Abstract/Summary

Atmospheric Rivers (ARs) are one of the main mechanisms of water vapour transport outside of the tropics. Defined as a region of strong horizontal water vapour flux typically located just ahead of the cold front of an extratropical cyclone, they can result in large quantities of precipitation when forced upward, for example by mountains or ascent in the Warm Conveyor Belt. Previous work has investigated the relationship between ARs and the strongest winter floods across a series of British river basins over the period 1979-2010, finding that between 40- 80% of all landfalling AR events were followed by a subsequent flooding event. Two Welsh catchments ∼70 km apart demonstrated the strongest and weakest relationships respectively; a surprising result given the typical width of ARs to be on the order of 1000 km. This thesis uses newly available high-resolution datasets to link the arrival of large-scale atmospheric features (in the form of ARs) to local hydrological observations. AR orientation in relation to the land-surface is found to be a primary control on AR impact potential. The role of additional catchment properties is quantified, allowing for the first time, a measure of the extent to which the land-surface modulates the impact of the strongest ARs. In combination with a refining of the AR detection tool, it is possible to predict where an AR will be the most impactful based on its orientation and magnitude alone. These results have developed an understanding of the extent to which ARs are responsible for winter flooding events across the UK as well as increasing flood forecasting potential.