Atmospheric river orientation determines flood occurrence

Helen V Griffith,David A Lavers,Glenn Watts,Andrew J Wade

doi:10.1002/hyp.13905

Abstract

AbstractAtmospheric Rivers (ARs) have been linked to many of the largest recorded UK winter floods. These large‐scale features can be 500–800 km in width but produce markedly different flood responses in adjacent catchments. Here we combine meteorological and hydrological data to examine why two impermeable catchments on the west coast of Britain respond differently to landfalling ARs. This is important to help better understand flood generation associated with ARs and improve flood forecasting and climate‐change impact assessment. Analysis of 32 years of a newly available ERA5 high‐resolution atmospheric reanalysis and corresponding 15‐min river flow data show that the most impactful ARs arise through a combination of the orientation and magnitude of their water vapour flux. At the Dyfi catchment, AR orientations of between 238–258° result in the strongest hydrological responses, whereas at the Teifi the range is 224–243°. We believe this differential flood response is the result of catchment orientation and topography enhancing or suppressing orographic rainfall totals, even in relatively low‐relief coastal catchments. Further to the AR orientation, ARs must have an average water vapour flux of 400–450 kg m−1 s−1 across their lifetime. Understanding the preferential properties of impactful ARs at catchments allows for the linking of large‐scale synoptic features, such as ARs, directly to winter flood impacts. These results using two test catchments suggest a novel approach to flood forecasts through the inclusion of AR activity.

Highlights

Atmospheric Rivers (ARs) are narrow regions of enhanced low-level moisture transport in mid-latitude cyclones (Browning & Pardoe, 1973; Newell, Newell, Zhu, & Scott, 1992) that are responsible for most of the meridional water vapour transport across the midlatitudes (Zhu & Newell, 1998)
In the UK, ARs are an important cause of floods, with some catchments having up to 80% of their largest winter floods associated with AR events (Lavers et al, 2011; Lavers, Villarini, Allan, Wood, & Wade, 2012)
Within the preferential orientation ranges identified above, we identify the mean IVT for the POT3 events to be 500 kg m−1 s−1 compared to 440 kg m−1 s−1 for the non-AR group, with all POT3 flood generating ARs existing above a threshold of 400 kg m−1 s−1

Summary

Introduction

Atmospheric Rivers (ARs) are narrow regions of enhanced low-level moisture transport in mid-latitude cyclones (Browning & Pardoe, 1973; Newell, Newell, Zhu, & Scott, 1992) that are responsible for most of the meridional water vapour transport across the midlatitudes (Zhu & Newell, 1998) These plumes of warm, moist air can generate very high rainfall totals as they cross elevated terrain and have been linked to floods in many coastal regions of the world (Barth, Villarini, Nayak, & White, 2017; Dettinger, 2011; Kingston, Lavers, & Hannah, 2016; Lavers et al, 2011; Lavers & Villarini, 2013; Paltan et al, 2017; Stohl, Forster, & Sodemann, 2008). Given the synoptic scale of ARs and their estimated widths of around 500–800 km, which means an AR would most likely affect both catchments, the aim of this study is to understand why these two catchments demonstrate such different hydrological responses to what may be expected a priori to be similar meteorological conditions

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Conclusion