Synoptic-scale precursors of extreme UK summer 3-hourly rainfall

Abstract

The synoptic-scale meteorological conditions leading up to the 30 most extreme sub-daily summer rain events for two regions of the UK (north-west and south-east) were examined for the period 1979-2013. Using a recently available, quality controlled, national hourly rain gauge dataset, we were able to identify extreme 3-hour rainfall accumulations that may be indicative of flash flooding. Composites of the state of the atmosphere leading up to these dates were produced to investigate synoptic-scale processes, thus potentially allowing for them to be identified in coarse resolution re-analyses and in climate models. The results show that the two regions have different dominant synoptic-scale conditions leading to extreme 3-hour rainfall, which is thought to be related to the type of rainfall typically experienced in each region. In particular, positive anomalies in mean sea level pressure and the geopotential height at 200hPa over the UK are associated with extreme rainfall in the north-west, where the position of the westerly jet is also important. For the south-east, no clear anomalous synoptic-scale conditions could be identified, however localised moisture sources and unstable air masses were observed in association with extremes. These results indicate the importance of better understanding of both synoptic-scale and thermodynamic drivers of short-duration extreme rainfall, with potential implications in forecasting and flood warning, as well as for understanding the representation of key processes by regional climate models.