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

欧洲的洪水事件可能造成巨大的经济损失，对基础设施的破坏，并可能致命。充分了解产生降水的物理过程对于预测降水和评估随后发生洪水的风险非常重要。然而，目前有一个关于产生极端降雨和洪水的机制的公开科学辩论。一些研究表明，英国的许多洪水事件与中纬度气旋内的低层潮湿气流有关。这些低层的凝结水蒸气丝被称为大气河流。大气河流被认为是水汽的河流，它将水汽从亚热带输送到热带外，然后在遇到地形时沉淀出来。然而，在阅读大学，我们已经确定了气旋输送水分的新机制。最近的研究表明，中纬度地区的降水与一种被称为“补给气流”的气流有关。这项研究表明，在一组有限的中纬度气旋，气旋当地的环境中的水分占主导地位的气旋的水分供应，并驱动当地的降水。这是一个新的科学领域，我们需要扩大这一分析，以检查在广泛的中纬度气旋的补给流机制，以进一步验证这一假设。我们将通过识别和分析再分析数据中过去中纬度气旋的气候学来进行这种分析。该博士生将使用阅读开发的方法来模拟和跟踪气旋，以了解产生气旋降水的动力学。该项目的第二部分将侧重于登陆中纬度气旋，以了解补给流如何与地形相互作用产生强降水。我们假设，极端的降水总量经常在英国产生时，降水下降从中纬度气旋的天气产生的上层云（播种云）福尔斯下降通过较低级别的地形云覆盖一座山（馈线云）。当这种情况发生时，山上的降水可以通过一种称为播种机-喂入机的机制迅速加强。学生将评估模型模拟如何以各种分辨率再现气旋内的物理过程。博士生将与气象局的环境预测小组和全球模型评估与开发小组的成员合作，研究各种案例研究，以调查支流如何与地形相互作用，以量化高强度降水如何导致洪水。

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.