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

欧洲的洪灾事件可能会造成巨大的经济损失，对基础设施造成破坏，并有可能致命。对产生降水的物理过程有一个很好的了解，这对于预报降水和评估后续洪水的风险很重要。然而，目前有一场关于极端降雨和洪水产生机制的公开科学辩论。一些研究表明，英国的许多洪灾事件与中纬度气旋内的低层潮湿气流有关。这些凝结水蒸气的低层细丝被称为大气河流。大气河流被认为是水分河流，将水分从亚热带地区输送到温带地区，当遇到地形时，水分在那里沉淀出来。然而，在雷丁大学，我们发现了气旋输送水分的一种新机制。最近的研究表明，中纬度降水与一种被称为“支线气流”的气流有关。这项研究表明，在一组有限的中纬度气旋中，气旋当地环境中的水分主导了气旋的水分供应，并推动了当地的降水。这是一个新的科学领域，我们需要扩展这一分析，以检查广泛的中纬度气旋的支流机制，以进一步检验这一假说。我们将通过在再分析资料中识别和分析过去中纬度气旋的气候学来执行这一分析。博士生将使用在阅读中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。项目的第二部分将专注于登陆中纬度气旋，以了解支流如何与地形相互作用产生强降水。我们假设，当从中纬度气旋的同光生成的高层云(播种云)落下的降水穿过覆盖山顶的低层地形云(支线云)时，英国经常会产生极端降水总量。当这种情况发生时，山上的降水可以通过一种被称为播种器-喂食器机制的机制迅速增强。学生将评估模型模拟在不同分辨率下重现气旋内部物理过程的效果。这位博士生将与英国气象局环境预测小组和全球模式评估和发展小组的成员合作，研究广泛的案例研究，以调查支流如何与地形相互作用，从而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.