A regional frequency analysis of UK sub-daily extreme precipitation and assessment of their seasonality

Floods related to extreme precipitation events, especially intense, short-duration precipitation, may cause significant damage in urbanized areas, including transport infrastructure, electricity networks, and property. These events are expected to increase in frequency with climate change but their characteristics, at either hourly or multi-hourly timescales, have been little studied due to short and poor quality data records. We examine annual maximum (AMAX) hourly and multi-hourly (3, 6, 12, and 24hr) precipitation accumulations in the United Kingdom using a quality-controlled hourly precipitation data set for the period 1992-2014. We describe their seasonality and diurnal cycle and use a regional frequency analysis (RFA) approach with L-moments to produce at-site return level estimates, and then use existing extreme precipitation regions to provide regional-scale return levels. The analysis shows a clear seasonality and the dominant occurrence of short-duration AMAX in summer with similar seasonality for 1 and 3 hr accumulation periods in some regions, while longer-duration AMAX (12 and 24hr) behave similarly to each other in all regions but are distributed over a longer period including late autumn and winter. The diurnal cycle of 1 hr AMAX indicates that most extremes occur during the afternoon, with a peak typically between 1400 and 1700, especially in southern and eastern regions. However, we also demonstrate that existing regions for UK daily extremes are not able to adequately reflect differences at shorter durations and that new regions should be created. These results provide new insights to help in designing urban drainage systems and infrastructure, including the need for new scaling relationships between sub-daily and design accumulation periods.