Clustering and stalling of North Atlantic cyclones: the influence on precipitation in England and Wales

Recently, several large-scale flooding events in England and Wales have been caused by multiple cyclones in a short period of time (clustering), or slow moving storms (stalling). The question of how much precipitation is associated with clustered or stalled extratropical cyclones is addressed using continuous areas of precipitation to associate extreme precipitation events to specific extratropical cyclones. This method is applied to ERAInterim/HadUKP data and the HadGEM2-ES Historical and RCP8.5 climate model experiments for 1, 7, 13 and 31-day precipitation accumulations. In ERA-Interim, extreme wet events (p ≥ 0.98) in England and Wales are associated with 20% to 45% more cyclones than wet events (p ≥ 0.5) in winter, spring and autumn. Mean cyclone residence times are generally longer in extreme wet events than wet events for all seasons. Longer residence times are associated with a quasi-stationary wavenumber 6 planetary wave in spring, summer and autumn. Clustering is a less important process for extreme England and Wales precipitation events in HadGEM2-ES than in ERA-Interim. Stalling as important for summer extreme wet events in HadGEM2-ES as in ERA-Interim, however, stalling in winter extreme events is under-represented. Projected increases in extreme winter England and Wales precipitation events in HadGEM2-ES are primarily associated with increased atmospheric moisture availability rather than changes to clustering or stalling. The ability of reanalyses to represent extreme England and Wales precipitation is also evaluated. ERA-Interim and 20CR only identify 45% to 55% of observed daily p98 precipitation events over England and Wales. Clustering and stalling are significant influences on England and Wales precipitation, mainly affecting winter and summer precipitation events respectively. However, future changes in England and Wales precipitation are likely to be governed more by thermodynamic changes than changes in circulation patterns