The creation and climatology of a large independent rainfall event database for Great Britain

Extreme rainfall studies generally focus on hourly or daily rainfall accumulations. Additionally, studies focus on annual maxima (AM) or 'peak intensities'. Although this approach is useful, the use of fixed-duration accumulations simplifies inherently continuous rainfall processes which, at a rain gauge, are experienced as rainstorms of varying duration. A gap also exists in the study of sub-hourly rainfall extremes which are an important source of flash floods. Here, we present the first large-scale study of rainstorms in the United Kingdom. We leverage a recently created sub-hourly resolution rain gauge dataset for Great Britain to identify over 70,000 rainstorms that generated AM rainfall intensities across a range of sub-hourly to daily durations at 1279 rain gauges up to 2018. Our findings are consistent with previous studies of UK rainfall extremes and their climatology, where strong east-west differences are found in the magnitude of autumn and winter long-duration (over 12-h) rainstorms. We observe broad similarities in the behaviour of short (under 4-h) rainstorms across GB, which are dominated by seasonal variations in convective activity rather than by geographical location. Our study reveals substantial overlaps between AM of different durations, with single rainstorms containing AM across a wide range of durations. We also demonstrate the substantial correlation between rainstorm duration and volume. These results challenge common independence assumptions used in the statistical modelling of rainfall extremes using extreme value theory. Our results represent an opportunity to refine rainfall design methods towards improved, robust representations of rainstorms.