Abstract/Summary

Both atmospheric rivers and the warm conveyor belts of extratropical cyclones have been linked to heavy precipitation and flooding in the UK. A low-level cyclone-relative moist airflow called the feeder airstream has been shown to transport moisture from the environment ahead of an intense cyclone to the base of the warm conveyor belt where it rises to form precipitation and enhances the poleward transport of moisture in an atmospheric river. In this study, the presence of a feeder airstream is investigated using a case study and a climatology of 1000 storms. The likelihood of a feeder airstream occurring, and the strength of the feeder airstream in relation to cyclone intensity, propagation velocity and life-cycle stage is determined. In the case study, a feeder-airstream is identified during the developing stage of the storm. The feeder airstream is also identified in the majority of the cyclones in the climatology. However, the strength of the moisture flux associated with the feeder airstream decreases with a decrease in maximum intensity, propagation velocity and, in the latter stages of the cyclone life-cycle. To quantify the contribution of synoptic scale and micro-scale processes leading to the precipitation associated with the case study, the proportion of rainfall attributed to cyclone ascent, orographic ascent and the seeder-feeder mechanism is calculated when the extratropical cyclone with a feeder airstream and an atmospheric river passes over the UK. Modifications to the height of the orography and the accretion and riming micro-physical processes are made in the Met Office Unified Model. It is found that the majority of the precipitation is produced by cyclone ascent and the accretion and riming related to the seeder-feeder mechanism. This suggests that heavy precipitation is formed by a combination of cyclone precipitation and low-level cloud formation associated with the feeder airstream.