Fundamental influences of large-scale wave dynamics on tropical weather systems

水资源短缺已被确定为热带地区居民健康和安全面临的最严重的环境威胁。供水方面的压力通过对粮食供应的压力和疟疾和脑膜炎等疾病的传播破坏了稳定。然而，在热带地区，特别是撒哈拉以南非洲等干旱气候带，降水预测显示出非常高的不确定性，这些地区的人口脆弱性是世界上最高的。在21世纪的耦合模拟中，萨赫勒地区降水的前景仍然非常不确定，对于未来降雨是多还是少，以及高影响天气的频率和强度将如何变化，没有达成共识。熟练的降雨预报将在从几小时到几十年的所有时间尺度上都有巨大的好处。特别是为公共和航空业提供短期预报（最多2天），为农业、水文和卫生信息提供中期预报（10-30天）。这是具有挑战性的，因为降雨是通过大尺度波浪模式、天气系统和孤立的深层对流上升气流的复杂相互作用组织起来的。个别对流上升气流的位置是不可预测的，甚至中尺度天气系统的发生和演变在数值天气预报（NWP）模式中也表现得很差。然而，由于对流活跃和不活跃区域的模式通常由大尺度波浪结构决定，因此有希望获得更大的可预测性。这种现象的例子包括非洲东风波和赤道波。由于波浪的存在，对于同时在几个地点同时发生的高影响天气事件具有潜在的可预测性。例如，如果在一个大范围的波浪中产生了几个风暴，每一个风暴都可能带来重大的自然灾害，并带来安全和经济影响（这种相关事件在保险业使用的风险模型中没有考虑到）。不幸的是，热带大尺度波浪的模拟存在严重缺陷，这些大尺度波浪通常在预报中衰减得太快，传播得太慢。然而，修正模式以改进大尺度热带波的模拟是困难的，因为目前热带波的基础理论太弱，无法解决这个问题。有必要更好地制定不同过程如何影响波的演变，以便修改可以旨在改善波的表现。拟议项目的目的是发展热带大尺度波浪背后的理论，使其能够应用于观测到的天气系统的定量诊断。在这样做的过程中，我们的目标是确定在波浪的产生、维持和传播中最重要的过程，以及它们在模型中被歪曲的方式，以期改善天气预报。该研究将通过三个复杂阶段来研究大尺度波浪与对流降雨之间的相互作用：A)假设小振幅的波浪动力学；B)大振幅方面，包括喷流强度和波浪振幅之间的波动；C)利用新开发的理论解释当前最先进的热带波浪预报的缺陷。这项研究的预期好处是改善了在一天到一个季节的主要时间内整个热带地区的降雨天气预报。C阶段将通过与两个世界领先的业务预报中心（英国气象局和欧洲中期天气预报中心）的项目伙伴合作来推进。

Scarcity of water has been identified as the most serious environmental threat facing the health and security of people living in the tropics. Pressures on water supply undermine stability through stresses on food availability and spread of disease e.g., malaria and meningitis. Yet predictions of precipitation show very high uncertainty in the Tropics and especially the arid climatic zones such as sub-Saharan Africa where the vulnerability of the population is amongst the highest in the world. The outlook for Sahel precipitation in coupled simulations of the twenty-first century remains very uncertain with no consensus as to whether there will be more or less rain in the future, or how the frequency and intensity of high impact weather will change. Skillful forecasts of rainfall would be of enormous benefit across all timescales ranging from hours to decades. In particular, short-range forecasts (up to 2 days) for the public and aviation industry, and medium-range forecasts (10-30 days) for agriculture, hydrology and health information. This is challenging since rainfall is organised through a complex interplay of large-scale wave patterns, weather systems and isolated deep convective updrafts. The locations of individual convective updrafts are not predictable and even the occurrence and evolution of mesoscale weather systems are poorly represented in Numerical Weather Prediction (NWP) models. However, there is some hope for greater predictability since the pattern of active and inactive regions of convection is often determined by large-scale wave structure. Examples of such phenomena include African Easterly Waves (AEWs) and equatorial waves. Owing to the waves, there is potential predictability for high impact weather events occurring simultaneously in several locations at once. For example, if several storms are spawned within a large-scale wave, each one of them could present a significant natural hazard, with safety and financial implications (such correlated events are not accounted for in the risk models used by the insurance industry). Unfortunately, there are severe deficiencies in the simulation of tropical large-scale waves which typically decay far too quickly in forecasts and propagate too slowly. However, it is difficult to amend models to improve the simulation of large-scale tropical waves because underpinning theory for tropical waves is currently too weak to unpick the problem. It is necessary to formulate better how different processes influence wave evolution so that modifications can be aimed at improving wave representation. The aim of the proposed project is to develop the theory behind large-scale waves in the tropics to the level where it can be applied in the quantitative diagnosis of observed weather systems. In doing so we aim to identify the processes that are most important in wave initiation, maintenance and propagation, and ways in which they are misrepresented in models, with a view to improving weather forecasts. The research will study the interplay between large-scale waves and convective rainfall through three stages of complexity: A) the dynamics of waves assuming small amplitude, B) large-amplitude aspects including vacillations between jet strength and wave amplitude, and C) explaining deficiencies in state-of-the-art forecasts of tropical waves using the new theory developed. The anticipated benefit of the research is improvement in weather forecasts of rainfall throughout the tropics at lead times of a day to a season. Stage C will be advanced through collaboration with project partners from two world-leading operational forecast centres: the Met Office and European Centre for Medium-Range Weather Forecasts.