Modelling crown snow loads in Finland: a comparison of two methods

Abstract

Modelling crown snow loads in Finland: a comparison of two methodsLehtonen I., Hoppula P., Pirinen P., Gregow H. (2014). Modelling crown snow loads in Finland: a comparison of two methods. Silva Fennica vol. 48 no. 3 article id 1120. 30 p. Highlights• A new method to model crown snow loads is presented and compared with a previously published simpler method.• The heaviest crown snow loads in Finland are found to typically occur in the eastern parts of the country.• The relative importance of different snow load types varies between different regions of Finland. AbstractThe spatial occurrence of heavy crown snow loads in Finland between 1961 and 2010 is studied by using for the first time a model that classifies the snow load into four different types: rime, dry snow, wet snow and frozen snow. In producing this climatology, we used meteorological observations made at 29 locations across Finland. The model performance is evaluated against classified daily images of canopy snow cover and with the help of two short case studies. The results are further compared to those achieved with a simpler method used in previous studies. The heaviest crown snow loads are found to occur typically in eastern Finland. The new method reveals that this holds not only for the total snow loads but also for the different snow load types, although there are certain differences in their geographical occurrence. The greatest benefit achieved with the new method is the inclusion of rime accretion. The forests most prone to heavy riming are those located on tree-covered hills in northern Finland, but as the terrain elevation affects riming efficiency greatly, these small-scale variations in the snow load amounts could not be described in this study in great detail. Moreover, the results are more inaccurate in northern Finland where variations in the terrain elevation are greater than elsewhere. Otherwise, the largest uncertainties in this study are related to wind speed measurements and possibly partly because of that, we were not able to detect any significant trends in the crown snow-load amounts over the study period.

Heavy snow loads in Finnish forests respond regionally asymmetrically to projected climate change

“…This method has been developed at FMI for operational purposes as an early-warning tool for heavy snow loads on tree crowns mainly threatening the functionality of electric power network. A detailed description of the method was presented by Lehtonen et al (2014) with discussion of the climatology of heavy crown snow loads in Finland. Lehtonen et al (2014) moreover briefly evaluated the model performance with the help of a few example cases and observational data from one station.…”

“…A detailed description of the method was presented by Lehtonen et al (2014) with discussion of the climatology of heavy crown snow loads in Finland. Lehtonen et al (2014) moreover briefly evaluated the model performance with the help of a few example cases and observational data from one station. The model assumes an exemplar tree having a cone-shaped crown with a projected catchment area of 1 m 2 from above and from the side in the direction of wind.…”

“…3). The division of Finland into subregions follows that by Lehtonen et al (2014). Temperature and relative humidity with respect to water are generally projected to increase towards end of the century, the increase being larger in the north than in the south.…”

“…The G08 method estimates the amount of snow load on tree crowns by applying precipitation, temperature and wind speed observations as input. However, Lehtonen et al (2014) showed that when the snow load is classified into different components (rime, dry snow, wet snow and frozen snow), the snow loads simulated by the G08 method correlate best with dry snow loads having little importance with regard to the forest damage. Therefore, it will be interesting to apply a more sophisticated method in studying the climate change impacts on heavy snow loads on tree crowns.…”

“…In this study, we apply the snow load calculation and classification method used by Lehtonen et al (2014), hereafter referred to as the FMI method. Our aim is to assess the climate change impacts on the occurrence of heavy snow loads on tree crowns in Finland.…”