Extreme sub-daily rainfall in Europe : climatology and dynamical drivers

Abstract

This thesis explores the large-scale dynamical drivers of sub-daily rainfall extremes in western Europe. Initially a sub-daily extreme rainfall climatology for the region is developed using a set of extreme indices for short duration rainfall. The spatiotemporal connections between synopticscale atmospheric circulation and summer (JJA) 3-hour extremes are then assessed, using a set of 30 weather patterns developed by the UK Met Office (MO30) and composites of geopotential height at 500hPa (z500). Finally, the relationships between Rossby wave patterns and extreme 3- hour rainfall are examined, using the local finite amplitude wave activity metric (LWA), and composites of 300hPa meridional wind (v300) anomalies. The results reveal sub-daily extremes are more intense and more frequent in southern and Alpine areas of western Europe, with greatest intensity and frequency of extremes in autumn in the Iberian Peninsula and in summer across the rest of the region. Other index results indicate these events are produced by very short or peaked storms which occur preferentially in the afternoon or evening. The spatiotemporal correlations between MO30 weather patterns (WP) and extreme sub-daily events show a strong preference for extremes to occur under just 5 or 6 of the 30 patterns. Comparing the composite z500 anomalies for all WP days against only WP days with an extreme rainfall event indicates there is significant intra-WP variability on the event days. WPs favouring sub-daily extremes are generally associated with a southerly or south-westerly flow over western Europe, along the leading edge of a trough. There is often also a blocking anomaly present over the Baltic Sea. Rossby wave patterns on the days of the most extreme 3-hour events in the UK and Germany (used as case study regions) show a clear wave train stretching westwards from Europe, with a positive anticyclonic anomaly over Scandinavia and a positive cyclonic anomaly to the west of the UK, echoing the patterns seen in the z500 analysis. While the group velocity (energy) of the wave train moves downstream over time, the phase speed appears to be very low, suggesting there is a level of stationarity to the wave train on days preceding the extreme events. Overall, the results show strong links between large-scale atmospheric dynamics and the occurrence of sub-daily extremes. As large-scale atmospheric variables are much better represented in both forecast and climate models than individual extreme rainfall events, the relationships identified here could be used to increase the lead time of probabilistic forecasts of periods with higher risk of sub-daily rainfall extremes. The UK Met Office Decider system calculates the daily WP probability from an ensemble prediction system and uses this to generate probabilistic forecasts of periods with higher risk of extreme daily rainfall. A similar system for forecasting periods with increased risk of sub-daily extremes could be developed from these results