Abstract
The synoptic‐scale meteorological conditions leading up to the 30 most extreme subdaily summer rain events for two regions of the United Kingdom (northwest and southeast) were examined for the period 1979–2013. Using a recently available, quality controlled, national hourly rain gauge data set, we were able to identify extreme 3‐hr 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 reanalyses and in climate models. The results show that the two regions have different dominant synoptic‐scale conditions leading to extreme 3‐hr 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 200 hPa over the United Kingdom are associated with extreme rainfall in the northwest, where the position of the westerly jet is also important. For the southeast, no clear anomalous synoptic‐scale conditions could be identified; however, localized 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.
Highlights
A warming climate is likely to lead to an intensification of heavy rainfall (Allan & Soden, 2008), which may in turn lead to an increased risk of flash flooding
The results presented suggest that there is greater moisture available in the immediate vicinity of the United Kingdom and leading up to northwest England (NW) region extreme rainfall, compared to both the monthly average and to southern England (SE) region extremes
The top 30 events for each region were identified and composites of the synoptic-scale atmospheric conditions were created from ERA-Interim reanalysis data
Summary
A warming climate is likely to lead to an intensification of heavy rainfall (Allan & Soden, 2008), which may in turn lead to an increased risk of flash flooding. It can be expected that locally, intense events are dominated by short-lived, small-scale meteorological (e.g., convective) processes but these events are typically difficult to identify in relatively coarse resolution reanalysis data sets. This is true for their identification in most climate model simulations due to their coarse resolution and the use of parameterized convection schemes, this is being addressed by the emerging generation of convection permitting regional climate models (Kendon et al, 2016; Prein et al, 2017). The spatial patterns of synoptic-scale atmospheric conditions leading up to, and associated with, these events are examined, using a composite approach to identify potential precursor signals in the synoptic conditions, atmospheric stability, and moisture availability fields
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