Atmospheric precursors for intense summer rainfall over the United Kingdom

Abstract

Intense sub‐daily summer rainfall is linked to flooding impacts in the United Kingdom. Characterizing the atmospheric conditions prior to the rainfall event can improve understanding of the large‐scale mechanisms involved. The most intense sub‐daily rainfall intensity data generated from rain gauge records across the United Kingdom over the period 1979–2014 are combined with fields from the ERA Interim reanalysis to characterize atmospheric conditions prior to heavy rainfall events. The 200 most intense 3‐hourly events for six UK regions are associated with negative anomalies in sea level pressure (<−2 hPa) and 200 hPa geopotential height (<−60 m) to the west or south west of the United Kingdom 1 day earlier, with above average moisture, evaporation and dew point temperature over North West Europe. Atmospheric precursors are more intense but less coherent between regions for composites formed of the 25 heaviest rainfall events but all display substantial moisture transport from the south or south east prior to their occurrence. Composites for the heaviest events are characterized by a tripole geopotential anomaly pattern across the North Atlantic. Above average geopotential height and dew point temperature over Newfoundland and below average geopotential height but elevated evaporation in the North Atlantic are found to be weakly associated with an increased chance of the most intense sub‐daily rainfall events 5–9 days later.

North Atlantic air pressure and temperature conditions associated with heavy rainfall in Great Britain

“…Similarly, we included part of North Africa to see if the temperature there would result in a separate summer cluster for a Spanish Plume mechanism. This means our study region is slightly larger than the one considered by, for example, Allan et al (2019).…”

“…The clustering results in a range of combined synoptic patterns, instead of a single composite pattern for each driver separately as in, for example, the detailed sub‐daily summer rainfall studies of Allan et al . (2019) and Champion et al . (2019).…”

“…Further to this, the atmospheric river contribution to rainfall mainly occurs during the winter half year when extratropical cyclones (within which atmospheric rivers occur) are more prevalent (Lavers and Villarini, 2015) and hence do not explain summer extremes. Allan et al (2019) investigated synoptic precursors of extreme short-duration (3-hr) summer rainfall events in the UK, including dew point temperature, evaporation, geopotential height at 200 hPa and sea-level pressure (SLP) anomalies, geopotential height anomalies, average moisture and evaporation patterns over northwest Europe and the North Atlantic. Their results reveal different conditions associated with intense rainfall events in different parts of the country.…”

“…Allan et al . (2019) investigated synoptic precursors of extreme short‐duration (3‐hr) summer rainfall events in the UK, including dew point temperature, evaporation, geopotential height at 200 hPa and sea‐level pressure (SLP) anomalies, geopotential height anomalies, average moisture and evaporation patterns over northwest Europe and the North Atlantic. Their results reveal different conditions associated with intense rainfall events in different parts of the country.…”

Identifying and interpreting extreme rainfall events using image classification

“…Further to this, both Figures 6 and 7 indicate on the day of occurrence the conditions across the rest of the North Atlantic are not contributing to the resulting classifications. This raises the question of how these contributions could change if the classifiers were trained using patterns of the days prior to an event, as it is known that certain prior meteorological conditions are common to some extreme rainfall events (Allan et al 2019).…”