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).