The first method is not able to detect any significant differences between the two different simulated periods. The second revealed a slight decrease in total storm frequency, most notable at occasions with a widespread low over Northern Scandinavia and for easterly storms with a high over northern Finland and Russia. No significant changes between HadAM3COM and HadAM3A2 can be seen in their main principal components.