How atmospheric rivers feed extreme flooding events

Atmospheric rivers are very aptly named, given that one can often carry a greater flux of water than the Amazon. They are responsible for 90% of the poleward transport of moisture on our planet. Thousands of miles long but only a few hundred miles wide, atmospheric rivers often appear on satellite imagery as elongated tendrils of moisture stretching from the tropics to the mid-latitudes.

When an atmospheric river makes landfall, often through interactions with mid-latitude cyclones, extreme flooding events can occur. The features act as a conveyor belt feeding huge amounts of tropical moisture into weather systems, intensifying the rainfall.

Perhaps the most famous example of atmospheric rivers is the “Pineapple Express”, which can deposit huge amounts of rainfall (and sometimes snow) to Pacific coastal regions of the US.

The effects of atmospheric rivers are also felt in the UK. On 4 and 5 December 2015, interactions between an atmospheric river and Storm Desmond led to record-breaking rainfall across north-west England. With the enhanced moisture supply, orographic rainfall across Cumbria broke the 24-hour UK rainfall record, with 341.4mm (13.4in) falling in Honister Pass.

Such huge rainfall totals led to the flooding of 5,200 properties in Lancashire and Cumbria, with a further 1,000 properties flooded in the Scottish Borders.

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