Wind-driven rain and future risk to built heritage in the United Kingdom: Novel metrics for characterising rain spells.

Wind-driven rain (WDR) is rain given a horizontal velocity component by wind and falling obliquely. It is a prominent environmental risk to built heritage, as it contributes to the damage of porous building materials and building element failure. While predicted climate trends are well-established, how they will specifically manifest in future WDR is uncertain. This paper combines UKCP09 Weather Generator predictions with a probabilistic process to create hourly time series of climate parameters under a high-emissions scenario for 2070-2099 at eight UK sites. Exposure to WDR at these sites for baseline and future periods is calculated from semi-empirical models based on long-term hourly meteorological data using ISO 15927-3:2009. Towards the end of the twenty-first century, it is predicted that rain spells will have higher volumes, i.e. a higher quantity of water will impact façades, across all 8 sites. Although the average number of spells is predicted to remain constant, they will be shorter with longer of periods of time between them and more intense with wind-driven rain occurring for a greater proportion of hours within them. It is likely that in this scenario building element failure - such as moisture ingress through cracks and gutter over-spill - will occur more frequently. There will be higher rates of moisture cycling and enhanced deep-seated wetting. These predicted changes require new metrics for wind-driven rain to be developed, so that future impacts can be managed effectively and efficiently.