Relating new theories of extratropical cyclone development to the present and future atmosphere

气旋对中纬度国家的人民和经济有重大影响。破坏性影响包括与暴雨相关的严重大风和洪水。这些天气系统对温度、云量和降雨量的影响也不那么戏剧性，但却是至关重要的。例如，2000年秋季给英国带来了前所未有的降雨量和持续时间，并引发了人们对其与气候变化关系的猜测。作为回应，DEFRA委托了一份报告(FD2304)来研究这种联系。这次降雨与异常持续和反复经过的锋面低压有关，而不是特别极端的气旋。在哈德利中心气候模式对本世纪末的模拟中，发现这种季节性极端降雨更为频繁，尽管自然变率如此之大，以至于2000年秋季的降雨量不能归因于人类对气候的影响。气候模型对地球系统的描述存在许多不确定性，特别是考虑到风暴的行为。然而，风暴频率和强度变化背后的物理机制从理论上还没有得到充分的理解，无法确定模拟结果是否在物理上是合理的。英国的天气预报几乎每一次都会参考即将到来或即将到来的气旋及其相关锋面。数值天气预报模式通常通过数值求解运动和热力学的原始方程，并将较小尺度的影响参数化，从而对未来5天以上的气旋发生提供良好的预测。虽然大尺度动力学理论涉及由称为位涡的单量演化控制的平衡流，但这种平衡模型不够准确，不能用于业务预报。此外，气旋发展的理论假设它们是从一个不随纬度圈变化的“背景状态”发展而来的，但大气层永远不会经过这样的状态。因此，理论能力与预报模式对大气的描述能力之间存在着不匹配。尽管如此，该理论确实为理解预测的演变及其误差提供了一个框架，而且该理论还有改进的余地，以至于它可以定量区分潜在的增长机制。在关于未来风暴行为改变的有力论据中，理论可以发挥更大的作用。例如，政策制定者和许多其他社会部门想知道，在温室气体强迫导致的全球变暖中，“极端风暴”是否会变得更加频繁--如果这些变化真的发生了，它们将产生重大的经济影响。该项目的目的是将中纬度气旋的一些最新理论应用于对实际大气的分析，试图确定对气旋特性的可靠预测，例如地面风强对背景状态特性的依赖，例如极点到赤道的温度梯度。通过建立背景状态(气候)和风暴行为之间的定量关系，该项目将产生一个框架，用于分析不同气候情景下极端风暴的变化。如果气候模型模拟的风暴变化符合从理论上讲合理的物理推理的预期，就有可能对气候预测产生更大的信心。

Cyclones have a major impact on people and the economies of countries in the mid-latitudes. Damaging impacts include severe gales and flooding associated with heavy rainfall. These weather systems also have less dramatic but nevertheless crucial impacts on temperature, cloudiness and rainfall. For example, autumn 2000 brought UK rainfall of unprecedented extent and duration, and prompted speculation about its relationship with climate change. In response, DEFRA commissioned a report (FD2304) to examine the link. The rainfall was associated with the unusually persistent and repeated passage of frontal depressions, rather than especially extreme cyclones. Such seasonal rainfall extremes were found to be more frequent in simulations of the end of the century by the Hadley Centre climate model, although the natural variability is so large that the autumn 2000 rainfall cannot be attributed to human influence on climate. There are many uncertainties associated with the representation of the Earth system by climate models, especially considering the behaviour of storms. However, the physical mechanisms behind changes in storm frequency and intensity are not sufficiently well understood from a theoretical perspective to determine whether the results of the simulations are physically reasonable. A weather forecast for the UK is hardly ever made without reference to a cyclone and its associated fronts that are with us or due to arrive soon. Numerical weather prediction models generally provide good forecasts of cyclone occurrence out to more than 5 days ahead by solving numerically the 'primitive equations' of motion and thermodynamics on 'resolved scales' and parameterising the effects of smaller scales. Although the theories of large-scale dynamics involve balanced flow governed by the evolution of single quantity called potential vorticity, such balanced models are not sufficiently accurate to be used in an operational forecasting context. Furthermore, theories of cyclone development assume that they grow from a 'background state' that does not vary around latitude circles, but the atmosphere never passes through such a state. Therefore, there is a mismatch between the capabilities of theory and the description of the atmosphere by forecast models. Nevertheless, the theory does provide a framework for understanding the evolution of forecasts and their errors and there is scope for improving theory to the point where it can distinguish quantitatively between potential mechanisms of growth. Theory has a greater role to play in robust arguments for changes in storm behaviour in the future. For example, policy makers and many other sectors of society would like to know whether 'extreme storms' will become more frequent in a warmer world resulting from greenhouse gas forcing - if these changes do occur they will have major economic impacts. The aim of this project is to bring some of the latest theories of mid-latitude cyclones to bear on analyses of the actual atmosphere in an attempt to identify robust predictions for the dependence of cyclone properties, such as surface wind strength, on background state properties, such as the pole to equator temperature gradient. By establishing quantitative relationships between background states (climate) and storm behaviour, this project will produce a framework for analysing changes to extreme storms under different climate scenarios. If the changes to storms simulated by a climate model accord with the expectations of sound physical reasoning, grounded in theory, it would be possible to place greater confidence in the climate projections.