Sub-task 5.1 Snow climatological modelling

The statistical model (Jackson, 1977) has been implemented using GIS and layers of snow incidence statistics have been calculated for the UK. However, there are some difficulties with model validation in that the available Meteorological Office Data is unsuitable because of the way in which it has been measured. Modelling snow incidence is a persistent problem and a special meeting was held to discuss the issues at the last EC meeting in Umea. The issues experienced are similar for Sweden, Finland and the UK, and generally stem from inadequate meteorological information. Several additional approaches are currently being considered with respect to the combination of snow and wind since the interaction of these two damage agents is critical for assessing damage risk, particularly in the UK.

The snow review paper (Nykanen et al., 1996). written in collaboration with three other participants (Joensuu, NRS and Umea) summarises the current state of knowledge of snow damage to trees, and has been submitted to Silva Fennica.

Having identified work by Jackson (1977) which could be used as a basic snow incidence model for the UK, the aim was to implement the model using GIS and generate layers of snow incidence for the UK, based on location and height. The model will be validated using independent meteorological data, and a means of analysing the combined incidence of snow and wind researched for the UK.

Sub-task 5.1

The snow model (Jackson 1977) has been implemented using ARC/INFO and AML (Arc Macro Language) programs to generate the statistics detailed below for each location on the digital elevation model (100m resolution) across the UK. Jackson's model can calculate the following statistics, for any location in the UK:

Mean Annual Days with Falling Sleet or Snow

Number of snow days in a 5, 10, 20, 50 year "snowy" winter

Number of snow days in a 5, 10, 20, 50 year "unsnowy" winter

Number of snow days in a median winter

(Snowy = more than Mean Annual Days of falling Snow, Unsnowy means less than the Mean etc.)

Hours of moderate or heavy falling snow in a 5, 10, 20, 50 year winter

For Birmingham only: Probability of hours of moderate or heavy falling snow being exceeded in a winter

For Birmingham and Eskdalemuir only:

Snowfall accumulations (cm) for given hours of snow falling with frequencies of once and twice per year

Snowfall accumulations (cm) for given hours of snow falling with return periods of 2, 5 and 10 years.

In order to investigate the joint occurrence of snow and wind, transects across the UK are being selected along which several meteorological stations lie, and the snow and wind layers are being used together with the meteorological information to make an assessment of the joint probabilities.

Sub-task 5.1

The statistics produced by the snow model are not as detailed as were originally hoped, and don't give actual snow accumulations, however it may be possible to make assumptions about snow distribution in order to use the Birmingham and Eskdalemuir values for making accumulation estimates for other parts of the UK. The estimates still give a useful overview of relative snow accumulation and snowfall in the UK in the absence of any better approaches to this problem. The issue of validation still remains to be tackled, since suitable data has not yet been identified. Available meteorological data does not, generally, record snow falling and snow accumulations. The position of many of these meteorological stations are not in or near forest areas, and therefore their validity in these areas may remain less well tested.

Sub-task 5.1

Closer collaboration with Finland and Sweden in tackling this problem is being pursued, since many of the problems and possible approaches seem to be comparable. In particular, Finland has developed a climate simulator which may provide a useful tool for investigating snow incidence.

Sub-task 5.1

Dissemination to date has involved the preparation of a snow damage review paper, which will shortly be submitted (Nykanen, Peltola, Kellomaki, Quine and Broadgate, 1996). The details of this paper and the abstract will appear on the project web site after submission.

Sub-task 5.1

Considering the difficulties associated with modelling the incidence of damaging snow, the approach identified appears to encompass the essential climatological elements. It is hoped that some further investigations will result in satisfactory progress in this area which is more complex and much less well researched than we originally anticipated.

References

Jackson, M.C. 1977 The occurrence of falling snow over the United Kingdom. Meteorological Magazine, 106 pp 26-38

Nykanen, M­L., Peltola, H., Kellomaki, S., Quine, C. and Broadgate,

M. L. 1996 Factors Affecting Snow Damage of Trees: A Literature

Review. (sent to be submitted to Silva Fennica 6/96).