The feeder airstream: investigating the mechanisms causing extreme precipitation in cyclones

Flooding events in Europe can cause huge economic losses, damage to infrastructure and are potentially fatal. A good understanding of the physical processes that generate precipitation is important for forecasting precipitation and assessing the risk of subsequent flooding. However, there I currently an open scientific debate regarding the mechanisms responsible for generating extreme rainfall and flooding.

Some studies have shown that many flooding events in the UK are associated with low-level moist airflows within mid-latitude cyclones. These low-level filaments of condensed water vapour are known as atmospheric rivers. Atmospheric rivers are thought to be rivers of moisture that transport moisture from the subtropics to the extratropics where it then precipitates out when it encounters orography. However, at Reading University, we have identified anew mechanism by which cyclones transport moisture. Recent research has shown that mid-latitude precipitation is linked to an airflow known as the 'feeder-airstream'. This research demonstrates, in a limited set of mid-latitude cyclones, that moisture in the environment local to the cyclone dominates the supply of moisture to the cyclone and drives local precipitation. This is a new area of science and we need to expand this analysis to examine the feeder-stream mechanism in a wide range of mid-latitude cyclones to test the hypothesis further. We will perform this analysis by identifying and analyzing a climatology of past mid-latitude cyclones in reanalysis data. The PhD student will use the methodology developed in Reading to model and track cyclones in order to understand the dynamics responsible for generating cyclone precipitation.

The second part of the project will focus on land-falling mid-latitude cyclones in order to understand how the feeder-stream interacts with orography to generate heavy precipitation. We hypothesise that extreme precipitation totals are frequently generated in the UK when precipitation falling from a mid-latitude cyclone's synoptically generated upper-level cloud (seeder cloud) falls through the lower-level orographic cloud capping a hill (feeder cloud). When this occurs, precipitation over the hill can intensify rapidly via a mechanism known as the seeder-feeder mechanism. The student will evaluate how well model simulations reproduce physical processes within the cyclone at various resolutions. The PhD student will work with members of the Environment Prediction group and Global Model Evaluation and Development group at the Met Office to examine a wide range of case studies to investigate how the feeder stream interacts with orography in order to quantify how high intensity precipitation leads to flooding.