Atmospheric driving mechanisms of flash floods in Portugal

Abstract

The present study aims to investigate how large‐scale atmospheric circulation may influence flash floods in Portugal. For this purpose, weather types (WTs) in a northeastern North Atlantic sector are used and flash floods between 1950 and 2003 were identified in six hydrographic basins: three in the north of Portugal and another three in the south of Portugal. An adapted methodology for flash flood detection on daily discharges is applied. A total of 131 flash floods were detected: 35 in the northern basins and 96 in the southern basins. WTs are identified using the National Oceanic and Atmospheric Administration 20th Century Reanalysis V2. The sea level pressure composites for flash flood days by WT reveal the large‐scale atmospheric patterns underlying their occurrence. The results show that the cyclonic (C) type was strongly associated with flash flood days in the northern basins. The C type pattern features deep low pressure systems north of Iberia, much stronger than average for this WT. For the southern basins, the C type still shows the highest relevance for flood occurrence, but to a lesser extent, since the easterly wind (E) and dual anticyclonic (AA) types also acquire some importance. The flash flood‐inducing AA‐type events are characterized by atypical largely zonal troughs extending towards Portugal. The flash flood‐inducing E type events hint at the occurrence of anomalously strong cut‐off low pressure systems over southern Portugal. Although the significance of these systems to precipitation in Portugal was addressed in previous studies, a systematized assessment of their role on flash flood occurrences is herein provided for the first time. The presented flash flood detection methodology can be applied to other regions where hourly flow discharge data are not available, thus being an alternative to the more conventional detection algorithms.

Regionalization and susceptibility assessment to daily precipitation extremes in mainland Portugal

“…In addition to extreme precipitation, there are other very important conditioning factors for landslides and floods, such as: physiographic conditions of the basins, soil types, relief, slope or lithological units, in mainland Portugal (e.g. Henriques, Santos, Santos, & Fragoso, 2017;Santos & Reis, 2017;Zêzere et al, 2015).…”

Synoptic‐Scale Precursors of Extreme U.K. Summer 3‐Hourly Rainfall

“…The synoptic‐scale atmospheric processes leading up to intense rainfall and flash flooding events have been investigated in a number of countries. Santos et al () used weather types to investigate flash flooding across three river basins in Portugal, although the study was not limited to the summer, while Barbero et al () found that disturbances associated with the jet stream and cutoff upper‐level lows were related to the occurrence of 1‐hr annual maximum precipitation across much of the United States. Summer flash flood studies in the United Kingdom have generally focused either on individual case studies or convective‐scale processes (e.g., Flack et al, ; Golding et al, ; Warren et al, ).…”

Meteorological Driving Mechanisms and Human Impacts of the February 1979 Extreme Hydro-Geomorphological Event in Western Iberia

“…It has been well known that extreme precipitation events in Portugal occurring during the extended winter months (October-March) are often associated with landslides and flooding in the major river basins [1][2][3] and, with flash floods in small watersheds or urban areas [4,5] which can produce socio-economic impacts. Thus, in order to prevent and manage the associated effects and risks, there is a clear need for better characterization of the associated extreme meteorological metrics and the nature of likely unusual strong atmospheric driving mechanisms.…”