Changing patterns of heavy rainfall in upland areas: A case study from northern England

摘要

There has recently been a widespread shift in the pattern of UK rainfall towards more heavy falls of rain in winter and fewer in summer. Here, this change is examined in the context of orographic enhancement for a transect of rain gauges running across northern England from coast to coast and including both the Lake District and Pennine uplands. Gauges have been selected where very long records of daily rainfall exist; where data are missing, these have been infilled using data from nearby gauges. The very long records for Armagh and Durham are also included to provide additional context in time and space. For the upland gauges, the increase in total winter rainfall in recent decades and the simultaneous decrease in total summer rainfall are reflected in the number of heavy falls of rain, as defined using two threshold indices. The 1990s saw record numbers of heavy falls in winter and an almost complete absence of heavy summer rainfall in the uplands, in marked contrast to lowland gauges. Comparison of the rainfall record with the Lamb Weather Catalogue suggests that increased winter rainfall is related to an increase in the rainfall provided by westerly weather types. Decreased summer rainfall is related to a reduction in rainfall associated with cyclonic weather types. The results presented here underline the value of long-term monitoring and the maintenance of records from key historic sites.