Winter Daily Precipitation Variability over Northern Scotland

Abstract

ABSTRACTDaily winter (December, January and February) precipitation variability over mainland Northern Scotland (1994–2004) is analysed using 99 rain gauges. Principal component analysis (PCA) is used to identify four main patterns of rainfall variability that collectively account for nearly 75% of the variance. The most important pattern (PC 1) shows a west to east decline in rainfall; and is associated with a westerly wind flow, orographic enhancement and rain shadow effects. PC 2 shows a south to north decline in rainfall, a distribution that is generated disproportionately by southerly winds and a more southerly storm track than PC 1's pattern. PC 3 produces the most rainfall in Aberdeenshire and the least in Inverness-shire and Argyll. This spatial distribution is caused mostly by south-easterly flows, with the lowest pressure occurring in eastern Scotland or over the North Sea. PC 4 exhibits a localised precipitation maximum in Nairnshire and Moray, which tends to occur on days with a north to north-easterly wind. Cluster analysis is used to identify six rainfall regions, with regionalisations being derived for each wind direction. The most important sub-division in the region's rainfall climatology is between Aberdeen and Banff and the remainder of the mainland; rainfall variability over time in these two eastern counties thus differs considerably from other parts of Northern Scotland. The Caithness Plain also emerges as a distinct region despite its small area. Distinct rainfall regions are most evident on days when the wind blows from the westerly quadrant.