In the first of a series of articles on how the mountains of Scotland influence our approach to monitoring and flood forecasting, Mike Kendon writes about their role on rainfall.

“Much of Scotland’s landscape is dominated by its mountains. Although relatively modest in height, reaching typically around 1000 metres in elevation, they nevertheless have a dramatic influence on rainfall.

Due to its northerly latitude, Scotland is located close to the North Atlantic jet stream and therefore lies in the path of low pressure systems and associated rain-bearing fronts. As the westerly airflow is forced to rise over this barrier, low-level clouds are formed over the mountains. When precipitation from higher level ‘seeder’ clouds fall through the lower level ‘feeder’ cloud, the droplets collide and coalesce. This mechanism, known as the ‘seeder-feeder’ effect, is responsible for greatly enhanced orographic precipitation.

As a consequence, the West Highlands receive rainfall in abundance. Annual average rainfall may exceed 3500mm per year, and there may be a ‘day of rain’ (exceeding 1mm) on average almost two days out of every three. However, this orographic effect is highly localised and rainfall rapidly diminishes from west to east across Scotland, so much so that parts of the east coast (such as Angus, Fife and East Lothian) receive less than 700mm – a five-fold reduction – and these areas are among the driest parts of the UK (figure 1).

Scotland’s rainfall is not evenly distributed through the year. The wettest months are from October to March, associated with the normal seasonal southerly shift in the Atlantic storm track – and it is not uncommon for 50mm to fall widely across the West Highlands in a day – locally totals sometimes exceed 100mm. The driest and more settled months tend to occur in late spring and early summer.

The seasonal influence on rainfall tends to reduce moving eastward, as this area increasingly falls within the rain-shadow of the mountains. In eastern Scotland, there may be a greater proportion of convective, rather than frontal rainfall, and rainfall totals on the east coast are more evenly distributed through the year. Figure 2 shows monthly average rainfall totals moving west to east – Glendessary, West Highlands (annual average 3508mm), Gairlochy, in the Great Glen (2168mm), Dalwhinnie, Central Highlands (1304mm), Braemar, Grampians (932mm) and Inverbervie, east coast (683mm).

The broad west-east contrast shows the most obvious influence of Scotland’s mountains on rainfall, but in reality the picture is much more complex. For example, in an easterly airstream the pattern may be reversed, with the wettest weather across the Grampians and the West Highlands falling into the rain-shadow. Scotland’s complex coastline also influences its rainfall patterns; for example coastal fringes of the West Highlands and Islands may be much drier than mountainous areas a few miles inland. Across upland areas, much of the precipitation falls as snow during the winter months, and lying snow often lasts well into the summer months. Over the mountain summits, snow may fall (if not lie) at almost any time of year.

Understanding Scotland’s rainfall distribution is important for many sectors – e.g. water resources, flood risk management – agriculture, ecology and hydro-power. A network of several hundred rain-gauges across the country provides data to help us monitor rainfall across Scotland, and understand trends, variability and extremes. Annual precipitation is, in general, dominated by a large annual variability about a relatively stable long-term mean, but with an increase in the latest few decades (Figure 3). Understanding these patterns in rainfall and how these may be changing as our climate changes, remains an important area of ongoing research for the Met Office and other research institutions. But for Scotland in particular, what makes monitoring precipitation so fascinating is the complexity of the climate across relatively short distances, and this complexity is in large part due to the influence of the mountains.”

References

Hollis, D, McCarthy, MP, Kendon, M, Legg, T, Simpson, I. HadUK‐Grid—A new UK dataset of gridded climate observations. Geosci Data J. 2019; 6: 151– 159. https://doi.org/10.1002/gdj3.78

Kendon, M., McCarthy, M., Jevrejeva, S., Matthews, A., Sparks, T. and Garforth, J. (2020), State of the UK Climate 2019. Int J Climatol, 40: 1-69. https://doi.org/10.1002/joc.6726