Atmospheric rivers and the land surface : drivers of extreme floods

The project aim is to improve our understanding of extreme flood occurrence and to incorporate this knowledge into UK flood forecast tools and longer-term assessments to improve flood preparedness. Flooding is the major UK natural hazard with the winter 2013-14 floods estimated to have caused economic damages of around £1.3 billion (Chatterton et al. 2016). Given this huge sum and evidence that severe flooding is likely to increase (Winsemius et al., 2015), there is a need to have better estimates of when, where and for how long flooding is likely to occur.
Atmospheric rivers (ARs), narrow bands of intense water vapour transport in extratropical cyclones, cause some of the biggest winter floods in western and northern Britain (Lavers et al., 2011), and in Europe, western North America, South America and New Zealand. For approximately 50-80% of observed extreme winter floods in western Britain, ARs are the causal factor (Lavers et al., 2012) and, as ARs can be identified in numerical weather prediction output, methods that detect their presence are being explored for use in operational flood forecasting to enhance forecast lead times (Lavers et al. 2014). To reduce false alarms and better define those areas that will, or will not, be flooded there is a need to understand why in the remaining severe winter floods there is no link to AR occurrence. Initial research suggests the relationship between AR occurrence and severe flooding is dependent on the river catchment characteristics (Lavers et al., 2012) but this has not been tested. Thus, the purpose of this project is to understand why extreme winter floods can occur when no AR is present, and if there is an AR, which catchment characteristics control whether a severe flood is generated or not. This will be achieved through re-examination and enhancement of current methods to identify ARs in datasets of atmospheric water vapour transport, wind speed and direction, consideration of other synoptic features, and a geomorphological assessment of flood generation in nine representative river catchments from Scotland to south west England. The findings will be consolidated into a rule set to describe flood occurrence for inclusion into flood forecast tools and longer-term assessments.
The Environment Agency (EA) initiated this application to integrate concepts of AR occurrence with those of flood generation, and incorporate the unified understanding into flood forecasting tools and longer-term (100 year) flood assessments. The results will be used to improve flood preparedness and policy.
NOVEL ASPECTS of the science addressed by this project includes:
To understand why extreme winter floods can occur when no AR is present, and if there is an AR, which catchment characteristics control whether a severe flood is generated or not. There are three objectives:
1. to identify enhancements to the AR detection algorithm of Lavers et al. (2012) to better detect AR events and/or to identify other atmospheric mechanisms initiating the largest winter floods over the past 100 years;
2. to determine the catchment characteristics that give rise to differences in the relationship between AR occurrence and flood generation (this analysis will help determine when floods do and do not occur in the presence of an AR);
3. to explore how an integrated understanding of flood generation from ARs and catchment characteristics could improve real-time flood forecasting and long-term (100 year) flood projections in operational and policy contexts.
The testing and development of AR detection algorithms and embedding a research student into a team working at the science-policy interface are also novel aspects of the project. Consideration of ARs and floods over the last 100 years is also new.